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

Matthew Brost
2021-09-14T05:07:12Z

On Mon, Sep 13, 2021 at 03:38:44PM -0700, John Harrison wrote:
> On 9/13/2021 09:54, Matthew Brost wrote:
>
> On Thu, Sep 09, 2021 at 03:51:27PM -0700, John Harrison wrote:
>
> 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.
>
>
> Yea, this need to be explained better. How about this?
>
> Calling switch_to_kernel_context isn't needed if the engine PM reference
> is take while all user contexts have scheduling enabled. Once scheduling
> is disabled on all user contexts the GuC is guaranteed to not touch any
> user context state which is effectively the same pointing to a kernel
> context.
>
> Matt
>
> I'm still not seeing how the PM reference is involved?
>

We shouldn't trap into the GT PM park code while a user context has
scheduling enabled as the GT PM park code may have side affects we don't
to execute if a user context still has scheduling enabled. I guess that
isn't explained very well.

> Also, IMHO the focus is wrong in the above text. The fundamental requirement is
> the ensure the hardware is idle. Execlist achieves this by switching to a safe
> context. GuC achieves it by disabling scheduling. Indeed, switching to a 'safe'
> context really has no effect with GuC submission. So 'effectively the same as
> pointing to a kernel context' is an incorrect description. I would go with
> something like:
>
> "This is execlist specific behaviour intended to ensure the GPU is idle by
> switching to a known 'safe' context. With GuC submission, the same idle
> guarantee is achieved by other means (disabling scheduling). Further,
> switching to a 'safe' context has no effect with GuC submission as the
> scheduler can just switch back again.
> FIXME: Move this backend scheduler specific behaviour into the scheduler
> backend."
>

That is worded better. Will pull into the next rev.

Matt

>
> John.
>
>
>
>
>
> v2:
> (Daniel Vetter)
> - Add FIXME comment about pushing switch_to_kernel_context to backend
>
> Signed-off-by: Matthew Brost <matthew.brost@intel.com>
> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> ---
> 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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