[Intel-gfx] [PATCH 2/3] drm/i915/guc: Keep the previous context pinned until the next one has been completed

[Tvrtko Ursulin]

From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>

In GuC mode submitting requests is only putting them into the
GuC queue with the actual submission to hardware following at
some future point. This makes the per engine last context
tracking insufficient for closing the premature context
unpin race.

Instead we need to make requests track and pin the previous
context on the same engine, and only unpin them when they
themselves are retired. This will ensure contexts remain pinned
in all cases until the are fully saved by the GPU.

v2: Only track contexts.
v3: Limit to GuC for now.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
Cc: Alex Dai <yu.dai at intel.com>
Cc: Dave Gordon <david.s.gordon at intel.com>
Cc: Nick Hoath <nicholas.hoath at intel.com>
Cc: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/i915_drv.h  |  4 +++-
 drivers/gpu/drm/i915/i915_gem.c  |  5 +++++
 drivers/gpu/drm/i915/intel_lrc.c | 15 +++++++++++++++
 3 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index b9ed1b305beb..80f2d959ed7b 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -2290,6 +2290,9 @@ struct drm_i915_gem_request {
 	struct intel_context *ctx;
 	struct intel_ringbuffer *ringbuf;
 
+	/** Context preceding this one on the same engine. Can be NULL. */
+	struct intel_context *prev_ctx;
+
 	/** Batch buffer related to this request if any (used for
 	    error state dump only) */
 	struct drm_i915_gem_object *batch_obj;
@@ -2325,7 +2328,6 @@ struct drm_i915_gem_request {
 
 	/** Execlists no. of times this request has been sent to the ELSP */
 	int elsp_submitted;
-
 };
 
 struct drm_i915_gem_request * __must_check
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index e9db56c7a31f..18e747132ce5 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -1413,6 +1413,11 @@ static void i915_gem_request_retire(struct drm_i915_gem_request *request)
 	list_del_init(&request->list);
 	i915_gem_request_remove_from_client(request);
 
+	if (request->prev_ctx) {
+		intel_lr_context_unpin(request->prev_ctx, request->engine);
+		request->prev_ctx = NULL;
+	}
+
 	i915_gem_request_unreference(request);
 }
 
diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index 2f49dfae5560..73be9eac8ecc 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -791,6 +791,21 @@ intel_logical_ring_advance_and_submit(struct drm_i915_gem_request *request)
 	if (intel_engine_stopped(engine))
 		return 0;
 
+	/*
+	 * Pin the previous context on this engine to ensure it is not
+	 * prematurely unpinned in GuC mode.
+	 * Previous context will be unpinned when this request is retired,
+	 * ensuring the GPU has switched out from the previous context and
+	 * into a new context at that point.
+	 */
+	if (dev_priv->guc.execbuf_client && engine->last_context) {
+		intel_lr_context_pin(engine->last_context, engine);
+		request->prev_ctx = engine->last_context;
+	}
+
+	/*
+	 * Track and pin the last context submitted on an engine.
+	 */
 	if (engine->last_context != request->ctx) {
 		if (engine->last_context)
 			intel_lr_context_unpin(engine->last_context, engine);
-- 
1.9.1