openwifi/doc

Openwifi document

Above figure shows software and hardware/FPGA modules that compose the openwifi design. The module name is equal/similar to the source code file name. Driver module source codes are in openwifi/driver/. FPGA module source codes are in openwifi-hw repository. The user space tool sdrctl source code are in openwifi/user_space/sdrctl_src/.

• Driver and software overall principle
• sdrctl command
• Rx packet flow and filtering config
• Tx packet flow and config
• Regulation and channel config
• Analog and digital frequency design
• Debug methods

Driver and software overall principle

Linux mac80211 subsystem, as a part of Linux wireless, defines a set of APIs (ieee80211_ops) to rule the Wi-Fi chip driver behavior. SoftMAC Wi-Fi chip driver implements (subset of) those APIs. That is why Linux can support so many Wi-Fi chips of different chip vendors.

openwifi driver (sdr.c) implements following APIs of ieee80211_ops:

• tx. It is called when upper layer has a packet to send
• start. It is called when NIC up. (ifconfig sdr0 up)
• stop. It is called when NIC down. (ifconfig sdr0 down)
• add_interface. It is called when NIC is created
• remove_interface. It is called when NIC is deleted
• config. It is called when upper layer wants to change channel/frequency (like the scan operation)
• bss_info_changed. It is called when upper layer believe some BSS parameters need to be changed (BSSID, TX power, beacon interval, etc)
• conf_tx. It is called when upper layer needs to config/change some tx parameters (AIFS, CW_MIN, CW_MAX, TXOP, etc)
• prepare_multicast. It is called when upper layer needs to prepare multicast, currently only a empty function hook is present.
• configure_filter. It is called when upper layer wants to config/change the frame filtering rule in FPGA.
• rfkill_poll. It is called when upper layer wants to know the RF status (ON/OFF).
• get_tsf. It is called when upper layer wants to get 64bit FPGA timer value (TSF - Timing synchronization function)
• set_tsf. It is called when upper layer wants to set 64bit FPGA timer value
• reset_tsf. It is called when upper layer wants to reset 64bit FPGA timer value
• set_rts_threshold. It is called when upper layer wants to change the threshold (packet length) for turning on RTS mechanism
• testmode_cmd. It is called when upper layer has test command for us. sdrctl command message is handled by this function.

Above APIs are called by upper layer (Linux mac80211 subsystem). When they are called, the driver (sdr.c) will do necessary job over SDR platform. If necessary, the driver will call other component drivers, like tx_intf_api/rx_intf_api/openofdm_tx_api/openofdm_rx_api/xpu_api, for help.

After receiving a packet from the air, FPGA will raise interrupt (if the frame filtering rule allows) to Linux, then the function openwifi_rx_interrupt() of openwifi driver (sdr.c) will be triggered. In that function, ieee80211_rx_irqsafe() API is used to give the packet and related information (timestamp, rssi, etc) to upper layer.

The packet sending is initiated by upper layer. After the packet is sent by the driver over FPGA to the air, the upper layer will expect a sending report from the driver. Each time FPGA sends a packet, an interrupt will be raised to Linux and trigger openwifi_tx_interrupt(). This function will report the sending result (failed? succeeded? number of retransmissions, etc.) to upper layer via ieee80211_tx_status_irqsafe() API.

sdrctl command

Besides the Linux native Wi-Fi control programs, such as ifconfig/iw/iwconfig/iwlist/wpa_supplicant/hostapd/etc, openwifi offers a user space tool sdrctl to access openwifi specific functionalities, such as time sharing of the interface between two network slices, you may find more details of the slicing mechanism here.

sdrctl is implemented as nl80211 testmode command and communicates with openwifi driver (function openwifi_testmode_cmd() in sdr.c) via Linux nl80211--cfg80211--mac80211 path

Get and set a parameter

sdrctl dev sdr0 get para_name
sdrctl dev sdr0 set para_name value 

para_name	meaning	comment
addr0	target MAC address of tx slice 0	32bit. for address 6c:fd:b9:4c:b1:c1, you set b94cb1c1
slice_total0	tx slice 0 cycle length in us	for length 50ms, you set 49999
slice_start0	tx slice 0 cycle start time in us	for start at 10ms, you set 10000
slice_end0	tx slice 0 cycle end time in us	for end at 40ms, you set 39999
addr1	target MAC address of tx slice 1	32bit. for address 6c:fd:b9:4c:b1:c1, you set b94cb1c1
slice_total1	tx slice 1 cycle length in us	for length 50ms, you set 49999
slice_start1	tx slice 1 cycle start time in us	for start at 10ms, you set 10000
slice_end1	tx slice 1 cycle end time in us	for end at 40ms, you set 39999

