genode/doc/porting_guide.txt

                                ====================
                                Genode Porting Guide
                                ====================

                                  Genode Labs GmbH


Overview
########

This document describes the basic workflows for porting applications, libraries,
and device drivers to the Genode framework. It consists of the following
sections:

:[http:porting_applications - Porting third-party code to Genode]:
  Overview of the general steps needed to use 3rd-party code on Genode.

:[http:porting_dosbox - Porting a program to natively run on Genode]:
  Step-by-step description of applying the steps described in the first
  section to port an application, using DosBox as an example.

:[http:porting_libraries - Native Genode port of a library]:
  Many 3rd-party applications have library dependencies. This section shows
  how to port a library using SDL_net (needed by DosBox) as an example.

:[http:porting_noux_packages - Porting an application to Genode's Noux runtime]:
  On Genode, there exists an environment specially tailored to execute
  command-line based Unix software, the so-called Noux runtime. This section
  demonstrates how to port and execute the tar program within Noux.

:[http:porting_device_drivers - Porting devices drivers]:
  This chapter describes the concepts of how to port a device driver to the
  Genode framework. It requires the basic knowledge introduced in the previous
  chapters and should be read last.

Before reading this guide, it is strongly advised to read the "The Genode
Build System" documentation:

:Build-system manual:

  [http://genode.org/documentation/developer-resources/build_system]


Porting third-party code to Genode
##################################

Porting an existing program or library to Genode is for the most part a
straight-forward task and depends mainly on the complexity of the program
itself. Genode provides a fairly complete libc based on FreeBSD's libc whose
functionality can be extended by so-called libc plugins. If the program one
wants to port solely uses standard libc functions, porting becomes easy. Every
porting task involves usually the same steps which are outlined below.


Steps in porting applications to Genode
=======================================

# Check requirements/dependencies (e.g. on Linux)

  The first step is gathering information about the application,
  e.g. what functionality needs to be provided by the target system and
  which libraries does it use.

# Create a port file

  Prepare the source code of the application for the use within Genode. The
  Genode build-system infrastructure uses fetch rules, so called port files,
  which declare where the source is obtained from, what patches are applied
  to the source code, and where the source code will be stored and
  configured.

# Check platform dependent code and create stub code

  This step may require changes to the original source code
  of the application to be compilable for Genode. At this point, it
  is not necessary to provide a working implementation for required
  functions. Just creating stubs of the various functions is fine.

# Create build-description file

  To compile the application we need build rules. Within these rules
  we also declare all dependencies (e.g. libraries) that are needed
  by it. The location of these rules depends on the type
  of the application. Normal programs on one hand use a _target.mk_ file,
  which is located in the program directory (e.g. _src/app/foobar_)
  within a given Genode repository. Libraries on the other hand use
  one or more _<library-name>.mk_ files that are placed in the _lib/mk_
  directory of a Genode repository. In addition, libraries have to
  provide _import-<library-name>.mk_ files. Amongst other things, these
  files are used by applications to find the associated header files
  of a library. The import files are placed in the _lib/import_
  directory.

# Create a run script to ease testing

  To ease the testing of applications, it is reasonable to write a run script
  that creates a test scenario for the application. This run script is used
  to automatically build all components of the Genode OS framework that are
  needed to run the application as well as the application itself. Testing
  the application on any of the kernels supported by Genode becomes just a
  matter of executing the run script.

# Compile the application

  The ported application is compiled from within the respective build
  directory like any other application or component of Genode. The build
  system of Genode uses the build rules created in the fourth step.

# Run the application

  While porting an application, easy testing is crucial. By using the run script
  that was written in the fifth step we reduce the effort.

# Debug the application

  In most cases, a ported application does not work right away. We have to
  debug misbehaviour and implement certain functionality in the platform-depending
  parts of the application so that is can run on Genode. There are
  several facilities available on Genode that help in the process. These are
  different on each Genode platform but basically break down to using either a
  kernel debugger (e.g., JDB on Fiasco.OC) or 'gdb(1)'. The reader of this guide
  is advised to take a look at the "User-level debugging on Genode via GDB"
  documentation.

_The order of step 1-4 is not mandatory but is somewhat natural._


Porting a program to natively run on Genode
###########################################

As an example on how to create a native port of a program for Genode, we will
describe the porting of DosBox more closely. Hereby, each of the steps
outlined in the previous section will be discussed in detail.


Check requirements/dependencies
===============================

In the first step, we build DosBox for Linux/x86 to obtain needed information.
Nowadays, most applications use a build-tool like Autotools or something
similar that will generate certain files (e.g., _config.h_). These files are
needed to successfully compile the program. Naturally they are required on
Genode as well. Since Genode does not use the original build tool of the
program for native ports, it is appropriate to copy those generated files
and adjust them later on to match Genode's settings.

We start by checking out the source code of DosBox from its subversion repository:

! $ svn export http://svn.code.sf.net/p/dosbox/code-0/dosbox/trunk@3837 dosbox-svn-3837
! $ cd dosbox-svn-3837

At this point, it is helpful to disable certain options that are not
available or used on Genode just to keep the noise down:

! $ ./configure --disable-opengl
! $ make > build.log 2>&1

After the DosBox binary is successfully built, we have a log file
(build.log) of the whole build process at our disposal. This log file will
be helpful later on when the _target.mk_ file needs to be created. In
addition, we will inspect the DosBox binary:

! $ readelf -d -t src/dosbox|grep NEEDED
!  0x0000000000000001 (NEEDED)  Shared library: [libasound.so.2]
!  0x0000000000000001 (NEEDED)  Shared library: [libdl.so.2]
!  0x0000000000000001 (NEEDED)  Shared library: [libpthread.so.0]
!  0x0000000000000001 (NEEDED)  Shared library: [libSDL-1.2.so.0]
!  0x0000000000000001 (NEEDED)  Shared library: [libpng12.so.0]
!  0x0000000000000001 (NEEDED)  Shared library: [libz.so.1]
!  0x0000000000000001 (NEEDED)  Shared library: [libSDL_net-1.2.so.0]
!  0x0000000000000001 (NEEDED)  Shared library: [libX11.so.6]
!  0x0000000000000001 (NEEDED)  Shared library: [libstdc++.so.6]
!  0x0000000000000001 (NEEDED)  Shared library: [libm.so.6]
!  0x0000000000000001 (NEEDED)  Shared library: [libgcc_s.so.1]
!  0x0000000000000001 (NEEDED)  Shared library: [libc.so.6]

Using _readelf_ on the binary shows all direct dependencies. We now know
that at least libSDL, libSDL_net, libstdc++, libpng, libz, and
libm are required by DosBox. The remaining libraries are mostly
mandatory on Linux and do not matter on Genode.  Luckily all of these
libraries are already available on Genode. For now all we have to do is to
keep them in mind.


