[RFC PATCH 2/5] drm/doc/rfc: i915 new parallel submission uAPI plan

[Matthew Brost <matthew.brost@intel.com>]

Add entry fpr i915 new parallel submission uAPI plan.

Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Tony Ye <tony.ye@intel.com>
CC: Carl Zhang <carl.zhang@intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
---
 Documentation/gpu/rfc/i915_scheduler.rst | 56 +++++++++++++++++++++++-
 1 file changed, 54 insertions(+), 2 deletions(-)

diff --git a/Documentation/gpu/rfc/i915_scheduler.rst b/Documentation/gpu/rfc/i915_scheduler.rst
index fa6780a11c86..e3455b33edfe 100644
--- a/Documentation/gpu/rfc/i915_scheduler.rst
+++ b/Documentation/gpu/rfc/i915_scheduler.rst
@@ -13,7 +13,8 @@ i915 with the DRM scheduler is:
 	  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 and not regress execlists
+	  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
@@ -67,4 +68,55 @@ levels too.
 
 New parallel submission uAPI
 ============================
-Details to come in a following patch.
+The existing bonding uAPI is completely broken with GuC submission because
+whether a submission is a single context submit or parallel submit isn't known
+until execbuf time activated via the I915_SUBMIT_FENCE. To submit multiple
+contexts in parallel with the GuC the context must be explictly registered with
+N contexts and all N contexts must be submitted in a single command to the GuC.
+This interfaces doesn't support dynamically changing between N contexts as the
+bonding uAPI does. Hence the need for a new parallel submission interface. Also
+the legacy bonding uAPI is quite confusing and not intuitive at all.
+
+The new parallel submission uAPI consists of 3 parts:
+
+* Export engines logical mapping
+* A 'set_parallel' extension to configure contexts for parallel
+  submission
+* Extend execbuf2 IOCTL to support submitting N BBs in a single IOCTL
+
+Export engines logical mapping
+------------------------------
+Certain use cases require BBs to be placed on engine instances in logical order
+(e.g. split-frame on gen11+). The logical mapping of engine instances can change
+based on fusing. Rather than making UMDs be aware of fusing, simply expose the
+logical mapping with the existing query engine info IOCTL. Also the GuC
+submission interface currently only supports submitting multiple contexts to
+engines in logical order.
+
+A single bit will be added to drm_i915_engine_info.flags indicating that the
+logical instance has been returned and a new field,
+drm_i915_engine_info.logical_instance, returns the logical instance.
+
+A 'set_parallel' extension to configure contexts for parallel submission
+------------------------------------------------------------------------
+The 'set_parallel' extension configures N contexts for parallel submission. It
+is setup step that should be called before using any of the contexts. See
+I915_CONTEXT_ENGINES_EXT_LOAD_BALANCE or I915_CONTEXT_ENGINES_EXT_BOND for
+similar existing examples. Once the N contexts are configured for parallel
+submission the execbuf2 IOCTL can be called submiting 1-N BBs in a single IOCTL.
+Although submitting less than N BBs is allowed it is not recommended as that
+will likely leave parts of the hardware reserved and idle. Initially only
+support GuC submission. Execlist support can be added later if needed.
+
+Add I915_CONTEXT_ENGINES_EXT_PARALLEL_SUBMIT and
+i915_context_engines_parallel_submit to the uAPI to implement this extension.
+
+Extend execbuf2 IOCTL to support submitting N BBs in a single IOCTL
+-------------------------------------------------------------------
+Contexts that have been configured with the 'set_parallel' extension are allowed
+to submit 1-N BBs in a single execbuf2 IOCTL. The BBs are either the last N
+objects in the drm_i915_gem_exec_object2 list or the first N if
+I915_EXEC_BATCH_FIRST is set.
+
+Add field 6 bit wide field to drm_i915_gem_exec_object2.flags which indicates
+the number of BBs - 1 included in the IOCTL.
-- 
2.28.0