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.pandi...@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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