drm/i915: Unify active context tracking between legacy/execlists/guc

The requests conversion introduced a nasty bug where we could generate a
new request in the middle of constructing a request if we needed to idle
the system in order to evict space for a context. The request to idle
would be executed (and waited upon) before the current one, creating a
minor havoc in the seqno accounting, as we will consider the current
request to already be completed (prior to deferred seqno assignment) but
ring->last_retired_head would have been updated and still could allow
us to overwrite the current request before execution.

We also employed two different mechanisms to track the active context
until it was switched out. The legacy method allowed for waiting upon an
active context (it could forcibly evict any vma, including context's),
but the execlists method took a step backwards by pinning the vma for
the entire active lifespan of the context (the only way to evict was to
idle the entire GPU, not individual contexts). However, to circumvent
the tricky issue of locking (i.e. we cannot take struct_mutex at the
time of i915_gem_request_submit(), where we would want to move the
previous context onto the active tracker and unpin it), we take the
execlists approach and keep the contexts pinned until retirement.
The benefit of the execlists approach, more important for execlists than
legacy, was the reduction in work in pinning the context for each
request - as the context was kept pinned until idle, it could short
circuit the pinning for all active contexts.

We introduce new engine vfuncs to pin and unpin the context
respectively. The context is pinned at the start of the request, and
only unpinned when the following request is retired (this ensures that
the context is idle and coherent in main memory before we unpin it). We
move the engine->last_context tracking into the retirement itself
(rather than during request submission) in order to allow the submission
to be reordered or unwound without undue difficultly.

And finally an ulterior motive for unifying context handling was to
prepare for mock requests.

v2: Rename to last_retired_context, split out legacy_context tracking
for MI_SET_CONTEXT.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161218153724.8439-3-chris@chris-wilson.co.uk

1 files changed, 22 insertions, 1 deletions

diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
index 3f43adefd1c0..4f1271821fa9 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.h
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
@@ -266,6 +266,10 @@ struct intel_engine_cs {
 	void		(*reset_hw)(struct intel_engine_cs *engine,
 				    struct drm_i915_gem_request *req);
 
+	int		(*context_pin)(struct intel_engine_cs *engine,
+				       struct i915_gem_context *ctx);
+	void		(*context_unpin)(struct intel_engine_cs *engine,
+					 struct i915_gem_context *ctx);
 	int		(*init_context)(struct drm_i915_gem_request *req);
 
 	int		(*emit_flush)(struct drm_i915_gem_request *request,
@@ -379,7 +383,24 @@ struct intel_engine_cs {
 	bool preempt_wa;
 	u32 ctx_desc_template;
 
-	struct i915_gem_context *last_context;
+	/* Contexts are pinned whilst they are active on the GPU. The last
+	 * context executed remains active whilst the GPU is idle - the
+	 * switch away and write to the context object only occurs on the
+	 * next execution.  Contexts are only unpinned on retirement of the
+	 * following request ensuring that we can always write to the object
+	 * on the context switch even after idling. Across suspend, we switch
+	 * to the kernel context and trash it as the save may not happen
+	 * before the hardware is powered down.
+	 */
+	struct i915_gem_context *last_retired_context;
+
+	/* We track the current MI_SET_CONTEXT in order to eliminate
+	 * redudant context switches. This presumes that requests are not
+	 * reordered! Or when they are the tracking is updated along with
+	 * the emission of individual requests into the legacy command
+	 * stream (ring).
+	 */
+	struct i915_gem_context *legacy_active_context;
 
 	struct intel_engine_hangcheck hangcheck;