Get and set a register of a module

sdrctl dev sdr0 get reg module_name reg_idx
sdrctl dev sdr0 set reg module_name reg_idx reg_value 

module_name refers to the name of driver functionality, can be drv_rx/drv_tx/drv_xpu. Related registers are defined in sdr.h (drv_rx_reg_val/drv_tx_reg_val/drv_xpu_reg_val)

module_name rf/rx_intf/tx_intf/rx/tx/xpu refer to RF (ad9xxx front-end) and FPGA modules (rx_intf/tx_intf/openofdm_rx/openofdm_tx/xpu). Related register addresses are defined in hw_def.h.

module_name: drv_rx

reg_idx	meaning	comment
1	rx antenna selection	0:rx1, 1:rx2. After this command, you should down and up sdr0 by ifconfig, but not reload sdr0 driver via ./wgd.sh

(In the comment column, you may get a list of decimalvalue(0xhexvalue):explanation for a register, only use the decimalvalue in the sdrctl command)

module_name: drv_tx

reg_idx	meaning	comment
0	override Linux rate control of tx unicast data packet	4:6M, 5:9M, 6:12M, 7:18M, 8:24M, 9:36M, 10:48M, 11:54M
1	tx antenna selection	0:tx1, 1:tx2. After this command, you should down and up sdr0 by ifconfig, but not reload sdr0 driver via ./wgd.sh

module_name: drv_xpu

reg_idx	meaning	comment
x	x	to be defined

module_name: rf

reg_idx	meaning	comment
x	x	to be defined

module_name: rx_intf

reg_idx	meaning	comment
2	enable/disable rx interrupt	256(0x100):disable, 0:enable

module_name: tx_intf

reg_idx	meaning	comment
13	tx I/Q digital gain before DUC	current optimal value: 237
14	enable/disable tx interrupt	196672(0x30040):disable, 64(0x40):enable

module_name: rx

reg_idx	meaning	comment
20	history of PHY rx state	read only. If the last digit readback is always 3, it means the Viterbi decoder stops working

module_name: tx

reg_idx	meaning	comment
1	pilot scrambler initial state	lowest 7 bits are used. 0x7E by default in openofdm_tx.c
2	data scrambler initial state	lowest 7 bits are used. 0x7F by default in openofdm_tx.c

module_name: xpu

reg_idx	meaning	comment
1	mute rx I/Q when tx	0:mute (default), 1:unmute, which means rx baseband will receive our own tx signal. Rx packet and tx packet (such as ACK) can be monitored in FPGA for timing analysis
2	TSF timer low 32bit write	only write this register won't trigger the TSF timer reload. should use together with register for high 32bit
3	TSF timer high 32bit write	falling edge of MSB will trigger the TSF timer reload, which means write '1' then '0' to MSB
4	band and channel number setting	see enum openwifi_band in hw_def.h. it will be set automatically by Linux. normally you shouldn't set it
11	max number of retransmission in FPGA	normally number of retransmissions controlled by Linux in real-time. If you write non-zeros value to this register, it will override Linux real-time setting
19	CSMA enable/disable	3758096384(0xe0000000): disable, 3:enable
20	tx slice 0 cycle length in us	for length 50ms, you set 49999
21	tx slice 0 cycle start time in us	for start at 10ms, you set 10000
22	tx slice 0 cycle end time in us	for end at 40ms, you set 39999
23	tx slice 1 cycle length in us	for length 50ms, you set 49999
24	tx slice 1 cycle start time in us	for start at 10ms, you set 10000
25	tx slice 1 cycle end time in us	for end at 40ms, you set 39999
27	FPGA packet filter config	check openwifi_configure_filter in sdr.c. also mac80211 frame filtering
28	BSSID address low 32bit for BSSID filtering	normally it is set by Linux in real-time automatically
29	BSSID address high 32bit for BSSID filtering	normally it is set by Linux in real-time automatically
30	openwifi MAC address low 32bit
31	openwifi MAC address high 32bit	check XPU_REG_MAC_ADDR_write in sdr.c to see how we set MAC address to FPGA when NIC start
58	TSF runtime value low 32bit	read only
59	TSF runtime value high 32bit	read only
63	version information	read only

Rx packet flow and filtering config

After FPGA receives a packet, no matter the FCS/CRC is correct or not it will raise interrupt to Linux if the frame filtering rule allows. openwifi_rx_interrupt() function in sdr.c will be triggered to do necessary operation and give the information to upper layer (Linux mac80211 subsystem).

• frame filtering

The FPGA frame filtering configuration is done in real-time by function openwifi_configure_filter() in sdr.c. The filter_flag together with HIGH_PRIORITY_DISCARD_FLAG finally go to pkt_filter_ctl.v of xpu module in FPGA, and control how FPGA does frame filtering. Openwifi has the capability to capture all received packets even if the CRC is bad. You just need to set the NIC to monitor mode by iwconfig command (check monitor_ch.sh in user_space directory). In monitor mode, openwifi_configure_filter() will set MONITOR_ALL to the frame filtering module pkt_filter_ctl.v in FPGA. This makes sure transfer all received packets to Linux mac80211 via rx interrupt.

• main rx interrupt operations in openwifi_rx_interrupt()
  • get raw content from DMA buffer. When Linux receives interrupt from FPGA rx_intf module, the content has been ready in Linux DMA buffer
  • parse extra information inserted by FPGA in the DMA buffer
    • TSF timer value
    • raw RSSI value that will be converted to actual RSSI in dBm by different correction in different bands/channels
    • packet length and MCS
    • FCS is valid or not
  • send packet content and necessary extra information to upper layer via ieee80211_rx_irqsafe()

Tx packet flow and config

Linux mac80211 subsystem calls openwifi_tx() to initiate a packet sending.

• main operations in openwifi_tx()
  • get necessary information from the packet header (struct ieee80211_hdr) for future FPGA configuration use
    • packet length and MCS
    • unicast or broadcast? does it need ACK? how many retransmissions at most are allowed to be tried by FPGA in case ACK is not received in time?
    • which time slice in FPGA the packet should go?
    • should RTS-CTS be used? (Send RTS and wait for CTS before actually send the data packet)
    • should CTS-to-self be used? (Send CTS-to-self packet before sending the data packet. You can force this on by force_use_cts_protect = true;)
    • should a sequence number be set for this packet?
  • generate SIGNAL field according to length and MCS information. Insert it before the packet for the future openofdm_tx FPGA module use
  • generate FPGA/PHY sequence number (priv->phy_tx_sn) for internal use (cross check between Linux and FPGA)
  • config FPGA register according to the above information to make sure FPGA do correct actions according to the packet specific requirement.
  • fire DMA transmission from Linux to one of FPGA tx queues. The packet may not be sent immediately if there are still some packets in FPGA tx queue (FPGA does the queue packet transmission according to channel and low MAC state)

Each time when FPGA sends a packet, an interrupt will be raised to Linux reporting the packet sending result. This interrupt handler is openwifi_tx_interrupt().

• main operations in openwifi_tx_interrupt()
  • get necessary information/status of the packet just sent by FPGA
    • packet length and sequence number to capture abnormal situation (cross checking between Linux and FPGA)
    • packet sending result: packet is sent successfully (FPGA receives ACK for this packet) or not. How many retransmissions are used for the packet sending (in case FPGA doesn't receive ACK in time, FPGA will do retransmission immediately)
  • send above information to upper layer (Linux mac80211 subsystem) via ieee80211_tx_status_irqsafe()

Regulation and channel config

SDR is a powerful tool for research. It is the user's duty to align with local spectrum regulation.

This section explains how openwifi config the frequency/channel range and change it in real-time. After knowing the mechanism, you can try to extend frequency/channel by yourself.

Frequency range

When openwifi driver is loaded, openwifi_dev_probe() will be executed. Following two lines configure the frequency range:

dev->wiphy->regulatory_flags = xxx
wiphy_apply_custom_regulatory(dev->wiphy, &sdr_regd);

sdr_regd is the predefined variable in sdr.h. You can search the definition/meaning of its type: struct ieee80211_regdomain. Then not difficult to find out how to change the frequency range in SDR_2GHZ_CH01_14 and SDR_5GHZ_CH36_64.

Supported channel

The supported channel list is defined in openwifi_2GHz_channels and openwifi_5GHz_channels in sdr.h. If you change the number of supported channels, make sure you also change the frequency range in sdr_regd accordingly and also array size of the following two fields in the struct openwifi_priv:

struct ieee80211_channel channels_2GHz[14];
struct ieee80211_channel channels_5GHz[11];

Finally, the supported channel list is transferred to Linux mac80211 when openwifi driver is loaded by following two lines in openwifi_dev_probe():

dev->wiphy->bands[NL80211_BAND_2GHZ] = &(priv->band_2GHz);
dev->wiphy->bands[NL80211_BAND_5GHZ] = &(priv->band_5GHz);

Real-time channel setting

Linux mac80211 (struct ieee80211_ops openwifi_ops in sdr.c) uses the "config" API to configure channel frequency and some other parameters in real-time (such as during scanning or channel setting by iwconfig). It is hooked to openwifi_config() in sdr.c, and supports only frequency setting currently. The real execution of frequency setting falls to ad9361_rf_set_channel() via the "set_chan" field of struct openwifi_rf_ops ad9361_rf_ops in sdr.c. Besides tuning RF front-end (AD9361), the ad9361_rf_set_channel() also handles RSSI compensation for different frequencies and FPGA configurations (SIFS, etc) for different bands.

