Re: [PATCH] drm/msm/dpu: Increment vsync_cnt before waking up userspace

[Dmitry Baryshkov <dmitry.baryshkov@linaro.org>]

On 22/06/2022 20:33, Rob Clark wrote:
> On Wed, Jun 22, 2022 at 10:24 AM Abhinav Kumar
> <quic_abhinavk@quicinc.com> wrote:
>>
>>
>>
>> On 6/21/2022 7:38 PM, Stephen Boyd wrote:
>>> The 'vsync_cnt' is used to count the number of frames for a crtc.
>>> Unfortunately, we increment the count after waking up userspace via
>>> dpu_crtc_vblank_callback() calling drm_crtc_handle_vblank().
>>> drm_crtc_handle_vblank() wakes up userspace processes that have called
>>> drm_wait_vblank_ioctl(), and if that ioctl is expecting the count to
>>> increase it won't.
>>>
>>> Increment the count before calling into the drm APIs so that we don't
>>> have to worry about ordering the increment with anything else in drm.
>>> This fixes a software video decode test that fails to see frame counts
>>> increase on Trogdor boards.
>>>
>>> Cc: Mark Yacoub <markyacoub@chromium.org>
>>> Cc: Jessica Zhang <quic_jesszhan@quicinc.com>
>>> Fixes: 885455d6bf82 ("drm/msm: Change dpu_crtc_get_vblank_counter to use vsync count.")
>>> Signed-off-by: Stephen Boyd <swboyd@chromium.org>
>>
>> This is right, we should increment before drm_crtc_handle_vblank() as
>> that will query the vblank counter. This also matches what we do
>> downstream, hence
>>
>> Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
>>
>> One small nit though, shouldnt the fixes tag be
>>
>> 25fdd5933e4c ("drm/msm: Add SDM845 DPU support")
> 
> *Kinda*.. but the sw vblank counter wasn't used for reporting frame nr
> to userspace until 885455d6bf82.  You could possibly list both,
> perhaps, but 885455d6bf82 is the important one for folks backporting
> to stable kernels to be aware of

I'd agree, the original Fixes tag seems good to me.

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>


-- 
With best wishes
Dmitry