Re: [Intel-gfx] [PATCH 11/46] drm/i915/guc: Don't call switch_to_kernel_context with GuC submission

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

On Mon, Aug 09, 2021 at 04:27:01PM +0200, Daniel Vetter wrote:
> On Tue, Aug 03, 2021 at 03:29:08PM -0700, 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.
> > 
> > Signed-off-by: Matthew Brost <matthew.brost@intel.com>
> > ---
> >  drivers/gpu/drm/i915/gt/intel_engine_pm.c | 4 ++++
> >  1 file changed, 4 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..58099de6bf07 100644
> > --- a/drivers/gpu/drm/i915/gt/intel_engine_pm.c
> > +++ b/drivers/gpu/drm/i915/gt/intel_engine_pm.c
> > @@ -162,6 +162,10 @@ 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 */
> 
> Maybe whack a big FIXME on here that we should unravel this properly.

Sure, can add a FIXME here.

> Currently the execlist backend assumptions are leaked all over the place,
> leading to stuff like this. Which means extremely fragile code.
>

Yes, this something required for execlists implemented in what should be
generic code. 

> I currently don't have a great idea on how exactly we should do that, but
> oh well.

Me either, it will be a process.

> 
> btw just in case we ever want to make guc lrc properly evictable (which as
> the og use-case for this function, way, way back), would we need to fully

Can you explain what you mean by fully evictable? Not getting what you
mean in this context.

> unregister them from guc? At least I'm assuming there's no other trick

If scheduling is disabled on the context (currently done on unpin) you are
free move anything around as the GuC is guaranteed not to touch the
context state. If on re-pin something has moved (e.g. the LRC vaddr is
different), you need to unregister and re-register the context with the
GuC.

> like the below one.
> 
> Another aside: How does the perf/OA patching work on GuC?
>

Not my area of expertise but perf somewhat a WIP. The plan is for the
GuC to write out some stats to HWSP I think? John Harrison is working to
get this fully implemented.

OA is working afaik, with Umesh Nerlige Ramappa being the expert here.

Matt

> Anyway, patch looks legit:
> 
> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> 
> 
> > +	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;
> > -- 
> > 2.28.0
> > 
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch