drm/i915: Expose one LRC function for GuC submission mode

GuC submission is basically execlist submission, but with the GuC
handling the actual writes to the ELSP and the resulting context
switch interrupts.  So to describe a context for submission via
the GuC, we need one of the same functions used in execlist mode.
This commit exposes one such function, changing its name to better
describe what it does (it's related to logical ring contexts rather
than to execlists per se).

v2:
    Replaces previous "drm/i915: Move execlists defines from .c to .h"

v3:
    Incorporates a change to one of the functions exposed here that was
        previously part of an internal patch, but which was omitted from
        the version recently committed to drm-intel-nightly:
	    7a01a0a drm/i915/lrc: Update PDPx registers with lri commands
        So we reinstate this change here.

v4:
    Drop v3 change, update function parameters due to collision with
        8ee3615 drm/i915: Convert execlists_ctx_descriptor() for requests

v5:
    Don't expose execlists_update_context() after all. The current
        version is no longer compatible with GuC submission; trying to
        share the execlist version of this function results in both GuC
        and CPU updating TAIL in the context image, with bad results when
        they get out of step. The GuC submission path now has its own
        private version that just updates the ringbuffer start address,
        and not TAIL or PDPx.

v6:
    Rebased

Issue: VIZ-4884
Signed-off-by: Dave Gordon <david.s.gordon@intel.com>
Reviewed-by: Tom O'Rourke <Tom.O'Rourke@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_lrc.c

@@ -270,11 +270,11 @@ u32 intel_execlists_ctx_id(struct drm_i915_gem_object *ctx_obj)
 	return lrca >> 12;
 }
 
-static uint64_t execlists_ctx_descriptor(struct drm_i915_gem_request *rq)
+uint64_t intel_lr_context_descriptor(struct intel_context *ctx,
+				     struct intel_engine_cs *ring)
 {
-	struct intel_engine_cs *ring = rq->ring;
 	struct drm_device *dev = ring->dev;
-	struct drm_i915_gem_object *ctx_obj = rq->ctx->engine[ring->id].state;
+	struct drm_i915_gem_object *ctx_obj = ctx->engine[ring->id].state;
 	uint64_t desc;
 	uint64_t lrca = i915_gem_obj_ggtt_offset(ctx_obj);
 
@@ -312,13 +312,13 @@ static void execlists_elsp_write(struct drm_i915_gem_request *rq0,
 	uint64_t desc[2];
 
 	if (rq1) {
-		desc[1] = execlists_ctx_descriptor(rq1);
+		desc[1] = intel_lr_context_descriptor(rq1->ctx, rq1->ring);
 		rq1->elsp_submitted++;
 	} else {
 		desc[1] = 0;
 	}
 
-	desc[0] = execlists_ctx_descriptor(rq0);
+	desc[0] = intel_lr_context_descriptor(rq0->ctx, rq0->ring);
 	rq0->elsp_submitted++;
 
 	/* You must always write both descriptors in the order below. */

drivers/gpu/drm/i915/intel_lrc.h

@@ -74,6 +74,8 @@ int intel_lr_context_deferred_create(struct intel_context *ctx,
 void intel_lr_context_unpin(struct drm_i915_gem_request *req);
 void intel_lr_context_reset(struct drm_device *dev,
 			struct intel_context *ctx);
+uint64_t intel_lr_context_descriptor(struct intel_context *ctx,
+				     struct intel_engine_cs *ring);
 
 /* Execlists */
 int intel_sanitize_enable_execlists(struct drm_device *dev, int enable_execlists);