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

[Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>]

On Mon, Jan 03, 2022 at 01:17:10PM -0800, Matt Roper wrote:
>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.
>

stream is not yet created at this point, so I would do this:

pass intel_gt to the helper instead of perf:
static u64 oa_exponent_to_ns(struct intel_gt *gt, int exponent)
{
	return intel_gt_clock_interval_to_ns(gt, 2ULL << exponent);
}

caller would then be:
oa_period = oa_exponent_to_ns(props->engine->gt, value);

Thanks,
Umesh

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