[Intel-gfx] [PATCH] drm/i915/dp: Increase cdclk when DP audio is enabled with 4 lanes and HBR2

[Pandiyan, Dhinakaran]

On Thu, 2016-10-13 at 11:30 -0700, Jim Bride wrote:
> On Thu, Oct 13, 2016 at 11:04:19AM -0700, Dhinakaran Pandiyan wrote:
> > According to BSpec, cdclk has to be not less than 432 MHz with DP audio
> > enabled, port width x4, and link rate HBR2 (5.4 GHz)
> > 
> > Having a lower cdclk triggers pipe underruns, which then lead to displays
> > continuously cycling off and on. This is essential for DP MST audio as the
> > link is trained at HBR2 and 4 lanes by default.
> > 
> > This should fix https://bugs.freedesktop.org/show_bug.cgi?id=97907
> > 
> > Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan at intel.com>
> > ---
> >  drivers/gpu/drm/i915/intel_display.c | 47 +++++++++++++++++++++++++++++++++---
> >  1 file changed, 43 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> > index cfcb03f..6a05183 100644
> > --- a/drivers/gpu/drm/i915/intel_display.c
> > +++ b/drivers/gpu/drm/i915/intel_display.c
> > @@ -6603,14 +6603,43 @@ static int valleyview_modeset_calc_cdclk(struct drm_atomic_state *state)
> >  	return 0;
> >  }
> >  
> > +static bool cdclk_min_for_dp_audio(struct drm_atomic_state *state)
> > +{
> > +
> > +	struct drm_crtc_state *crtc_state;
> > +	struct drm_crtc *crtc;
> > +	int i;
> > +
> > +	/* BSpec says "Do not use DisplayPort with CDCLK less than 432 MHz,
> > +	 * audio enabled, port width x4, and link rate HBR2 (5.4 GHz), or else
> > +	 * there may be audio corruption or screen corruption."
> > +	 */
> 
> It's probably worth mentioning that this limitation really only applies for
> SKL (at least that I can see) and BXT A (which I don't think we even care
> about anymore since it's not a production part.)  Were we seeing underruns
> on non-SKL platforms that increasing CDCLK fixed?   In any event, I'd rather
> see this only implemented for SKL (see BDW code below) unless we've empirically
> noticed a similar problem on BDW (or the B-Spec says something about this and I
> missed it.)
> 
> Jim
> 
> 
Yes, Libin saw the problem when testing a BDW NUC. And, the patch is
applicable to BDW according to the BSpec. I will try to get a
confirmation on whether this is needed BXT production parts.

> > +
> > +	for_each_crtc_in_state(state, crtc, crtc_state, i) {
> > +		struct intel_crtc_state *pipe_config =
> > +			to_intel_crtc_state(crtc_state);
> > +
> > +		return (intel_crtc_has_dp_encoder(pipe_config) &&
> > +			pipe_config->has_audio &&
> > +			pipe_config->port_clock == 540000 &&
> > +			pipe_config->lane_count == 4);
> > +	}
> > +
> > +	return false;
> > +}
> > +
> >  static int bxt_modeset_calc_cdclk(struct drm_atomic_state *state)
> >  {
> >  	int max_pixclk = ilk_max_pixel_rate(state);
> > +	int cdclk, min_cdclk = 0;
> >  	struct intel_atomic_state *intel_state =
> >  		to_intel_atomic_state(state);
> >  
> > -	intel_state->cdclk = intel_state->dev_cdclk =
> > -		bxt_calc_cdclk(max_pixclk);
> > +	if (cdclk_min_for_dp_audio(state))
> > +		min_cdclk = bxt_calc_cdclk(432000);
> > +
> > +	cdclk = bxt_calc_cdclk(max_pixclk);
> > +	intel_state->cdclk = intel_state->dev_cdclk = max(min_cdclk, cdclk);
> >  
> >  	if (!intel_state->active_crtcs)
> >  		intel_state->dev_cdclk = bxt_calc_cdclk(0);
> > @@ -10374,7 +10403,10 @@ static int broadwell_modeset_calc_cdclk(struct drm_atomic_state *state)
> >  	struct drm_i915_private *dev_priv = to_i915(state->dev);
> >  	struct intel_atomic_state *intel_state = to_intel_atomic_state(state);
> >  	int max_pixclk = ilk_max_pixel_rate(state);
> > -	int cdclk;
> > +	int cdclk, min_cdclk = 0;
> > +
> > +	if (cdclk_min_for_dp_audio(state))
> > +		min_cdclk = broadwell_calc_cdclk(432000);
> >  
> >  	/*
> >  	 * FIXME should also account for plane ratio
> > @@ -10382,6 +10414,8 @@ static int broadwell_modeset_calc_cdclk(struct drm_atomic_state *state)
> >  	 */
> >  	cdclk = broadwell_calc_cdclk(max_pixclk);
> >  
> > +	cdclk =	max(min_cdclk, cdclk);
> > +
> >  	if (cdclk > dev_priv->max_cdclk_freq) {
> >  		DRM_DEBUG_KMS("requested cdclk (%d kHz) exceeds max (%d kHz)\n",
> >  			      cdclk, dev_priv->max_cdclk_freq);
> > @@ -10411,7 +10445,10 @@ static int skl_modeset_calc_cdclk(struct drm_atomic_state *state)
> >  	struct drm_i915_private *dev_priv = to_i915(state->dev);
> >  	const int max_pixclk = ilk_max_pixel_rate(state);
> >  	int vco = intel_state->cdclk_pll_vco;
> > -	int cdclk;
> > +	int cdclk, min_cdclk = 0;
> > +
> > +	if (cdclk_min_for_dp_audio(state))
> > +		min_cdclk = skl_calc_cdclk(432000, vco);
> >  
> >  	/*
> >  	 * FIXME should also account for plane ratio
> > @@ -10419,6 +10456,8 @@ static int skl_modeset_calc_cdclk(struct drm_atomic_state *state)
> >  	 */
> >  	cdclk = skl_calc_cdclk(max_pixclk, vco);
> >  
> > +	cdclk = max(min_cdclk, cdclk);
> > +
> >  	/*
> >  	 * FIXME move the cdclk caclulation to
> >  	 * compute_config() so we can fail gracegully.
> > -- 
> > 2.7.4
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