drm/doc/rfc: i915 GuC submission / DRM scheduler

Add entry for i915 GuC submission / DRM scheduler integration plan.
Follow up patch with details of new parallel submission uAPI to come.

v2:
 (Daniel Vetter)
  - Expand explaination of why bonding isn't supported for GuC
    submission
  - CC some of the DRM scheduler maintainers
  - Add priority inheritance / boosting use case
  - Add reasoning for removing in order assumptions
 (Daniel Stone)
  - Add links to priority spec
v4:
 (Tvrtko)
  - Add TODOs section
 (Daniel Vetter)
  - Pull in 1 line from following patch
v5:
 (Checkpatch)
  - Fix typos

Cc: Christian König <christian.koenig@amd.com>
Cc: Luben Tuikov <luben.tuikov@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Steven Price <steven.price@arm.com>
Cc: Jon Bloomfield <jon.bloomfield@intel.com>
Cc: Dave Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210629193511.124099-2-matthew.brost@intel.com

Documentation/gpu/rfc/i915_scheduler.rst

+=========================================
+I915 GuC Submission/DRM Scheduler Section
+=========================================
+
+Upstream plan
+=============
+For upstream the overall plan for landing GuC submission and integrating the
+i915 with the DRM scheduler is:
+
+* Merge basic GuC submission
+	* Basic submission support for all gen11+ platforms
+	* Not enabled by default on any current platforms but can be enabled via
+	  modparam enable_guc
+	* Lots of rework will need to be done to integrate with DRM scheduler so
+	  no need to nit pick everything in the code, it just should be
+	  functional, no major coding style / layering errors, and not regress
+	  execlists
+	* Update IGTs / selftests as needed to work with GuC submission
+	* Enable CI on supported platforms for a baseline
+	* Rework / get CI heathly for GuC submission in place as needed
+* Merge new parallel submission uAPI
+	* Bonding uAPI completely incompatible with GuC submission, plus it has
+	  severe design issues in general, which is why we want to retire it no
+	  matter what
+	* New uAPI adds I915_CONTEXT_ENGINES_EXT_PARALLEL context setup step
+	  which configures a slot with N contexts
+	* After I915_CONTEXT_ENGINES_EXT_PARALLEL a user can submit N batches to
+	  a slot in a single execbuf IOCTL and the batches run on the GPU in
+	  paralllel
+	* Initially only for GuC submission but execlists can be supported if
+	  needed
+* Convert the i915 to use the DRM scheduler
+	* GuC submission backend fully integrated with DRM scheduler
+		* All request queues removed from backend (e.g. all backpressure
+		  handled in DRM scheduler)
+		* Resets / cancels hook in DRM scheduler
+		* Watchdog hooks into DRM scheduler
+		* Lots of complexity of the GuC backend can be pulled out once
+		  integrated with DRM scheduler (e.g. state machine gets
+		  simplier, locking gets simplier, etc...)
+	* Execlists backend will minimum required to hook in the DRM scheduler
+		* Legacy interface
+		* Features like timeslicing / preemption / virtual engines would
+		  be difficult to integrate with the DRM scheduler and these
+		  features are not required for GuC submission as the GuC does
+		  these things for us
+		* ROI low on fully integrating into DRM scheduler
+		* Fully integrating would add lots of complexity to DRM
+		  scheduler
+	* Port i915 priority inheritance / boosting feature in DRM scheduler
+		* Used for i915 page flip, may be useful to other DRM drivers as
+		  well
+		* Will be an optional feature in the DRM scheduler
+	* Remove in-order completion assumptions from DRM scheduler
+		* Even when using the DRM scheduler the backends will handle
+		  preemption, timeslicing, etc... so it is possible for jobs to
+		  finish out of order
+	* Pull out i915 priority levels and use DRM priority levels
+	* Optimize DRM scheduler as needed
+
+TODOs for GuC submission upstream
+=================================
+
+* Need an update to GuC firmware / i915 to enable error state capture
+* Open source tool to decode GuC logs
+* Public GuC spec
+
+New uAPI for basic GuC submission
+=================================
+No major changes are required to the uAPI for basic GuC submission. The only
+change is a new scheduler attribute: I915_SCHEDULER_CAP_STATIC_PRIORITY_MAP.
+This attribute indicates the 2k i915 user priority levels are statically mapped
+into 3 levels as follows:
+
+* -1k to -1 Low priority
+* 0 Medium priority
+* 1 to 1k High priority
+
+This is needed because the GuC only has 4 priority bands. The highest priority
+band is reserved with the kernel. This aligns with the DRM scheduler priority
+levels too.
+
+Spec references:
+----------------
+* https://www.khronos.org/registry/EGL/extensions/IMG/EGL_IMG_context_priority.txt
+* https://www.khronos.org/registry/vulkan/specs/1.2-extensions/html/chap5.html#devsandqueues-priority
+* https://spec.oneapi.com/level-zero/latest/core/api.html#ze-command-queue-priority-t
+
+New parallel submission uAPI
+============================
+Details to come in a following patch.

Documentation/gpu/rfc/index.rst

@@ -19,3 +19,7 @@ host such documentation:
 .. toctree::
 
     i915_gem_lmem.rst
+
+.. toctree::
+
+    i915_scheduler.rst