[Intel-gfx] [PATCH] drm/i915/glk: limit pixel clock to 99% of cdclk workaround

[Chauhan, Madhav]

> -----Original Message-----
> From: Nikula, Jani
> Sent: Tuesday, April 4, 2017 3:48 PM
> To: Ander Conselvan De Oliveira <conselvan2 at gmail.com>; intel-
> gfx at lists.freedesktop.org
> Cc: Chauhan, Madhav <madhav.chauhan at intel.com>; Ville Syrjälä
> <ville.syrjala at linux.intel.com>
> Subject: Re: [PATCH] drm/i915/glk: limit pixel clock to 99% of cdclk
> workaround
> 
> On Tue, 04 Apr 2017, Ander Conselvan De Oliveira <conselvan2 at gmail.com>
> wrote:
> > On Tue, 2017-04-04 at 11:40 +0300, Jani Nikula wrote:
> >> On Tue, 04 Apr 2017, Ander Conselvan De Oliveira
> <conselvan2 at gmail.com> wrote:
> >> > On Tue, 2017-04-04 at 11:15 +0300, Jani Nikula wrote:
> >> > > From: Madhav Chauhan <madhav.chauhan at intel.com>
> >> > >
> >> > > As per BSPEC, valid cdclk values for glk are 79.2, 158.4, 316.8 Mhz.
> >> > > Practically we can achive only 99% of these cdclk values (HW team
> >> > > checking on this). So cdclk should be calculated for the given
> >> > > pixclk as per that otherwise it may lead to screen corruption for some
> scenarios.
> >> > >
> >> > > v2: Rebased to new CDLCK code framework
> >> > > v3: Addressed review comments from Ander/Jani
> >> > >     - Add comment in code about 99% usage of CDCLK
> >> > >     - Calculate max dot clock as well with 99% limit
> >> > > v4 by Jani:
> >> > >     - drop superfluous whitespace change
> >> > >     - rewrite code comments to clarify
> >> > >
> >> > > Cc: Ander Conselvan de Oliveira
> >> > > <ander.conselvan.de.oliveira at intel.com>
> >> > > Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
> >> > > Signed-off-by: Madhav Chauhan <madhav.chauhan at intel.com>
> >> > > Signed-off-by: Jani Nikula <jani.nikula at intel.com>
> >> > > ---
> >> > >  drivers/gpu/drm/i915/intel_cdclk.c | 16 +++++++++++++---
> >> > >  1 file changed, 13 insertions(+), 3 deletions(-)
> >> > >
> >> > > diff --git a/drivers/gpu/drm/i915/intel_cdclk.c
> >> > > b/drivers/gpu/drm/i915/intel_cdclk.c
> >> > > index dd3ad52b7dfe..763010f8ad89 100644
> >> > > --- a/drivers/gpu/drm/i915/intel_cdclk.c
> >> > > +++ b/drivers/gpu/drm/i915/intel_cdclk.c
> >> > > @@ -1071,9 +1071,15 @@ static int bxt_calc_cdclk(int max_pixclk)
> >> > >
> >> > >  static int glk_calc_cdclk(int max_pixclk)  {
> >> > > -	if (max_pixclk > 2 * 158400)
> >> > > +	/*
> >> > > +	 * FIXME: Avoid using a pixel clock that is more than 99% of the cdclk
> >> > > +	 * as a temporary workaround. Use a higher cdclk instead. (Note
> >> > > +that
> >> >
> >> > Temporary workaround for what? Neither the comment nor the commit
> >> > message explicitly lists the scenario that triggers this issue.
> >>
> >> Frankly I did not know what to write.
> >
> >
> >> There are issues with pixel clocks
> >> near cdclk that shouldn't happen. People are looking into this, but
> >> in the mean time dodge the issues by using higher cdclk. The issue
> >> should not be encoder specific, but in practice this has only been
> >> seen with DSI because there were some modes with pixel clocks that
> >> are near the cdclk.
> >
> > The above plus the model number of the DSI panel with which the issue
> > has been seen would be good enough IMO.
> 
> Madhav?

Issue is generic (valid for DP/HDMI) not just MIPI DSI specific. Particular DSI panel 
exposing this issue as described below:
1. For XXXX panel (19X12) required pixclk is 157100 KHZ
2. glk_calc_cdclk returns 79200 KHZ for this pixclk. For 2PPC it will be 158400 KHZ
3. Practically 100% of the cdclk can’t be achieved, so 99% of 158400 KHZ  = 156816 which is less than the desired pixlclk.
     This causes the corruption. 

So why do we need to mention a particular panel??

> 
> 
> >
> > Ander
> >
> >
> >> > With that fixed,
> >> >
> >> > Reviewed-by: Ander Conselvan de Oliveira <conselvan2 at gmail.com>
> >> >
> >> > > +	 * intel_compute_max_dotclk() limits the max pixel clock to 99% of
> max
> >> > > +	 * cdclk.)
> >> > > +	 */
> >> > > +	if (max_pixclk > DIV_ROUND_UP(2 * 158400 * 99, 100))
> >> > >  		return 316800;
> >> > > -	else if (max_pixclk > 2 * 79200)
> >> > > +	else if (max_pixclk > DIV_ROUND_UP(2 * 79200 * 99, 100))
> >> > >  		return 158400;
> >> > >  	else
> >> > >  		return 79200;
> >> > > @@ -1664,7 +1670,11 @@ static int intel_compute_max_dotclk(struct
> drm_i915_private *dev_priv)
> >> > >  	int max_cdclk_freq = dev_priv->max_cdclk_freq;
> >> > >
> >> > >  	if (IS_GEMINILAKE(dev_priv))
> >> > > -		return 2 * max_cdclk_freq;
> >> > > +		/*
> >> > > +		 * FIXME: Limiting to 99% as a temporary workaround. See
> >> > > +		 * glk_calc_cdclk() for details.
> >> > > +		 */
> >> > > +		return 2 * max_cdclk_freq * 99 / 100;
> >> > >  	else if (INTEL_INFO(dev_priv)->gen >= 9 ||
> >> > >  		 IS_HASWELL(dev_priv) || IS_BROADWELL(dev_priv))
> >> > >  		return max_cdclk_freq;
> >>
> >>
> 
> --
> Jani Nikula, Intel Open Source Technology Center