Creating the port file
======================

Since DosBox is an application, which depends on several ported
libraries (e.g., libSDL), the _ports_ repository within the Genode
source tree is a natural fit. On that account, the port file
_ports/ports/dosbox.port_ is created.

For DosBox the _dosbox.port_ looks as follows:

! LICENSE   := GPLv2
! VERSION   := svn
! DOWNLOADS := dosbox.svn
!
! URL(dosbox) := http://svn.code.sf.net/p/dosbox/code-0/dosbox/trunk
! DIR(dosbox) := src/app/dosbox
! REV(dosbox) := 3837

First, we define the license, the version and the type of the source code
origin. In case of DosBox, we checkout the source code from a Subversion
repository. This is denoted by the '.svn' suffix of the item specified in
the 'DOWNLOADS' declaration. Other valid types are 'file' (a plain file),
'archive' (an archive of the types tar.gz, tar.xz, tgz, tar.bz2, or zip)
or 'git' (a Git repository).
To checkout the source code out from the Subversion repository, we also need
its URL, the revision we want to check out and the destination directory
that will contain the sources afterwards. These declarations are mandatory and
must always be specified. Otherwise the preparation of the port will fail.

! PATCHES := $(addprefix src/app/dosbox/patches/,\
!                        $(notdir $(wildcard $(REP_DIR)/src/app/dosbox/patches/*.patch)))
!
! PATCH_OPT := -p2 -d src/app/dosbox

As next step, we declare all patches that are needed for the DosBox port.
Since in this case, the patches are using a different path format, we have
to override the default patch settings by defining the _PATCH_OPT_ variable.

Each port file comes along with a hash file. This hash is generated by taking
several sources into account. For one, the port file, each patch and the
port preparation tool (_tool/ports/prepare_port_) are the ingredients for
the hash value. If any of these files is changed, a new hash will be generated,
For now, we just write "dummy" in the '_ports/ports/dosbox.hash_ file.

The DosBox port can now be prepared by executing

! $ <genode-dir>/tool/ports/prepare_port dosbox

However, we get the following error message:

! Error: <rep-dir>/ports/dosbox.port is out of date, expected <fingerprint>

We get this message because we had specified the "dummy" hash value in
the _dosbox.hash_ file. The prepare_port tool computes a fingerprint
of the actual version of the port and compares this fingerprint with the
hash value specified in _dosbox.hash_. The computed fingerprint can
be found at _<genode-dir>/contrib/dosbox-dummy/dosbox.hash_. In the final
step of the port, we will replace the dummy fingerprint with the actual
fingerprint of the port. But before finalizing the porting work, it is
practical to keep using the dummy hash and suppress the fingerprint check.
This can be done by adding 'CHECK_HASH=no' as argument to the prepare_port tool:

! $ <genode-dir>/tool/ports/prepare-port dosbox CHECK_HASH=no


Check platform-dependent code
=============================

At this point, it is important to spot platform-dependent source files or
rather certain functions that are not yet available on Genode. These source
files should be omitted. Of course they may be used as a guidance when
implementing the functionality for Genode later on, when creating the
_target.mk_ file. In particular the various 'cdrom_ioctl_*.cpp' files are such
candidates in this example.


Creating the build Makefile
===========================

Now it is time to write the build rules into the _target.mk_, which will be
placed in _ports/src/app/dosbox_.

Armed with the _build.log_ that we created while building DosBox on Linux,
we assemble a list of needed source files. If an application just
uses a simple Makefile and not a build tool, it might be easier to just
reuse the contents of this Makefile instead.

First of all, we create a shortcut for the source directory of DosBox by calling
the 'select_from_ports' function:

! DOSBOX_DIR := $(call select_from_ports,dosbox)/src/app/dosbox

Under the hood, the 'select_from_ports' function looks up the
fingerprint of the specified port by reading the corresponding
<port-name>.hash file. It then uses this hash value to construct the
directory path within the _<genode-dir>contrib/_ directory that belongs to
the matching version of the port. If there is no hash file that matches the
port name, or if the port directory does not exist, the build system
will back out with an error message.

Examining the log file leaves us with the following list of source files:

! SRC_CC_cpu      = $(notdir $(wildcard $(DOSBOX_DIR)/src/cpu/*.cpp))
! SRC_CC_debug    = $(notdir $(wildcard $(DOSBOX_DIR)/src/debug/*.cpp))
! FILTER_OUT_dos  = cdrom_aspi_win32.cpp cdrom_ioctl_linux.cpp cdrom_ioctl_os2.cpp \
!                   cdrom_ioctl_win32.cpp
! SRC_CC_dos      = $(filter-out $(FILTER_OUT_dos), \
!                   $(notdir $(wildcard $(DOSBOX_DIR)/src/*.cpp)))
! […]
! SRC_CC          = $(DOSBOX_DIR)/src/dosbox.cpp
! SRC_CC         += $(SRC_CC_cpu) $(SRC_CC_debug) $(SRC_CC_dos) $(SRC_CC_fpu) \
!                   $(SRC_CC_gui) $(SRC_CC_hw) $(SRC_CC_hw_ser) $(SRC_CC_ints) \
!                   $(SRC_CC_ints) $(SRC_CC_misc) $(SRC_CC_shell)
!
! vpath %.cpp $(DOSBOX_DIR)/src
! vpath %.cpp $(DOSBOX_DIR)/src/cpu
! […]

_The only variable here that is actually evaluated by Genode's build-system is_
'SRC_CC'. _The rest of the variables are little helpers that make our_
_life more comfortable._

In this case, it is mandatory to use GNUMake's 'notdir' file name function
because otherwise the compiled object files would be stored within
the _contrib_ directories. Genode runs on multiple platforms with varying
architectures and mixing object files is considered harmful, which can happen
easily if the application is build from the original source directory. That's
why you have to use a build directory for each platform. The Genode build
system will create the needed directory hierarchy within the build directory
automatically. By combining GNUMake's 'notdir' and 'wildcard' function, we
can assemble a list of all needed source files without much effort. We then
use 'vpath' to point GNUMake to the right source file within the dosbox
directory.

