[PATCH v9 2/6] drm/i915: Use to_gt() helper for GGTT accesses

[Matt Roper]

On Tue, Dec 21, 2021 at 09:46:29PM +0200, Andi Shyti wrote:
> Hi Matt,
> 
> > > diff --git a/drivers/gpu/drm/i915/i915_perf.c b/drivers/gpu/drm/i915/i915_perf.c
> > > index 170bba913c30..128315aec517 100644
> > > --- a/drivers/gpu/drm/i915/i915_perf.c
> > > +++ b/drivers/gpu/drm/i915/i915_perf.c
> > > @@ -1630,7 +1630,7 @@ static int alloc_noa_wait(struct i915_perf_stream *stream)
> > >  	struct drm_i915_gem_object *bo;
> > >  	struct i915_vma *vma;
> > >  	const u64 delay_ticks = 0xffffffffffffffff -
> > > -		intel_gt_ns_to_clock_interval(stream->perf->i915->ggtt.vm.gt,
> > > +		intel_gt_ns_to_clock_interval(to_gt(stream->perf->i915)->ggtt->vm.gt,
> > 
> > I'm not too familiar with the perf code, but this looks a bit roundabout
> > since we're ultimately trying to get to a GT...do we even need to go
> > through the ggtt structure here or can we just pass
> > "to_gt(stream->perf->i915)" as the first parameter?
> > 
> > >  					      atomic64_read(&stream->perf->noa_programming_delay));
> > >  	const u32 base = stream->engine->mmio_base;
> > >  #define CS_GPR(x) GEN8_RING_CS_GPR(base, x)
> > > @@ -3542,7 +3542,7 @@ i915_perf_open_ioctl_locked(struct i915_perf *perf,
> > >  
> > >  static u64 oa_exponent_to_ns(struct i915_perf *perf, int exponent)
> > >  {
> > > -	return intel_gt_clock_interval_to_ns(perf->i915->ggtt.vm.gt,
> > > +	return intel_gt_clock_interval_to_ns(to_gt(perf->i915)->ggtt->vm.gt,
> > 
> > Ditto; this looks like "to_gt(perf->i915)" might be all we need?
> 
> I think this function is looking for the GT coming from the VM,
> otherwise originally it could have taken it from &i915->gt. In my
> first version I proposed a wrapper around this but it was
> rejected by Lucas.
> 
> Besides, as we discussed earlier when I was proposed the static
> allocation, the ggtt might not always be linked to the same gt,
> so that I assumed that sometimes:
> 
>    to_gt(perf->i915)->ggtt->vm.gt != to_gt(perf->i915)
> 
> if two GTs are sharing the same ggtt, what would the ggtt->vm.gt
> link be?

>From the git history, it doesn't look like this really needs to care
about the GGTT at all; I think it was just unintentionally written in a
roundabout manner when intel_gt was first being introduced in the code.
The reference here first showed up in commit f170523a7b8e ("drm/i915/gt:
Consolidate the CS timestamp clocks").

Actually the most correct thing to do is probably to use
'stream->engine->gt' to ensure we grab the GT actually associated with
the stream's engine.


Matt


> 
> Thanks,
> Andi

-- 
Matt Roper
Graphics Software Engineer
VTT-OSGC Platform Enablement
Intel Corporation
(916) 356-2795