[Intel-gfx] [PATCH] drm/i915/perf: don't read head/tail pointers outside critical section

[Chris Wilson]

Quoting Lionel Landwerlin (2020-03-30 10:14:11)
> Reading or writing those fields should only happen under
> stream->oa_buffer.ptr_lock.

Writing, yes. Reading as a pair, sure. There are other ways you can
ensure that the tail/head are read as one, but fair enough.

> Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin at intel.com>
> Fixes: d1df41eb72ef ("drm/i915/perf: rework aging tail workaround")
> ---
>  drivers/gpu/drm/i915/i915_perf.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_perf.c b/drivers/gpu/drm/i915/i915_perf.c
> index c74ebac50015..ec9421f02ebd 100644
> --- a/drivers/gpu/drm/i915/i915_perf.c
> +++ b/drivers/gpu/drm/i915/i915_perf.c
> @@ -463,6 +463,7 @@ static bool oa_buffer_check_unlocked(struct i915_perf_stream *stream)
>         u32 gtt_offset = i915_ggtt_offset(stream->oa_buffer.vma);
>         int report_size = stream->oa_buffer.format_size;
>         unsigned long flags;
> +       bool pollin;
>         u32 hw_tail;
>         u64 now;
>  
> @@ -532,10 +533,13 @@ static bool oa_buffer_check_unlocked(struct i915_perf_stream *stream)
>                 stream->oa_buffer.aging_timestamp = now;
>         }
>  
> +       pollin = OA_TAKEN(stream->oa_buffer.tail - gtt_offset,
> +                         stream->oa_buffer.head - gtt_offset) >= report_size;
> +
> +

Bonus \n

Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>

>         spin_unlock_irqrestore(&stream->oa_buffer.ptr_lock, flags);
>  
> -       return OA_TAKEN(stream->oa_buffer.tail - gtt_offset,
> -                       stream->oa_buffer.head - gtt_offset) >= report_size;
> +       return pollin;

You could always leave the calculation here, and just have the read
inside.
-Chris