The remaining thing to do now is setting the right include directories and proper
compiler flags:

! INC_DIR += $(PRG_DIR)
! INC_DIR += $(DOSBOX_DIR)/include
! INC_DIR += $(addprefix $(DOSBOX_DIR)/src, cpu debug dos fpu gui hardware \
!                                           hardware/serialport ints misc shell)

'PRG_DIR' _is a special variable of Genode's build-system_
_and its value is always the absolute path to the directory containing_
_the 'target.mk' file._

We copy the _config.h_ file, which was generated in the first step to this
directory and change certain parts of it to better match Genode's
environment. Below is a skimmed diff of these changes:

! --- config.h.orig       2013-10-21 15:27:45.185719517 +0200
! +++ config.h    2013-10-21 15:36:48.525727975 +0200
! @@ -25,7 +25,8 @@
!  /* #undef AC_APPLE_UNIVERSAL_BUILD */
!
!  /* Compiling on BSD */
! -/* #undef BSD */
! +/* Genode's libc is based on FreeBSD 8.2 */
! +#define BSD 1
!
! […]
!
! /* The type of cpu this target has */
! -#define C_TARGETCPU X86_64
! +/* we define it ourself */
! +/* #undef C_TARGETCPU */
!
! […]

Thereafter, we specify the compiler flags:

! CC_OPT  = -DHAVE_CONFIG_H -D_GNU_SOURCE=1 -D_REENTRANT
! ifeq ($(filter-out $(SPECS),x86_32),)
! INC_DIR += $(PRG_DIR)/x86_32
! CC_OPT += -DC_TARGETCPU=X86
! else ifeq ($(filter-out $(SPECS),x86_64),)
! INC_DIR += $(PRG_DIR)/x86_64
! CC_OPT += -DC_TARGETCPU=X86_64
! endif
!
! CC_WARN  = -Wall
! #CC_WARN += -Wno-unused-variable -Wno-unused-function -Wno-switch \
!            -Wunused-value -Wno-unused-but-set-variable

As noted in the commentary seen in the diff we define 'C_TARGETCPU'
and adjust the include directories ourselves according to the target
architecture.

While debugging, compiler warnings for 3rd-party code are really helpful but
tend to be annoying after the porting work is finished, we can
remove the hashmark to keep the compiler from complaining too
much.

Lastly, we need to add the required libraries, which we acquired in step 1:

! LIBS += libc libm libpng sdl stdcxx zlib
! LIBS += libc_lwip_nic_dhcp config_args

In addition to the required libraries, a few Genode specific
libraries are also needed. These libraries implement certain
functions in the libc via the libc's plugin mechanism.
libc_lwip_nic_dhcp, for example, is used to connect the BSD socket interface
to a NIC service such as a network device driver.


Creating the run script
=======================

To ease compiling, running and debugging DosBox, we create a run script
at _ports/run/dosbox.run_.

First, we specify the components that need to be built

! set build_components {
!   core init drivers/audio drivers/framebuffer drivers/input
!   drivers/pci drivers/timer app/dosbox
! }
! build $build_components

and instruct _tool/run_ to create the boot directory that hosts
all binaries and files which belong to the DosBox scenario.

As the name 'build_components' suggests, you only have to declare
the components of Genode, which are needed in this scenario. All
dependencies of DosBox (e.g. libSDL) will be built before DosBox
itself.

Nextm we provide the scenario's configuration 'config':

! append config {
! <config>
!   <parent-provides>
!     <service name="ROM"/>
!     <service name="RAM"/>
!     <service name="IRQ"/>
!     <service name="IO_MEM"/>
!     <service name="IO_PORT"/>
!     <service name="CAP"/>
!     <service name="PD"/>
!     <service name="RM"/>
!     <service name="CPU"/>
!     <service name="LOG"/>
!     <service name="SIGNAL"/>
!   </parent-provides>
!   <default-route>
!     <any-service> <parent/> <any-child/> </any-service>
!   </default-route>
!   <start name="audio_drv">
!     <resource name="RAM" quantum="6M"/>}
!     <provides><service name="Audio_out"/></provides>
!   </start>
!   <start name="fb_drv">
!     <resource name="RAM" quantum="4M"/>
!     <provides><service name="Framebuffer"/></provides>
!   </start>
!   <start name="ps2_drv">
!     <resource name="RAM" quantum="1M"/>
!     <provides><service name="Input"/></provides>
!   </start>
!   <start name="timer">
!     <resource name="RAM" quantum="1M"/>
!     <provides><service name="Timer"/></provides>
!   </start>
!   <start name="dosbox">
!     <resource name="RAM" quantum="128M"/>
!     <config>
!       <sdl_audio_volume value="100"/>
!       <libc stdout="/dev/log" stderr="/dev/log">
!         <vfs>
!           <tar name="dosbox.tar"/>
!           <dir name="dev"> <log/> </dir>
!         </vfs>
!       </libc>
!     </config>
!   </start>
! </config>}
! install_config $config

The _config_ file will be used by the init program to start all
components and application of the scenario, including DosBox.

Thereafter we declare all boot modules:

! set boot_modules {
!   core init timer audio_drv fb_drv ps2_drv ld.lib.so
!   libc.lib.so libm.lib.so
!   lwip.lib.so libpng.lib.so stdcxx.lib.so sdl.lib.so
!   pthread.lib.so zlib.lib.so dosbox dosbox.tar
! }
! build_boot_image $boot_modules

The boot modules comprise all binaries and other files like
the tar archive that contains DosBox' configuration file _dosbox.conf_
that are needed for this scenario to run sucessfully.

Finally, we set certain options, which are used when Genode is executed
in Qemu and instruct _tool/run_ to keep the scenario executing as long
as it is not manually stopped:

! append qemu_args " -m 256 -soundhw ac97 "
! run_genode_until forever

_It is reasonable to write the run script in a way that makes it possible_
_to use it for multiple Genode platforms. Debugging is often done on_
_Genode/Linux or on another Genode platform running in Qemu but testing_
_is normally done using actual hardware._


Compiling the program
=====================

To compile DosBox and all libraries it depends on, we execute

! $ make app/dosbox

from within Genode's build directory.

_We could also use the run script that we created in the previous step but_
_that would build all components that are needed to actually run_ DosBox
_and at this point our goal is just to get_ DosBox _compiled._

At the first attempt, the compilation stopped because g++ could not find
the header file _sys/timeb.h_:

! /src/genode/ports/contrib/dosbox-svn-3837/src/ints/bios.cpp:35:23: fatal error:
!         sys/timeb.h: No such file or directory

This header is part of the libc but until now there was no program, which
actually used this header. So nobody noticed that it was missing. This
can happen all time when porting a new application to Genode because most
functionality is enabled or rather added on demand. Someone who is
porting applications to Genode has to be aware of the fact that it might be
necessary to extend Genode functionality by enabling so far disabled
bits or implementing certain functionality needed by the
application that is ported.

Since 'ftime(3)' is a deprecated function anyway we change the code of
DosBox to use 'gettimeofday(2)'.

After this was fixed, we face another problem:

! /src/genode/ports/contrib/dosbox-svn-3837/src/ints/int10_vesa.cpp:48:33: error:
!         unable to find string literal operator ‘operator"" VERSION’

The fix is quite simple and the compile error was due to the fact
that Genode uses C++11 by now. It often happens that 3rd party code
is not well tested with a C++11 enabled compiler. In any case, a patch file
should be created which will be applied when preparing the port.

Furthermore it would be reasonable to report the bug to the DosBox
developers so it can be fixed upstream. We can then get rid of our
local patch.

The next show stoppers are missing symbols in Genode's SDL library port.
As it turns out, we never actually compiled and linked in the cdrom dummy
code which is provided by SDL.


Running the application
=======================

DosBox was compiled successfully. Now it is time to execute the binary
on Genode. Hence we use the run script we created in step 5:

! $ make run/dosbox

This may take some time because all other components of the Genode OS
Framework that are needed for this scenario have to be built.


Debugging the application
=========================

DosBox was successfully compiled but unfortunately it did not run.
To be honest that was expected and here the fun begins.

At this point, there are several options to chose from. By running
Genode/Fiasco.OC within Qemu, we can use the kernel debugger (JDB)
to take a deeper look at what went wrong (e.g., backtraces of the
running processes, memory dumps of the faulted DosBox process etc.).
Doing this can be quite taxing but fortunately Genode runs on multiple
kernels and often problems on one kernel can be reproduced on another
kernel. For this reason, we choose Genode/Linux where we can use all
the normal debugging tools like 'gdb(1)', 'valgrind(1)' and so on. Luckily
for us, DosBox also fails to run on Genode/Linux. The debugging steps
are naturally dependent on the ported software. In the case of DosBox,
the remaining stumbling blocks were a few places where DosBox assumed
Linux as a host platform.

For the sake of completeness here is a list of all files that were created by
porting DosBox to Genode:

! ports/ports/dosbox.hash
! ports/ports/dosbox.port
! ports/run/dosbox.run
! ports/src/app/dosbox/config.h
! ports/src/app/dosbox/patches/bios.patch
! ports/src/app/dosbox/patches/int10_vesa.patch
! ports/src/app/dosbox/target.mk
! ports/src/app/dosbox/x86_32/size_defs.h
! ports/src/app/dosbox/x86_64/size_defs.h

[image dosbox]
  DosBox ported to Genode

Finally, after having tested that both the preparation-step and the
build of DosBox work as expected, it is time to
finalize the fingerprint stored in the _<genode-dir>/ports/ports/dosbox.hash_
file. This can be done by copying the content of the
_<genode-dir>/contrib/dosbox-dummy/dosbox.hash file_.
Alternatively, you may invoke the _tool/ports/update_hash_ tool with the
port name "dosbox" as argument. The next time, you
invoke the prepare_port tool, do not specify the 'CHECK_HASH=no' argument.
So the fingerprint check will validate that the _dosbox.hash_ file
corresponds to your _dosbox.port_ file. From now on, the
_<genode-dir>/contrib/dosbox-dummy_ directory will no longer be used because
the _dosbox.hash_ file points to the port directory named after the real
fingerprint.


Native Genode port of a library
###############################

Porting a library to be used natively on Genode is similar to porting
an application to run natively on Genode. The source codes have to be
obtained and, if needed, patched to run on Genode.
As an example on how to port a library to natively run on Genode, we
will describe the porting of SDL_net in more detail. Ported libraries
are placed in the _libports_ repository of Genode. But this is just a
convention. Feel free to host your library port in a custom repository
of your's.


Checking requirements/dependencies
==================================

We will proceed as we did when we ported DosBox to run natively on Genode.
First we build SDL_net on Linux to obtain a log file of the whole build
process:

! $ wget http://www.libsdl.org/projects/SDL_net/release/SDL_net-1.2.8.tar.gz
! $ tar xvzf SDL_net-1.2.8.tar.gz
! $ cd SDL_net-1.2.8
! $ ./configure
! $ make > build.log 2>&1


Creating the port file
======================

We start by creating _<genode-dir>/libports/ports/sdl_net.port:

! LICENSE   := BSD
! VERSION   := 1.2.8
! DOWNLOADS := sdl_net.archive
!
! URL(sdl_net) := http://www.libsdl.org/projects/SDL_net/release/SDL_net-$(VERSION).tar.gz
! SHA(sdl_net) := fd393059fef8d9925dc20662baa3b25e02b8405d
! DIR(sdl_net) := src/lib/sdl_net
!
! PATCHES := src/lib/sdl_net/SDLnet.patch src/lib/sdl_net/SDL_net.h.patch

In addition to the URL the SHA1 checksum of the SDL_net archive needs to
specified because _tool/prepare_port_ validates the downloaded archive
by using this hash.

Applications that want to use SDL_net have to include the 'SDL_net.h' header
file. Hence it is necessary to make this file visible to applications. This is
done by populating the _<genode-dir>/contrib/sdl-<hash>/include_ directory:

! DIRS := include/SDL
! DIR_CONTENT(include/SDL) := src/lib/sdl_net/SDL_net.h

For now, we also use a dummy hash in the _sdl_net.hash_ file like it was done
while porting DosBox. We will replace the dummy hash with the proper one at
the end.


Creating the build Makefile
===========================

We create the build rules in _libports/lib/mk/sdl_net.mk_:

! SDL_NET_DIR := $(call select_from_ports,sdl_net)/src/lib/sdl_net
!
! SRC_C = $(notdir $(wildcard $(SDL_NET_DIR)/SDLnet*.c))
!
! vpath %.c $(SDL_NET_DIR)
!
! INC_DIR += $(SDL_NET_DIR)
!
! LIBS += libc sdl

'SDL_net' should be used as shared library. To achieve this, we
have to add the following statement to the 'mk' file:

! SHARED_LIB = yes

_If we omit this statement, Genode's build system will automatically_
_build SDL_net as a static library called_ 'sdl_net.lib.a' _that_
_is linked directly into the application._

It is reasonable to create a dummy application that uses the
library because it is only possible to build libraries automatically
as a dependency of an application.

Therefore we create
_libports/src/test/libports/sdl_net/target.mk_ with the following content:

! TARGET = test-sdl_net
! LIBS   = libc sdl_net
! SRC_CC = main.cc

! vpath main.cc $(PRG_DIR)/..

At this point we also create _lib/import/import-sdl_net.mk_
with the following content:

! SDL_NET_PORT_DIR := $(call select_from_ports,sdl_net)
! INC_DIR += $(SDL_NET_PORT_DIR)/include $(SDL_NET_PORT_DIR)/include/SDL

Each port that depends on SDL_net and has added it to its LIBS variable
will automatically include the _import-sdl_net.mk_ file and therefore
will use the specified include directory to find the _SDL_net.h_ header.


