Re: [PATCH v2 2/2] drm/i915: Implement cdclk restrictions based on Azalia BCLK

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Wed, Mar 29, 2017 at 09:16:36PM +0000, Pandiyan, Dhinakaran wrote:
> On Wed, 2017-03-29 at 11:50 +0300, Ville Syrjälä wrote:
> > On Tue, Mar 07, 2017 at 04:12:52PM -0800, Dhinakaran Pandiyan wrote:
> > > According to BSpec, "The CD clock frequency must be at least twice the
> > > frequency of the Azalia BCLK." and BCLK is configured to 96 MHz by
> > > default. This check is needed because BXT and GLK support cdclk
> > > frequencies less than 192 MHz.
> > > 
> > > Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
> > > ---
> > >  drivers/gpu/drm/i915/intel_cdclk.c | 12 ++++++++++++
> > >  1 file changed, 12 insertions(+)
> > > 
> > > diff --git a/drivers/gpu/drm/i915/intel_cdclk.c b/drivers/gpu/drm/i915/intel_cdclk.c
> > > index e8c1181..7b1ac1d 100644
> > > --- a/drivers/gpu/drm/i915/intel_cdclk.c
> > > +++ b/drivers/gpu/drm/i915/intel_cdclk.c
> > > @@ -1458,6 +1458,18 @@ static int bdw_adjust_min_pipe_pixel_rate(struct intel_crtc_state *crtc_state,
> > >  			pixel_rate = max(432000, pixel_rate);
> > >  	}
> > >  
> > > +	/* According to BSpec, "The CD clock frequency must be at least twice
> > > +	 * the frequency of the Azalia BCLK." and BCLK is 96 MHz by default.
> > > +	 * The check for GLK has to be adjusted as the platform can output
> > > +	 * two pixels per clock.
> > > +	 */
> > > +	if (crtc_state->has_audio) {
> > > +		if (IS_GEMINILAKE(dev_priv))
> > > +			pixel_rate = max(2 * 2 * 96000, pixel_rate);
> > 
> > BTW that x2 factor for GLK looks wrong. It should be /2.
> > 
> > https://bugs.freedesktop.org/show_bug.cgi?id=100439
> > 
> 
> BSpec[1] says cdclk needs to be > 2*96 MHz (azalia bclk). To set a cdclk
> > 192 MHz on GLK, we need to pass in max_pixclk = 2*192 MHz. But with a
> factor of /2, we can end up picking 158.4 MHz or 79.2 MHz for cdclk,
> which are lower than 192 MHz.
> 
> 
> 	static int glk_calc_cdclk(int max_pixclk)
> 	{
> 	        if (max_pixclk > 2 * 158400)
>         	        return 316800;
> 	        else if (max_pixclk > 2 * 79200)
>         	        return 158400;
>         	else
>         	        return 79200;
> 	}
> 
> [1] Geminilake Clocks
> "158.4 MHz CD (cannot be used when audio is enabled and Azalia BCLK is
> 96 MHz)
> 316.8 MHz CD
> 79.2 MHz CD (exclusively for resolutions up to 1080p in low power single
> pipe eDP/MIPI configurations, no audio support)"

Argh. So it's really about the cdclk freq vs. bclk. Apparently the fact
that the pipe can output two pixels per clock doesn't extend to the HDA
side. OK, then the code looks correct.

But how do we know the bclk is really 96Mhz? Or are we just making a
worst case estimate here? Base on the hda spec I think bclk should typically
be 24 MHz, but of course the hda spec is so old that it probably has
little to do with today's realities. I guess ideally we'd like the audio
driver to tell us what the frequency is, or we'd dig that up from
somewhere ourselves.

Hmm. AUD_FREQ_CNTRL seems to have something. 96 vs. 48 MHz. But I'm not sure
we could trust that at boot time. Not sure if the audio driver will make
its own choice if BCLK somewhere. Can any alsa folks help us?

> 
> 
> 
> -DK
> 
> > > +		if (IS_BROXTON(dev_priv))
> > > +			pixel_rate = max(2 * 96000, pixel_rate);
> > > +	}
> > > +
> > >  	return pixel_rate;
> > >  }
> > >  
> > > -- 
> > > 2.7.4
> > > 
> > > _______________________________________________
> > > Intel-gfx mailing list
> > > Intel-gfx@lists.freedesktop.org
> > > https://lists.freedesktop.org/mailman/listinfo/intel-gfx
> > 
> 

-- 
Ville Syrjälä
Intel OTC
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