nic/ipxe: add throttle RX interrupt support

The commit adds support to throttle the rate of the RX IRQs to a specified
value. The effect is, that no RX IRQs below the time threshold will fire and
therefore the CPU load gets reduced on the host. Trade-off gaming between
cpu load, throughput, overload.

Modular Sculpt 23.10 on S938 as testcase. In brackets the CPU affinity is
denoted.

ipxe (0,0) -> nic_router (1,0) -> Debian VM vbox6 (3,0) and (3,1)

VM: iperf -C X.X.X.X -t 60 -R

iperf server X.X.X.X is outside Sculpt and sends data due to '-R' to VM

Non representative measure points:

cpu load   - ipxe  - nic_router - iperf throughput
--------------------------------------------------
w/o  patch -  ~80% -       ~50% - ~706 MBit/s - 0    -> throttling off by default on S938
patch 651  -  ~20% -       ~35% - ~763 MBit/s - 651  -> 0.166ms throttle RX IRQ
patch 5580 -  ~15% -       ~25% - ~650 MBit/s - 5580 -> 1.4ms   throttle RX IRQ

Issue #5149

repos/dde_ipxe/patches/intel_rx_throttle.patch

@@ -0,0 +1,42 @@
+--- a/src/drivers/net/intel.c
++++ b/src/drivers/net/intel.c
+@@ -681,6 +681,27 @@
+ 		intel->force_icr = INTEL_IRQ_RXT0;
+ 	}
+ 
++	/*
++	 * Intel PCI/PCI-X family of Gigabit Ethernet control Software Developers's
++	 * Manual Rev 4.0
++	 *
++	 * 13.4.18 Interrupt Throttling Register
++	 * advises - 651 to 5580 - for "optimal performance setting"
++	 */
++
++	uint32_t const throttle_before = readl ( intel->regs + INTEL_ITR );
++	uint32_t const throttle = 651; /* 651 * 256µs -> 0.166 ms */
++
++	writel( throttle, intel->regs + INTEL_ITR );
++
++	uint64_t const value = readl ( intel->regs + INTEL_ITR );
++
++	dde_printf("throttle RX interrupt %luµs%s -> %luµs - %s\n",
++	           256ul * throttle_before / 1000ul,
++	           throttle_before ? "" : "(off)",
++	           256ul * value / 1000ul,
++	           value == throttle ? "succeeded" : "failed");
++
+ 	return 0;
+ 
+ 	intel_destroy_ring ( intel, &intel->rx );
+--- a/src/drivers/net/intel.h
++++ b/src/drivers/net/intel.h
+@@ -102,6 +102,9 @@
+ #define INTEL_IRQ_RXO		0x00000040UL	/**< Receive overrun */
+ #define INTEL_IRQ_RXT0		0x00000080UL	/**< Receive timer */
+ 
++/** Interrupt throttling Register */
++#define INTEL_ITR 0x000C4UL
++
+ /** Interrupt Mask Set/Read Register */
+ #define INTEL_IMS 0x000d0UL
+ 

repos/dde_ipxe/ports/dde_ipxe.hash

@@ -1 +1 @@
-c20a39b17bca4d9cefcee43f366f5c23296396e0
+a7206d6c2a1b2de7fa7fde6733a3500411a64de2

repos/dde_ipxe/ports/dde_ipxe.port

@@ -11,7 +11,8 @@ PATCHES   := patches/dde_ipxe.patch \
              patches/intel_update.patch \
              patches/tg3.patch \
              patches/realtek.patch \
-             patches/intel_tx_batch.patch
+             patches/intel_tx_batch.patch \
+             patches/intel_rx_throttle.patch
 
 PATCH_OPT := -p1 -d ${DIR(ipxe)}