Re: [PATCH] drm/msm/mdp5: fix missing CTL flush

[Sean Paul]

On Tue, Jul 03, 2018 at 12:43:50PM -0400, Rob Clark wrote:
> f9cb8d8d836e fixed various race conditions with CTL flush, in particular
> flushing and sending the START signal before encoder state was updated.
> But it did this a little too well in some cases that don't trigger
> encoder->enable(), and CTL[n].FLUSH would never be set.  When page flips
> happen it would paper over the bug, since the first plag flip would
> flush out the state to the hardware.
> 
> The issue could be reproduced with, for example, modetest (without the
> '-v' argument).
> 
> Fixes: f9cb8d8d836e drm/msm/mdp5: rework CTL START signal handling
> Signed-off-by: Rob Clark <robdclark@xxxxxxxxx>

Reviewed-by: Sean Paul <seanpaul@xxxxxxxxxxxx>

> ---
>  drivers/gpu/drm/msm/disp/mdp5/mdp5_encoder.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/msm/disp/mdp5/mdp5_encoder.c b/drivers/gpu/drm/msm/disp/mdp5/mdp5_encoder.c
> index 9af94e35f678..fcd44d1d1068 100644
> --- a/drivers/gpu/drm/msm/disp/mdp5/mdp5_encoder.c
> +++ b/drivers/gpu/drm/msm/disp/mdp5/mdp5_encoder.c
> @@ -319,7 +319,17 @@ static int mdp5_encoder_atomic_check(struct drm_encoder *encoder,
>  
>  	mdp5_cstate->ctl = ctl;
>  	mdp5_cstate->pipeline.intf = intf;
> -	mdp5_cstate->defer_start = true;
> +
> +	/*
> +	 * This is a bit awkward, but we want to flush the CTL and hit the
> +	 * START bit at most once for an atomic update.  In the non-full-
> +	 * modeset case, this is done from crtc->atomic_flush(), but that
> +	 * is too early in the case of full modeset, in which case we
> +	 * defer to encoder->enable().  But we need to *know* whether
> +	 * encoder->enable() will be called to do this:
> +	 */
> +	if (drm_atomic_crtc_needs_modeset(crtc_state))
> +		mdp5_cstate->defer_start = true;
>  
>  	return 0;
>  }
> -- 
> 2.17.1
> 

-- 
Sean Paul, Software Engineer, Google / Chromium OS