Compiling the library
=====================

We compile the SDL_net library as a side effect of building our dummy test
program by executing

! $ make test/libports/sdl_net

All source files are compiled fine but unfortunately the linking of the
library does not succeed:

! /src/genodebuild/foc_x86_32/var/libcache/sdl_net/sdl_net.lib.so:
!     undefined reference to `gethostbyaddr'

The symbol 'gethostbyaddr' is missing, which is often a clear sign
of a missing dependency. In this case however 'gethostbyaddr(3)' is
missing because this function does not exist in Genode's libc _(*)_.
But 'getaddrinfo(3)' exists. We are now facing the choice of implementing
'gethostbyaddr(3)' or changing the code of SDL_net to use 'getaddrinfo(3)'.
Porting applications or libraries to Genode always may involve this kind of
choice. Which way is the best has to be decided by closely examining the
matter at hand. Sometimes it is better to implement the missing functions
and sometimes it is more beneficial to change the contributed code.
In this case, we opt for changing SDL_net because the former function is
obsolete anyway and implementing 'gethostbyaddr(3)' involves changes to
several libraries in Genode, namely libc and the network related
libc plugin. Although we have to keep in mind that it is likely to encounter
another application or library that also uses this function in the future.

With this change in place, SDL_net compiles fine.

_(*) Actually this function is implemented in the Genode's_ libc _but is_
_only available by using libc_resolv which we did not do for the sake of_
_this example._


Testing the library
===================

The freshly ported library is best tested with the application, which was the
reason the library was ported in the first place, since it is unlikely that
we port a library just for fun and no profit. Therefore, it is not necessary to
write a run script for a library alone.

For the records, here is a list of all files that were created by
porting SDL_net to Genode:

! libports/lib/mk/sdl_net.mk
! libports/lib/mk/import/import-sdl_net.mk
! libports/ports/sdl_net.hash
! libports/ports/sdl_net.port
! libports/src/lib/sdl_net/SDLnet.patch
! libports/test/libports/sdl_net/target.mk


Porting an application to Genode's Noux runtime
###############################################

Porting an application to Genode's Noux runtime is basically the same as
porting a program to natively run on Genode. The source code has to be
prepared and, if needed, patched to run in Noux. However in contrast to
this, there are Noux build rules (_ports/mk/noux.mk_) that enable us to use
the original build-tool if it is based upon Autotools. Building the
application is done within a cross-compile environment. In this environment
all needed variables like 'CC', 'LD', 'CFLAGS' and so on are set to their
proper values. In addition to these precautions, using _noux.mk_ simplifies certain things.
The system-call handling/functions is/are implemented in the libc plugin
_libc_noux_ (the source code is found in _ports/src/lib/libc_noux_). All
applications running in Noux have to be linked against this library which is
done implicitly by using the build rules of Noux.

As an example on how to port an application to Genode's Noux runtime, we
will describe the porting of GNU's 'tar' tool in more detail. A ported
application is normally referred to as a Noux package.

Checking requirements/dependencies
==================================

As usual, we first build GNU tar on Linux/x86 and capture the build
process:

! $ wget http://ftp.gnu.org/gnu/tar/tar-1.27.tar.xz
! $ tar xJf tar-1.27.tar.xz
! $ cd tar-1.27
! $ ./configure
! $ make > build.log 2>&1


Creating the port file
======================

We start by creating the port Makefile _ports/ports/tar.mk_:

! LICENSE   := GPLv3
! VERSION   := 1.27
! DOWNLOADS := tar.archive
!
! URL(tar)  := http://ftp.gnu.org/gnu/tar/tar-$(VERSION).tar.xz
! SHA(tar)  := 790cf784589a9fcc1ced33517e71051e3642642f
! SIG(tar)  := ${URL(tar)}.sig
! KEY(tar)  := GNU
! DIR(tar)  := src/noux-pkg/tar

_As of version 14.05, Genode does not check the signature specified via_
_the SIG and KEY declaration but relies the SHA checksum only. However,_
_as signature checks are planned in the future, we use to include the_
_respective declarations if signature files are available._

While porting GNU tar we will use a dummy hash as well.


Creating the build rule
=======================

Build rules for Noux packages are located in _<genode-dir>/ports/src/noux-pkgs_.

The _tar/target.mk_ corresponding to GNU tar looks like this:

! CONFIGURE_ARGS = --bindir=/bin \
!                  --libexecdir=/libexec
!
! include $(REP_DIR)/mk/noux.mk

The variable 'CONFIGURE_ARGS' contains the options that are
passed on to Autoconf's configure script. The Noux specific build
rules in _noux.mk_ always have to be included last.

The build rules for GNU tar are quite short and therefore at the end
of this chapter we take a look at a much more extensive example.


Creating a run script
=====================

Creating a run script to test Noux packages is the same as it is
with running natively ported applications. Therefore we will only focus
on the Noux-specific parts of the run script and omit the rest.

First, we add the desired Noux packages to 'build_components':

! set noux_pkgs "bash coreutils tar"
!
! foreach pkg $noux_pkgs {
!   lappend_if [expr ![file exists bin/$pkg]] build_components noux-pkg/$pkg }
!
! build $build_components

Since each Noux package is, like every other Genode binary, installed to the
_<build-dir>/bin_ directory, we create a tar archive of each package from
each directory:

! foreach pkg $noux_pkgs {
!   exec tar cfv bin/$pkg.tar -h -C bin/$pkg . }

_Using noux.mk makes sure that each package is always installed to_
_<build-dir>/bin/<package-name>._

Later on, we will use these tar archives to assemble the file system
hierarchy within Noux.

Most applications ported to Noux want to read and write files. On that
matter, it is reasonable to provide a file-system service and the easiest
way to do this is to use the ram_fs server. This server provides a RAM-backed
file system, which is perfect for testing Noux applications. With
the help of the session label we can route multiple directories to the
file system in Noux:

! append config {
! <config>
! […]
!   <start name="ram_fs">
!     <resource name="RAM" quantum="32M"/>
!     <provides><service name="File_system"/></provides>
!     <config>
!       <content>
!         <dir name="tmp"> </dir>
!         <dir name="home"> </dir>
!       </content>
!       <policy label="noux -> root" root="/" />
!       <policy label="noux -> home" root="/home" writeable="yes" />
!       <policy label="noux -> tmp"  root="/tmp"  writeable="yes" />
!     </config>
!   </start>
! […]

The file system Noux presents to the running applications is constructed
out of several stacked file systems. These file systems have to be
registered in the 'fstab' node in the configuration node of Noux:

!   <start name="noux">
!     <resource name="RAM" quantum="256M" />
!     <config>
!       <fstab>}

Each Noux package is added

! foreach pkg $noux_pkgs {
!   append config {
!         <tar name=\"$pkg.tar\" />" }}

and the routes to the ram_fs file system are configured:

! append config {
!         <dir name="home"> <fs label="home" /> </dir>
!         <dir name="ram"> <fs label="root" /> </dir>
!         <dir name="tmp"> <fs label="tmp" /> </dir>
!       </fstab>
!       <start name="/bin/bash">
!         <env name="TERM" value="linux" />
!       </start>
!     </config>
!   </start>}

In this example we save the run script as _ports/run/noux_tar.run_.


