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

[Matthew Brost <matthew.brost@intel.com>]

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.
> 
> 
> > 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"
> 

Missed this. Will fix in next rev.

Matt

> 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;
>