Analog and digital frequency design

Following figure shows the current openwifi analog and digital frequency design strategy. The Tx RF center frequency is tuned with 10MHz offset deliberately to ease Tx Lo leakage suppressed by Rx filter. This RF offset is pre-compensated by Tx DUC (Digital Up Converter) in FPGA (duc_bank_core.bd used by tx_intf.v). It combines AD9361's bandwidth, frequency, sampling rate and FPGA's digital down/up converter (ddc_bank_core.bd/duc_bank_core.bd) setting to achieve this example spectrum arrangement. Values in the figure are configurable in the openwifi design.

Above spectrum setting has two benefits:

• The Tx Lo leakage is suppressed by Rx filter
• The centered Rx Lo and single channel Rx analog filter leads to more easy/accurate RSSI estimation in FPGA (together with real-time AD9361 AGC gain value accessed via FPGA GPIO)

Following figure shows the detailed configuration point in AD9361, driver (sdr.c/tx_intf.c/rx_intf.c/ad9361.c/etc) and related FPGA modules.

Debug methods

dmesg

To debug/see the basic driver behaviour, you could use dmesg command in Linux. openwifi driver prints normal tx/rx packet information when a packet is sent or received. The driver also prints WARNING information if it feels something abnormal happens. You can search "printk" in sdr.c to see all the printing points.

• tx printing example

  sdr,sdr openwifi_tx:  116bytes 48M FC0208 DI002c addr1/2/3:b827ebe65f1e/66554433224c/66554433224c SC1df0 flag40000012 retry2 ack1 q0 sn1075 R/CTS 00 1M 0us wr/rd 19/19
  

  • printing from sdr driver, openwifi_tx function.
  • 116bytes: packet size (length field in SIGNAL) is 116 bytes.
  • 48M: MCS (rate field in SIGNAL) is 48Mbps.
  • FC0208: Frame Control field 0x0208, which means type data, subtype data, to DS 0, from DS 1 (a packet from AP to client).
  • DI002c: Duration/ID field 0x002c. How many us this packet will occupy the channel (including waiting for ACK).
  • addr1/2/3: address fields. Target MAC address b827ebe65f1e, source MAC address 66554433224c (openwifi).
  • SC1df0: Sequence Control field 0x1df0, which means that the driver inserts sequence number 0x1df0 to the packet under request of upper layer.
  • flag40000012: flags field from upper layer struct ieee80211_tx_info (first fragment? need ACK? need sequence number insertion? etc.). Here is 0x40000012.
  • retry2: upper layer tells us the maximum number of retransmissions for this packet is 2.
  • ack1: upper layer tells us this packet needs ACK.
  • q0: the packet goes to FPGA queue 0.
  • sn1075: PHY/FPGA sequence number 1075. This is different from Sequence Control asked by upper layer. This is for cross check between FPGA/interrupt and driver.
  • R/CTS 00: upper layer believes this packet doesn't need RTS/CTS mechanism (Because the packet size is below the RTS threshold).
  • 1M 0us: if RTS/CTS is asked to be used by upper layer, it should use xM rate and Xus duration.
  • wr/rd 19/19: the write/read index of buffer (shared buffer between the active openwifi_tx and background openwifi_tx_interrupt).

• rx printing example

  sdr,sdr openwifi_rx_interrupt: 120bytes ht0 54M FC0108 DI002c addr1/2/3:66554433224c/b827ebe65f1e/66554433224c SCcf20 fcs1 sn117 i117 -36dBm
  

  • printing from sdr driver, openwifi_rx_interrupt function.
  • 120bytes: packet size (length field in SIGNAL) is 120 bytes.
  • ht0: this is non-ht packet.
  • 54M: MCS (rate field in SIGNAL) is 54Mbps.
  • FC0108: Frame Control field 0x0208, which means type data, subtype data, to DS 1, from DS 0 (a packet client to openwifi AP).
  • DI002c: Duration/ID field 0x002c. How many us this packet will occupy the channel (including waiting for ACK).
  • addr1/2/3: address fields. Target MAC address 66554433224c (openwifi), source MAC address b827ebe65f1e.
  • SCcf20: Sequence Control field 0x1df0, which means that the packet includes sequence number 0xcf20 (under request of upper layer of the peer).
  • fcs1: FCS/CRC is OK.
  • sn117: HY/FPGA sequence number 117. This is different from Sequence Control asked by upper layer. This is for cross check between FPGA/interrupt and driver.
  • i117: current rx packet DMA buffer index 117.
  • -36dBm: signal strength of this received packet.

Native Linux tools

For protocol, many native Linux tools you still could rely on. Such as tcpdump.

FPGA

For FPGA itself, FPGA developer could use Xilinx ILA tools to analyze FPGA signals. Spying on those state machines in xpu/tx_intf/rx_intf would be very helpful for understanding/debugging Wi-Fi low level funtionalities.