Compiling the Noux package
==========================

Now we can trigger the compilation of tar by executing

! $ make VERBOSE= noux-pkg/tar

_At least on the first compilation attempt, it is wise to unset_ 'VERBOSE'
_because it enables us to see the whole output of the_ 'configure' _process._

By now, Genode provides almost all libc header files that are used by
typical POSIX programs. In most cases, it is rather a matter of enabling
the right definitions and compilation flags. It might be worth to take a
look at FreeBSD's ports tree because Genode's libc is based upon the one
of FreeBSD 8.2.0 and if certain changes to the contributed code are needed,
they are normally already done in the ports tree.

The script _noux_env.sh_ that is used to create the cross-compile
environment as well as the famous _config.log_ are found
in _<build-dir>/noux-pkg/<package-name>_.


Running the Noux package
========================

We use the previously written run script to start the scenario, in which we
can execute and test the Noux package by issuing:

! $ make run/noux_tar

After the system has booted and Noux is running, we first create some test
files from within the running bash process:

! bash-4.1$ mkdir /tmp/foo
! bash-4.1$ echo 'foobar' > /tmp/foo/bar

Following this we try to create a ".tar" archive of the directory _/tmp/foo_

! bash-4.1$ cd /tmp
! bash-4.1$ tar cvf foo.tar foo/
! tar: /tmp/foo: Cannot stat: Function not implemented
! tar: Exiting with failure status due to previous errors
! bash-4.1$

Well, this does not look too good but at least we have a useful error message
that leads (hopefully) us into the right direction.


Debugging an application that uses the Noux runtime
===================================================

Since the Noux service is basically the kernel part of our POSIX runtime
environment, we can ask Noux to show us the system calls executed by tar.
We change its configuration in the run script to trace all system calls:

! […]
! <start name="noux">
! 	<config trace_syscalls="yes">
! […]

We start the runscript again, create the test files and try to create a
".tar" archive. It still fails but now we have a trace of all system calls
and know at least what is going in Noux itself:

! […]
! [init -> noux] PID 0 -> SYSCALL FORK
! [init -> noux] PID 0 -> SYSCALL WAIT4
! [init -> noux] PID 5 -> SYSCALL STAT
! [init -> noux] PID 5 -> SYSCALL EXECVE
! [init -> noux] PID 5 -> SYSCALL STAT
! [init -> noux] PID 5 -> SYSCALL GETTIMEOFDAY
! [init -> noux] PID 5 -> SYSCALL STAT
! [init -> noux] PID 5 -> SYSCALL OPEN
! [init -> noux] PID 5 -> SYSCALL FTRUNCATE
! [init -> noux] PID 5 -> SYSCALL FSTAT
! [init -> noux] PID 5 -> SYSCALL GETTIMEOFDAY
! [init -> noux] PID 5 -> SYSCALL FCNTL
! [init -> noux] PID 5 -> SYSCALL WRITE
! [init -> noux -> /bin/tar] DUMMY fstatat(): fstatat called, not implemented
! [init -> noux] PID 5 -> SYSCALL FCNTL
! [init -> noux] PID 5 -> SYSCALL FCNTL
! [init -> noux] PID 5 -> SYSCALL WRITE
! [init -> noux] PID 5 -> SYSCALL FCNTL
! [init -> noux] PID 5 -> SYSCALL WRITE
! [init -> noux] PID 5 -> SYSCALL GETTIMEOFDAY
! [init -> noux] PID 5 -> SYSCALL CLOSE
! [init -> noux] PID 5 -> SYSCALL FCNTL
! [init -> noux] PID 5 -> SYSCALL WRITE
! [init -> noux] PID 5 -> SYSCALL CLOSE
! [init -> noux] child /bin/tar exited with exit value 2
! […]

_The trace log was shortened to only contain the important information._

We now see at which point something went wrong. To be honest, we see the
'DUMMY' message even without enabling the tracing of system calls. But
there are situations where a application is actually stuck in a (blocking)
system call and it is difficult to see in which.

Anyhow, 'fstatat' is not properly implemented. At this point, we either have
to add this function to the Genode's libc or rather add it to libc_noux.
If we add it to the libc, not only applications running in Noux will
benefit but all applications using the libc. Implementing it in
libc_noux is the preferred way if there are special circumstances because
we have to treat the function differently when used in Noux (e.g. 'fork').

For the sake of completeness here is a list of all files that were created by
porting GNU tar to Genode's Noux runtime:

! ports/ports/tar.hash
! ports/ports/tar.port
! ports/run/noux_tar.run
! ports/src/noux-pkg/tar/target.mk


Extensive build rules example
=============================

The build rules for OpenSSH are much more extensive than the ones in
the previous example. Let us take a quick look at those build rules to
get a better understanding of possible challenges one may encounter while
porting a program to Noux:

! # This prefix 'magic' is needed because OpenSSH uses $exec_prefix
! # while compiling (e.g. -DSSH_PATH) and in the end the $prefix and
! # $exec_prefix path differ.
!
! CONFIGURE_ARGS += --disable-ip6 \
!                   […]
!                   --exec-prefix= \
!                   --bindir=/bin \
!                   --sbindir=/bin \
!                   --libexecdir=/bin

In addition to the normal configure options, we have to also define the
path prefixes. The OpenSSH build system embeds certain paths in the
ssh binary, which need to be changed for Noux.

! INSTALL_TARGET  = install

Normally the Noux build rules (_noux.mk_) execute 'make install-strip' to
explicitly install binaries that are stripped of their debug symbols. The
generated Makefile of OpenSSH does not use this target. It automatically
strips the binaries when executing 'make install'. Therefore, we set the
variable 'INSTALL_TARGET' to override the default behaviour of the
Noux build rules.

! LIBS += libcrypto libssl zlib libc_resolv

As OpenSSH depends on several libraries, we need to include these in the
build Makefile. These libraries are runtime dependencies and need to be
present when running OpenSSH in Noux.

Sometimes it is needed to patch the original build system. One way to do
this is by applying a patch while preparing the source code. The other
way is to do it before building the Noux package:

! noux_built.tag: Makefile Makefile_patch
!
! Makefile_patch: Makefile
!         @#
!         @# Our $(LDFLAGS) contain options which are usable by gcc(1)
!         @# only. So instead of using ld(1) to link the binary, we have
!         @# to use gcc(1).
!         @#
!         $(VERBOSE)sed -i 's|^LD=.*|LD=$(CC)|' Makefile
!         @#
!         @# We do not want to generate host-keys because we are crosscompiling
!         @# and we can not run Genode binaries on the build system.
!         @#
!         $(VERBOSE)sed -i 's|^install:.*||' Makefile
!         $(VERBOSE)sed -i 's|^install-nokeys:|install:|' Makefile
!         @#
!         @# The path of ssh(1) is hardcoded to $(bindir)/ssh which in our
!         @# case is insufficient.
!         @#
!         $(VERBOSE)sed -i 's|^SSH_PROGRAM=.*|SSH_PROGRAM=/bin/ssh|' Makefile

The target _noux_built.tag_ is a special target defined by the Noux build
rules. It will be used by the build rules when building the Noux package.
We add the 'Makefile_patch' target as a dependency to it. So after configure
is executed, the generated Makefile will be patched.

