[Intel-gfx] [PATCH 2/2] drm/i915: Suppress underruns during DP link retraining

[Chris Wilson]

On Fri, Oct 14, 2016 at 08:02:54PM +0300, ville.syrjala at linux.intel.com wrote:
> From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> 
> DP link retraining causes (spurious?) underruns. We can't really avoid
> them, except perhaps by doing a full modeset (which has its own underrun
> suppression anyway). So let's just hide them.
> 
> MST still has its own logic for retrainin, but a bigger hpd handling
> cleanup/unification is needed there anyway, so let's leave that be for now.
> 
> References: https://bugs.freedesktop.org/show_bug.cgi?id=98251
> Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
> ---
>  drivers/gpu/drm/i915/intel_dp.c | 29 +++++++++++++++++++++++++++--
>  1 file changed, 27 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> index bc03f61d94f1..780691a34133 100644
> --- a/drivers/gpu/drm/i915/intel_dp.c
> +++ b/drivers/gpu/drm/i915/intel_dp.c
> @@ -3988,6 +3988,31 @@ go_again:
>  }
>  
>  static void
> +intel_dp_retrain_link(struct intel_dp *intel_dp)
> +{
> +	struct intel_encoder *encoder = &dp_to_dig_port(intel_dp)->base;
> +	struct drm_i915_private *dev_priv = to_i915(encoder->base.dev);
> +	struct intel_crtc *crtc = to_intel_crtc(encoder->base.crtc);
> +
> +	/* Suppress underruns caused by re-training */
> +	intel_set_cpu_fifo_underrun_reporting(dev_priv, crtc->pipe, false);
> +	if (crtc->config->has_pch_encoder)
> +		intel_set_pch_fifo_underrun_reporting(dev_priv,
> +						      intel_crtc_pch_transcoder(crtc), false);
> +
> +	intel_dp_start_link_train(intel_dp);
> +	intel_dp_stop_link_train(intel_dp);
> +
> +	/* Keep underrun reporting disabled until things are stable */
> +	intel_wait_for_vblank(&dev_priv->drm, crtc->pipe);

I panicked over the irqreturn for hpd_pulse. That's a scary way of
saying the return type is boolean.

> +	intel_set_cpu_fifo_underrun_reporting(dev_priv, crtc->pipe, true);
> +	if (crtc->config->has_pch_encoder)
> +		intel_set_pch_fifo_underrun_reporting(dev_priv,
> +						      intel_crtc_pch_transcoder(crtc), true);

These are becoming quite popular. Is has_pch_encoder generic enough for
all crtc?

intel_crtc_suppress_underruns(struct intel_crtc *crtc, bool state)
{
	struct drm_i915_private *dev_priv = to_i915(crtc->base.dev);

	if (!state)
		intel_wait_for_vblank(&dev_priv->drm, crtc->pipe);

	intel_set_cpu_fifo_underrun_reporting(dev_priv, crtc->pipe, state);
	if (crtc->config->has_pch_encoder)
		intel_set_pch_fifo_underrun_reporting(dev_priv,
						      intel_crtc_pch_transcoder(crtc),
						      state);
}

If this was igt, we would be writing all kinds of silly

#define intel_crtc_suppress_underruns(crtc, state) \
	for (state = true; __intel_crtc_suppress_underruns(crtc, state); state = false)


intel_crtc_suppress_underruns(crtc, state) {
	intel_dp_start_link_train(intel_dp);
	intel_dp_stop_link_train(intel_dp);
}

Other than bikeshedding to see if we can trim down the profileration,
this looks inline with the suppression everywhere else, so
Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>

Main concern was over the vblank wait making short/long pulse handling
slower, but the code looks like it will survive - and the only
noticeable artifacts would be if link training failed anyway with the
small delay before recovery.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre