Re: [Intel-gfx] [PATCH 07/27] drm/i915/guc: Don't call switch_to_kernel_context with GuC submission

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On 8/20/2021 15:44, Matthew Brost wrote:
Calling switch_to_kernel_context isn't needed if the engine PM reference
is taken while all contexts are pinned. By not calling
switch_to_kernel_context we save on issuing a request to the engine.
I thought the intention of the switch_to_kernel was to ensure that the GPU is not touching any user context and is basically idle. That is not a valid assumption with an external scheduler such as GuC. So why is the description above only mentioning PM references? What is the connection between the PM ref and the switch_to_kernel?

Also, the comment in the code does not mention anything about PM references, it just says 'not necessary with GuC' but no explanation at all.


v2:
  (Daniel Vetter)
   - Add FIXME comment about pushing switch_to_kernel_context to backend

Signed-off-by: Matthew Brost <matthew.brost@xxxxxxxxx>
Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
---
  drivers/gpu/drm/i915/gt/intel_engine_pm.c | 9 +++++++++
  1 file changed, 9 insertions(+)

diff --git a/drivers/gpu/drm/i915/gt/intel_engine_pm.c b/drivers/gpu/drm/i915/gt/intel_engine_pm.c
index 1f07ac4e0672..11fee66daf60 100644
--- a/drivers/gpu/drm/i915/gt/intel_engine_pm.c
+++ b/drivers/gpu/drm/i915/gt/intel_engine_pm.c
@@ -162,6 +162,15 @@ static bool switch_to_kernel_context(struct intel_engine_cs *engine)
  	unsigned long flags;
  	bool result = true;
+ /*
+	 * No need to switch_to_kernel_context if GuC submission
+	 *
+	 * FIXME: This execlists specific backend behavior in generic code, this
"This execlists" -> "This is execlist"

"this should be" -> "it should be"

John.

+	 * should be pushed to the backend.
+	 */
+	if (intel_engine_uses_guc(engine))
+		return true;
+
  	/* GPU is pointing to the void, as good as in the kernel context. */
  	if (intel_gt_is_wedged(engine->gt))
  		return true;




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