meta-lxatac-bsp: lxatac-net-switch-hacks: increase prio for SPI irq t…

…hread

When the system is under high load some SPI transfers with the ethernet
switch will time out before they are handled.

Increase the priority of the kernel thread that handles the SPI transfer
to work around the issue.

It does not make a lot of sense for a SPI transfer, that is 100% under the
hosts control (it does not and can not wait for the device for example) to
time out in the first place.
This means we are only fighting symptoms here, which is why this change
is also marked as a hack.

Signed-off-by: Leonard Göhrs <l.goehrs@pengutronix.de>

meta-lxatac-bsp/recipes-core/lxatac-net-switch-hacks/files/60-spi-device.rules

@@ -0,0 +1 @@
+SUBSYSTEM=="platform", DRIVER=="spi_stm32", TAG+="systemd"

meta-lxatac-bsp/recipes-core/lxatac-net-switch-hacks/files/spi-irq-prio-44009000.service

@@ -0,0 +1,11 @@
+[Unit]
+Description=Increase kernel thread priority for the SPI bus to prevent timeouts
+Requires=sys-devices-platform-soc-5c007000.bus-44009000.spi.device
+After=sys-devices-platform-soc-5c007000.bus-44009000.spi.device
+
+[Service]
+Type=oneshot
+ExecStart=/usr/bin/sh -c "chrt --pid 55 `pgrep irq/[0-9]+-44009000`"
+
+[Install]
+WantedBy=multi-user.target

meta-lxatac-bsp/recipes-core/lxatac-net-switch-hacks/lxatac-net-switch-hacks.bb

@@ -1,10 +1,12 @@
 LICENSE = "MIT"
 LIC_FILES_CHKSUM = "file://${COMMON_LICENSE_DIR}/MIT;md5=0835ade698e0bcf8506ecda2f7b4f302"
 
-inherit allarch
+inherit allarch systemd
 
 SRC_URI += " \
     file://01-increase-atomic-mem-pool-size.conf \
+    file://60-spi-device.rules \
+    file://spi-irq-prio-44009000.service \
 "
 
 do_install() {
@@ -14,8 +16,28 @@ do_install() {
     # under high load.
     install -D -m0600 ${WORKDIR}/01-increase-atomic-mem-pool-size.conf \
         ${D}${libdir}/sysctl.d/01-increase-atomic-mem-pool-size.conf
+
+    # Tag the SPI bus controllers with the `systemd` udev tag.
+    # This makes systemd create units like
+    #   sys-devices-platform-soc-5c007000.bus-44009000.spi.device
+    # that we can use as `Requires=` and `After=` in other units.
+    install -D -m0644 ${WORKDIR}/60-spi-device.rules \
+        ${D}${sysconfdir}/udev/rules.d/60-spi-device.rules
+
+    # Increase the priority of the kernel thread that handles the
+    # interrupts for the SPI controler the ethernet switch is connected to.
+    # This prevents timeouts when communicating with the switch while the
+    # system is under high load.
+    install -D -m0600 ${WORKDIR}/spi-irq-prio-44009000.service \
+        ${D}${systemd_system_unitdir}/spi-irq-prio-44009000.service
 }
 
+SYSTEMD_SERVICE:${PN} = "spi-irq-prio-44009000.service"
+
+# For chrt and pgrep in spi-irq-prio-44009000.service
+RDEPENDS:${PN} += "util-linux busybox"
+
 FILES:${PN} += "\
     ${libdir}/sysctl.d/ \
+    ${sysconfdir} \
 "