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

[Daniel Vetter <daniel@ffwll.ch>]

On Fri, Oct 14, 2016 at 07:27:39PM +0000, Pandiyan, Dhinakaran wrote:
> 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@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.

Please also check docs for hsw, that's where dp mst support started.
-Daniel

> 
> > > +
> > > +	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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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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