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

[Abhinav Kumar <quic_abhinavk@quicinc.com>]

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")

This code has been this way since that commit itself.

> ---
>   drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c b/drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c
> index 3a462e327e0e..a1b8c4592943 100644
> --- a/drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c
> +++ b/drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c
> @@ -1251,12 +1251,13 @@ static void dpu_encoder_vblank_callback(struct drm_encoder *drm_enc,
>   	DPU_ATRACE_BEGIN("encoder_vblank_callback");
>   	dpu_enc = to_dpu_encoder_virt(drm_enc);
>   
> +	atomic_inc(&phy_enc->vsync_cnt);
> +
>   	spin_lock_irqsave(&dpu_enc->enc_spinlock, lock_flags);
>   	if (dpu_enc->crtc)
>   		dpu_crtc_vblank_callback(dpu_enc->crtc);
>   	spin_unlock_irqrestore(&dpu_enc->enc_spinlock, lock_flags);
>   
> -	atomic_inc(&phy_enc->vsync_cnt);
>   	DPU_ATRACE_END("encoder_vblank_callback");
>   }
>   
> 
> base-commit: f2906aa863381afb0015a9eb7fefad885d4e5a56