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@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; > > > SECURITY NOTE: file ~/.netrc must not be accessible by others
