drm/i915: Improve long running compute w/a for GuC submission

A workaround was added to the driver to allow compute workloads to run
'forever' by disabling pre-emption on the RCS engine for Gen12.
It is not totally unbound as the heartbeat will kick in eventually
and cause a reset of the hung engine.

However, this does not work well in GuC submission mode. In GuC mode,
the pre-emption timeout is how GuC detects hung contexts and triggers
a per engine reset. Thus, disabling the timeout means also losing all
per engine reset ability. A full GT reset will still occur when the
heartbeat finally expires, but that is a much more destructive and
undesirable mechanism.

The purpose of the workaround is actually to give compute tasks longer
to reach a pre-emption point after a pre-emption request has been
issued. This is necessary because Gen12 does not support mid-thread
pre-emption and compute tasks can have long running threads.

So, rather than disabling the timeout completely, just set it to a
'long' value.

v2: Review feedback from Tvrtko - must hard code the 'long' value
instead of determining it algorithmically. So make it an extra CONFIG
definition. Also, remove the execlist centric comment from the
existing pre-emption timeout CONFIG option given that it applies to
more than just execlists.

Signed-off-by: John Harrison <[e-mail]>
Reviewed-by: Daniele Ceraolo Spurio <[e-mail]>
Acked-by: Michal Mrozek <[e-mail]>
Acked-by: Tvrtko Ursulin <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/i915/Kconfig.profile

···
 	default 640 # milliseconds
 	help
 	  How long to wait (in milliseconds) for a preemption event to occur
-	  when submitting a new context via execlists. If the current context
-	  does not hit an arbitration point and yield to HW before the timer
-	  expires, the HW will be reset to allow the more important context
-	  to execute.
+	  when submitting a new context. If the current context does not hit
+	  an arbitration point and yield to HW before the timer expires, the
+	  HW will be reset to allow the more important context to execute.
+
+	  This is adjustable via
+	  /sys/class/drm/card?/engine/*/preempt_timeout_ms
+
+	  May be 0 to disable the timeout.
+
+	  The compiled in default may get overridden at driver probe time on
+	  certain platforms and certain engines which will be reflected in the
+	  sysfs control.
+
+config DRM_I915_PREEMPT_TIMEOUT_COMPUTE
+	int "Preempt timeout for compute engines (ms, jiffy granularity)"
+	default 7500 # milliseconds
+	help
+	  How long to wait (in milliseconds) for a preemption event to occur
+	  when submitting a new context to a compute capable engine. If the
+	  current context does not hit an arbitration point and yield to HW
+	  before the timer expires, the HW will be reset to allow the more
+	  important context to execute.
 
 	  This is adjustable via
 	  /sys/class/drm/card?/engine/*/preempt_timeout_ms

drivers/gpu/drm/i915/gt/intel_engine_cs.c

···
 	engine->props.timeslice_duration_ms =
 		CONFIG_DRM_I915_TIMESLICE_DURATION;
 
-	/* Override to uninterruptible for OpenCL workloads. */
+	/*
+	 * Mid-thread pre-emption is not available in Gen12. Unfortunately,
+	 * some compute workloads run quite long threads. That means they get
+	 * reset due to not pre-empting in a timely manner. So, bump the
+	 * pre-emption timeout value to be much higher for compute engines.
+	 */
 	if (GRAPHICS_VER(i915) == 12 && (engine->flags & I915_ENGINE_HAS_RCS_REG_STATE))
-		engine->props.preempt_timeout_ms = 0;
+		engine->props.preempt_timeout_ms = CONFIG_DRM_I915_PREEMPT_TIMEOUT_COMPUTE;
 
 	/* Cap properties according to any system limits */
 #define CLAMP_PROP(field) \