[Intel-gfx] [PATCH 1/2] drm/i915: Try to suppress more spurious PCH underruns on ILK-IVB

[Chris Wilson]

Quoting Ville Syrjala (2018-05-24 20:04:05)
> From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> 
> My ILK seems to generate a spurious PCH underrun with most interlaced
> HDMI modes. Add a second vblank wait to avoid it.
> 
> We have seen some spurious PCH underruns still in CI as well, some
> of which seem to be progressive DP. The logs also point towards some
> spurious underrins with progressive HDMI on SNB. While I don't have
> a solid explanation for those let's try to kill all the birds with one
> stone and always do the double wait.
> 
> Buzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106387
> Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
> ---
>  drivers/gpu/drm/i915/intel_display.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> index 7604fbda0607..b5fa4943372a 100644
> --- a/drivers/gpu/drm/i915/intel_display.c
> +++ b/drivers/gpu/drm/i915/intel_display.c
> @@ -5528,9 +5528,16 @@ static void ironlake_crtc_enable(struct intel_crtc_state *pipe_config,
>         if (HAS_PCH_CPT(dev_priv))
>                 cpt_verify_modeset(dev, intel_crtc->pipe);
>  
> -       /* Must wait for vblank to avoid spurious PCH FIFO underruns */
> -       if (intel_crtc->config->has_pch_encoder)
> +       /*
> +        * Must wait for vblank to avoid spurious PCH FIFO underruns.
> +        * And a second vblank wait is needed at least on ILK with
> +        * some interlaced HDMI modes. Let's do the double wait always
> +        * in case there are more corner cases we don't know about.
> +        */
> +       if (intel_crtc->config->has_pch_encoder) {
> +               intel_wait_for_vblank(dev_priv, pipe);
>                 intel_wait_for_vblank(dev_priv, pipe);

The only purpose for the double wait here is for delaying the switching
on of underrun reporting, right? It doesn't accidentally fix anything
else?
-Chris