[Intel-gfx] [PATCH 1/3] drm/i915/perf: Subtract gtt_offset from hw_tail

[Umesh Nerlige Ramappa]

On Fri, Sep 08, 2023 at 06:16:24PM -0700, Ashutosh Dixit wrote:
>The code in oa_buffer_check_unlocked() is correct only if the OA buffer is
>16 MB aligned (which seems to be the case today in i915). However when the
>16 MB alignment is dropped, when we "Subtract partial amount off the tail",
>the "& (OA_BUFFER_SIZE - 1)" operation in OA_TAKEN() will result in an
>incorrect hw_tail value.
>
>Therefore hw_tail must be brought to the same base as head and read_tail
>prior to OA_TAKEN by subtracting gtt_offset from hw_tail.
>
>Signed-off-by: Ashutosh Dixit <ashutosh.dixit at intel.com>
>---
> drivers/gpu/drm/i915/i915_perf.c | 1 +
> 1 file changed, 1 insertion(+)
>
>diff --git a/drivers/gpu/drm/i915/i915_perf.c b/drivers/gpu/drm/i915/i915_perf.c
>index 018f42fff4cc0..ec0fc2934045a 100644
>--- a/drivers/gpu/drm/i915/i915_perf.c
>+++ b/drivers/gpu/drm/i915/i915_perf.c
>@@ -565,6 +565,7 @@ static bool oa_buffer_check_unlocked(struct i915_perf_stream *stream)
> 	partial_report_size %= report_size;
>
> 	/* Subtract partial amount off the tail */
>+	hw_tail -= gtt_offset;
> 	hw_tail = OA_TAKEN(hw_tail, partial_report_size);

I see partial_report_size is a value in the 0 - report_size range and it 
may not have the gtt_offset added to it, so I guess the OA_TAKEN may 
result in a bad value, but I am not able to visualize what the specific 
issue is. Can you please provide an example with numbers?

Also, slightly confused about the need for this patch. Are we dropping 
the 16 MB alignment for some reason?  If not, I suggest we can add this 
patch later with any series that drops it.

Thanks,
Umesh

>
> 	/* NB: The head we observe here might effectively be a little
>-- 
>2.41.0
>