[PATCH 2/2] drm/i915: Handle Intel igfx + Intel dgfx hybrid graphics setup

[Tvrtko Ursulin]

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

In short this makes i915 work for hybrid setups (DRI_PRIME=1 with Mesa)
when rendering is done on Intel dgfx and scanout/composition on Intel
igfx.

Before this patch the driver was not quite ready for that setup, mainly
because it was able to emit a semaphore wait between the two GPUs, which
results in deadlocks because semaphore target location in HWSP is neither
shared between the two, nor mapped in both GGTT spaces.

To fix it the patch adds an additional check to a couple of relevant code
paths in order to prevent using semaphores for inter-engine
synchronisation when relevant objects are not in the same GGTT space.

v2:
 * Avoid adding rq->i915. (Chris)

v3:
 * Use GGTT which describes the limit more precisely.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
---
 drivers/gpu/drm/i915/i915_request.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
index ce446716d092..e671f4e993cb 100644
--- a/drivers/gpu/drm/i915/i915_request.c
+++ b/drivers/gpu/drm/i915/i915_request.c
@@ -1152,6 +1152,12 @@ __emit_semaphore_wait(struct i915_request *to,
 	return 0;
 }
 
+static bool
+can_use_semaphore_wait(struct i915_request *to, struct i915_request *from)
+{
+	return to->engine->gt->ggtt == from->engine->gt->ggtt;
+}
+
 static int
 emit_semaphore_wait(struct i915_request *to,
 		    struct i915_request *from,
@@ -1160,6 +1166,9 @@ emit_semaphore_wait(struct i915_request *to,
 	const intel_engine_mask_t mask = READ_ONCE(from->engine)->mask;
 	struct i915_sw_fence *wait = &to->submit;
 
+	if (!can_use_semaphore_wait(to, from))
+		goto await_fence;
+
 	if (!intel_context_use_semaphores(to->context))
 		goto await_fence;
 
@@ -1263,7 +1272,8 @@ __i915_request_await_execution(struct i915_request *to,
 	 * immediate execution, and so we must wait until it reaches the
 	 * active slot.
 	 */
-	if (intel_engine_has_semaphores(to->engine) &&
+	if (can_use_semaphore_wait(to, from) &&
+	    intel_engine_has_semaphores(to->engine) &&
 	    !i915_request_has_initial_breadcrumb(to)) {
 		err = __emit_semaphore_wait(to, from, from->fence.seqno - 1);
 		if (err < 0)
-- 
2.30.2