Re: [Intel-gfx] [PATCH 06/19] drm/i915/perf: Use helpers to process reports w.r.t. OA buffer size

["Dixit, Ashutosh" <ashutosh.dixit@intel.com>]

On Wed, 14 Sep 2022 11:19:30 -0700, Umesh Nerlige Ramappa wrote:
>
> On Wed, Sep 14, 2022 at 09:04:10AM -0700, Dixit, Ashutosh wrote:
> > On Tue, 23 Aug 2022 13:41:42 -0700, Umesh Nerlige Ramappa wrote:
> >>
> >
> > Hi Umesh,
> >
> >> diff --git a/drivers/gpu/drm/i915/i915_perf.c b/drivers/gpu/drm/i915/i915_perf.c
> >> index 6fc4f0d8fc5a..bbf1c574f393 100644
> >> --- a/drivers/gpu/drm/i915/i915_perf.c
> >> +++ b/drivers/gpu/drm/i915/i915_perf.c
> >> @@ -385,6 +385,21 @@ static struct ctl_table_header *sysctl_header;
> >>
> >>  static enum hrtimer_restart oa_poll_check_timer_cb(struct hrtimer *hrtimer);
> >>
> >> +static u32 _oa_taken(struct i915_perf_stream * stream, u32 tail, u32 head)
> >
> > nit: no space between * and stream.
> >
> >> +{
> >> +	u32 size = stream->oa_buffer.vma->size;
> >> +
> >> +	return tail >= head ? tail - head : size - (head - tail);
> >> +}
> >
> > If we are doing this we should probably eliminate references to OA_TAKEN
> > which serves an identical purpose (I think there is one remaining
> > reference) and also delete OA_TAKEN #define.
> >
> >> +
> >> +static u32 _rewind_tail(struct i915_perf_stream * stream, u32 relative_hw_tail,
> >> +			u32 rewind_delta)
> >> +{
> >> +	return rewind_delta > relative_hw_tail ?
> >> +	       stream->oa_buffer.vma->size - (rewind_delta - relative_hw_tail) :
> >> +	       relative_hw_tail - rewind_delta;
> >> +}
> >
> > Also are we really saying here that we are supporting non-power-of-2 OA
> > buffer sizes? Because if we stayed with power-of-2 sizes the expression
> > above are nice and elegant and actually closer to the previous code being
> > changed in this patch. For example:
> >
> > #include <linux/circ_buf.h>
> >
> > static u32 _oa_taken(struct i915_perf_stream *stream, u32 tail, u32 head)
> > {
> >	return CIRC_CNT(tail, head, stream->oa_buffer.vma->size);
> > }
> >
> > static u32 _rewind_tail(struct i915_perf_stream *stream, u32 relative_hw_tail,
> >				u32 rewind_delta)
> > {
> >	return CIRC_CNT(relative_hw_tail, rewind_delta, stream->oa_buffer.vma->size);
> > }
> >
> > Note that for power-of-2 sizes the two functions above are identical but we
> > should keep them separate for clarity (as is done in the patch) since they
> > are serving two different functions in the OA code.
> >
> > Also another assumption in the code seems to be:
> >
> >	stream->oa_buffer.vma->size == OA_BUFFER_SIZE
> >
> > which I am pretty sure will not hold for arbitrary non-power-of-2 OA buffer
> > sizes? So we might as well stick with power-of-2 sizes and change later in
> > a separate patch only if needed?
>
> Most changes here are related to the OA buffer size issue and that is
> specific to xehpsdv where the size is not a power of 2. I am thinking of
> dropping these changes in the next revision since DG2 is fixed and OA
> buffer sizes are power of 2.

In the code stream->oa_buffer.vma->size and OA_BUFFER_SIZE are both used,
if we want to clean that up and only use stream->oa_buffer.vma->size, we
could still do soemthing like I suggested with just power-of-2 sizes and
keep this patch. If we ever have to support non-power-of-2 sizes in the
future we'll just need to change _oa_taken and _rewind_tail
functions. Anyway your call.

Thanks.
--
Ashutosh