Autoconf's configure script checks if all requirements are fulfilled and
therefore, tests if all required libraries are installed on the host system.
This is done by linking a small test program against the particular library.
Since these libraries are only build-time dependencies, we fool the configure
script by providing dummy libraries:

! #
! # Make the zlib linking test succeed
! #
! Makefile: dummy_libs
!
! LDFLAGS += -L$(PWD)
!
! dummy_libs: libz.a libcrypto.a libssl.a
!
! libcrypto.a:
!         $(VERBOSE)$(AR) -rc $@
! libssl.a:
!         $(VERBOSE)$(AR) -rc $@
! libz.a:
!         $(VERBOSE)$(AR) -rc $@


Porting devices drivers
#######################

Even though Genode encourages writing native device drivers, this task sometimes
becomes infeasible. Especially if there is no documentation available for a
certain device or if there are not enough programming resources at hand to
implement a fully fledged driver. Examples of ported drivers can be found in
the 'dde_linux', 'dde_bsd', and 'dde_ipxe' repositories.

In this chapter we will exemplary discuss how to port a Linux driver for an ARM
based SoC to Genode. The goal is to execute driver code in user land directly on
Genode while making the driver believe it is running within the Linux kernel.
Traditionally there have been two approaches to reach this goal in Genode. In
the past, Genode provided a Linux environment, called 'dde_linux26', with the
purpose to offer just enough infrastructure to easily port drivers. However,
after adding more drivers it became clear that this repository grew extensively,
making it hard to maintain. Also updating the environment to support newer
Linux-kernel versions became a huge effort which let the repository to be neglected
over time.

Therefore we choose the path to write a customized environment for each driver,
which provides a specially tailored infrastructure. We found that the
support code usually is not larger than a couple of thousand lines of code,
while upgrading to newer driver versions, as we did with the USB drivers, is
feasible.


Basic driver structure
======================

The first step in porting a driver is to identify the driver code that has to be
ported. Once the code is located, we usually create a new Genode repository and
write a port file to download and extract the code. It is good practice to name
the port and the hash file like the new repository, e.g. _dde_linux.port_ if
the repository directory is called _<genode-dir>/repos/dde_linux_.
Having the source code ready, there are three main tasks the environment must
implement. The first is the driver back end, which is responsible for raw device
access using Genode primitives, the actual environment that emulates Linux
function calls the driver code is using, and the front end, which exposes for
example some Genode-session interface (like NIC or block session) that client
applications can connect to.


Further preparations
====================

Having the code ready, the next step is to create an _*.mk_ file that actually
compiles the code. For a driver library _lib/mk/<driver name>.mk_ has to be
created and for a stand-alone program _src/<driver name>/target.mk_ is created
within the repository. With the _*.mk_ file in place, we can now start the
actual compilation. Of course this will cause a whole lot of errors and
warnings. Most of the messages will deal with implicit declarations of functions
and unknown data types. What we have to do now is to go through each warning and
error message and either add the header file containing the desired function or
data type to the list of files that will be extracted to the _contrib_ directory
or create our own prototype or data definition.

When creating our own prototypes, we put them in a file called _lx_emul.h_. To
actually get this file included in all driver files we use the following code in
the _*.mk_ file:

! CC_C_OPT  += -include $(INC_DIR)/lx_emul.h

where 'INC_DIR' points to the include path of _lx_emul.h_.

The hard part is to decide which of the two ways to go for a specific function
or data type, since adding header files also adds more dependencies and often
more errors and warnings. As a rule of thumb, try adding as few headers as
possible.

The compiler will also complain about a lot of missing header files. Since we do
not want to create all these header files, we use a trick in our _*.mk_ file that
extracts all header file includes from the driver code and creates symbolic
links that correspond to the file name and links to _lx_emul.h_. You can put the
following code snippet in your _*.mk_ file which does the trick:

!#
!# Determine the header files included by the contrib code. For each
!# of these header files we create a symlink to _lx_emul.h_.
!#
!GEN_INCLUDES := $(shell grep -rh "^\#include .*\/" $(DRIVER_CONTRIB_DIR) |\
!                        sed "s/^\#include [^<\"]*[<\"]\([^>\"]*\)[>\"].*/\1/" | \
!                        sort | uniq)
!
!#
!# Filter out original Linux headers that exist in the contrib directory
!#
!NO_GEN_INCLUDES := $(shell cd $(DRIVER_CONTRIB_DIR); find -name "*.h" | sed "s/.\///" | \
!                           sed "s/.*include\///")
!GEN_INCLUDES    := $(filter-out $(NO_GEN_INCLUDES),$(GEN_INCLUDES))
!
!#
!# Put Linux headers in 'GEN_INC' dir, since some include use "../../" paths use
!# three level include hierarchy
!#
!GEN_INC         := $(shell pwd)/include/include/include
!
!$(shell mkdir -p $(GEN_INC))
!
!GEN_INCLUDES    := $(addprefix $(GEN_INC)/,$(GEN_INCLUDES))
!INC_DIR         += $(GEN_INC)
!
!#
!# Make sure to create the header symlinks prior building
!#
!$(SRC_C:.c=.o) $(SRC_CC:.cc=.o): $(GEN_INCLUDES)
!
!$(GEN_INCLUDES):
!  $(VERBOSE)mkdir -p $(dir $@)
!  $(VERBOSE)ln -s $(LX_INC_DIR)/lx_emul.h $@

Make sure 'LX_INC_DIR' is the directory containing the _lx_emul.h_ file. Note
that 'GEN_INC' is added to your 'INC_DIR' variable.

The 'DRIVER_CONTRIB_DIR' variable is defined by calling the _select_from_port_
function at the beginning of a Makefile or a include file, which is used by
all other Makefiles:

! DRIVER_CONTRIB_DIR := $(call select_from_ports,driver_repo)/src/lib/driver_repo

The process of function definition and type declaration continues until the code
compiles. This process can be quite tiresome. When the driver code finally compiles, the
next stage is linking. This will of course lead to another whole set of errors
that complain about undefined references. To actually obtain a linked binary we
create a _dummies.cc_ file. To ease things up we suggest to create a macro called
'DUMMY' and implement functions as in the example below:

! /*
!  * Do not include 'lx_emul.h', since the implementation will most likely clash
!  * with the prototype
!  */
!
!#define DUMMY(retval, name) \
!	DUMMY name(void) { \
!		PDBG( #name " called (from %p) not implemented", __builtin_return_address(0)); \
!	return retval; \
!}
!
! DUMMY(-1, kmalloc)
! DUMMY(-1, memcpy)
! ...

