[Intel-gfx] [PATCH] drm/i915/gt: Add a comment about how to use udev for configuring engines

[Chris Wilson]

We expose engine properties under sysfs so that the sysadmin can
configure the driver according to their requirements. We can also use
udev rules to then apply that configuration anytime a device is
reloaded. Include a udev snippet provided by Joonas as an example.

v2: Update the snippet

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
---
 drivers/gpu/drm/i915/gt/sysfs_engines.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/drivers/gpu/drm/i915/gt/sysfs_engines.c b/drivers/gpu/drm/i915/gt/sysfs_engines.c
index 9cf943b6a44b..37f078e69a42 100644
--- a/drivers/gpu/drm/i915/gt/sysfs_engines.c
+++ b/drivers/gpu/drm/i915/gt/sysfs_engines.c
@@ -11,6 +11,22 @@
 #include "intel_engine_heartbeat.h"
 #include "sysfs_engines.h"
 
+/*
+ * The sysfs provides a means for configuring each engine for the intended
+ * usecase, and by utilising a udev the configuration can be made persistent
+ * across reboots and device rebinding.
+ *
+ * An example udev rule to run a custom sysadmin script would be,
+ * /etc/udev/rules.d/50-intel-gpu.rules:
+ *
+ * ACTION=="bind|add",SUBSYSTEM=="pci",DRIVER=="i915",RUN+="/usr/local/libexec/i915-disable-preempt-timeout.sh"
+ *
+ * where the script receives the device name and can open the sysfs, e.g.:
+ * for ENGINE in $(find /sys/$DEVPATH/drm/card?/engine -maxdepth 1) do
+ *   echo 0 > $ENGINE/preempt_timeout_ms # Disable reset on preemption failure
+ * done
+ */
+
 struct kobj_engine {
 	struct kobject base;
 	struct intel_engine_cs *engine;
-- 
2.20.1