Create a 'DUMMY' for each undefined reference until the binary links. We now
have a linked binary with a dummy environment.


Debugging
=========

From here on, we will actually start executing code, but before we do that, let us
have a look at the debugging options for device drivers. Since drivers have to
be tested on the target platform, there are not as many debugging options
available as for higher level applications, like running applications on the
Linux version of Genode while using GDB for debugging. Having these
restrictions, debugging is almost completely performed over the serial line and
on rare occasions with an hardware debugger using JTAG.

For basic Linux driver debugging it is useful to implement the 'printk'
function (use 'dde_kit_printf' or something similar) first. This way, the driver
code can output something and additions for debugging can be made. The
'__builtin_return_address' function is also useful in order to determine where a
specific function was called from. 'printk' may become a problem with devices
that require certain time constrains because serial line output is very slow. This is
why we port most drivers by running them on top of the Fiasco.OC version of
Genode. There you can take advantage of Fiasco's debugger (JDB) and trace buffer
facility.

The trace buffer can be used to log data and is much faster than 'printk' over
serial line. Please inspect the 'ktrace.h' file (at
_base-foc/contrib/l4/pkg/l4sys/include/ARCH-*/ktrace.h_)
that describes the complete interface. A very handy function there is

!fiasco_tbuf_log_3val("My message", variable1, variable2, variable3);

which stores a message and three variables in the trace buffer. The trace buffer
can be inspected from within JDB by pressing 'T'.

JDB can be accessed at any time by pressing the 'ESC' key. It can be used to
inspect the state of all running threads and address spaces on the system. There
is no recent JDB documentation available, but

:Fiasco kernel debugger manual:

  [http://os.inf.tu-dresden.de/fiasco/doc/jdb.pdf]

should be a good starting point. It is also possible to enter the debugger at
any time from your program calling the 'enter_kdebug("My breakpoint")' function
from within your code. The complete JDB interface can be found in
_base-foc/contrib/l4/pkg/l4sys/include/ARCH-*/kdebug.h_.

Note that the backtrace ('bt') command does not work out of the box on ARM
platforms. We have a small patch for that in our Fiasco.OC development branch
located at GitHub: [http://github.com/ssumpf/foc/tree/dev]


The back end
============

To ease up the porting of drivers and interfacing Genode from C code, Genode offers a
library called DDE kit. DDE kit provides access to common functions required
by drivers like device memory, virtual memory with physical-address lookup,
interrupt handling, timers, etc. Please inspect _os/include/dde_kit_ to see the
complete interface description. You can also use 'grep -r dde_kit_ *' to see
usage of the interface in Genode.

As an example for using DDE kit we implement the 'kmalloc' call:

!void *kmalloc(size_t size, gfp_t flags)
!{
!  return dde_kit_simple_malloc(size);
!}

It is also possible to directly use Genode primitives from C++ files, the
functions only have to be declared as 'extern "C"' so they can be called from C
code.


The environment
===============

Having a dummy environment we may now begin to actually execute driver code.

Driver initialization
~~~~~~~~~~~~~~~~~~~~~

Most Linux drivers will have an initialization routine to register itself within
the Linux kernel and do other initializations if necessary. In order to be
initialized, the driver will register a function using the 'module_init' call.
This registered function must be called before the driver is actually used. To
be able to call the registered function from Genode, we define the 'module_init'
macro in _lx_emul.h_ as follows:

! #define module_init(fn) void module_##fn(void) { fn(); }

when a driver now registers a function like

! module_init(ehci_hcd_init);

we would have to call

! module_ehci_hcd_init();

during driver startup. Having implemented the above, it is now time to start our
ported driver on the target platform and check if the initialization function is
successful. Any important dummy functions that are called must be implemented
now. A dummy function that does not do device related things, like Linux book
keeping, may not be implemented. Sometimes Linux checks the return values of
functions we might not want to implement, in this case it is sufficient to simply
adjust the return value of the affected function.

Device probing
~~~~~~~~~~~~~~
Having the driver initialized, we will give the driver access to the device
resources. This is performed in two steps. In the case of ARM SoC's we have to
check in which state the boot loader (usually U-Boot) left the device. Sometimes
devices are already setup by the boot loader and only a simple device reset is
necessary to proceed. If the boot loader did not touch the device, we most
likely have to check and setup all the necessary clocks on the platform and may
have to perform other low level initializations like PHY setup.

If the device is successfully (low level) initialized, we can hand it over to
the driver by calling the 'probe' function of the driver. For ARM platforms the
'probe' function takes a 'struct platform_device' as an argument and all
important fields, like device resources and interrupt numbers, should be set to
the correct values before calling 'probe'. During 'probe' the driver will most
likely map and access device memory, request interrupts, and reset the device.
All dummy functions that are related to these tasks should be implemented or
ported at this point.

When 'probe' returns successful, you may either test other driver functions by
hand or start building the front-end.


The front end
=============

An important design question is how the front end is attached to the driver. In
some cases the front end may not use the driver directly, but other Linux
subsystems that are ported or emulated by the environment. For example, the USB
storage driver implements parts of the SCSI subsystem, which in turn is used
by the front end. The whole decision depends on the kind of driver that is
ported and on how much additional infrastructure is needed to actually make use
of the data. Again an USB example: For USB HID, we needed to port the USB controller
driver, the hub driver, the USB HID driver, and the generic HID driver in order
to retrieve keyboard and mouse events from the HID driver.

The last step in porting a device driver is to make it accessible to other
Genode applications. Typically this is achieved by implementing one of Genode's
session interfaces, like a NIC session for network adapters or a block session for
block devices. You may also define your own session interfaces. The
implementation of the session interface will most likely trigger driver calls,
so you have to have to keep an eye on the dummy functions. Also make sure that calls to the
driver actually do what they are supposed to, for example, some wrong return value
of a dummy function may cause a function to return without performing any work.


Notes on synchronization
========================

After some experiences with Linux drivers and multi-threading, we lately
choose to have all Linux driver code executed by a single thread only. This way no Linux
synchronization primitives have to be implemented and we simply don't have to
worry about subtle pre- and postconditions of many functions (like "this
function has to be called with lock 'x' being held").

Unfortunately we cannot get rid of all threads within a device-driver server,
there is at least one waiting for interrupts and one for the entry point that
waits for client session requests. In order to synchronize these threads, we use
Genode's signalling framework. So when, for example, the IRQ thread receives an
interrupt it will send a signal. The Linux driver thread will at certain points
wait for these signals (e.g., functions like 'schedule_timeout' or
'wait_for_completion') and execute the right code depending on the kind of
signal delivered or firmly speaking the signal context. For this to work, we use
a class called 'Signal_dispatcher' (_base/include/base/signal.h_) which inherits
from 'Signal_context'. More than one dispatcher can be bound to a signal
receiver, while each dispatcher might do different work, like calling the
Linux interrupt handler in the IRQ example.