[Intel-gfx] [PATCH] Revert "drm/i915: Remove unneeded hack now for CDCLK"

[Lisovskiy, Stanislav]

On Mon, Jun 08, 2020 at 11:21:14AM +0300, Jani Nikula wrote:
> On Mon, 08 Jun 2020, Stanislav Lisovskiy <stanislav.lisovskiy at intel.com> wrote:
> > This reverts commit 82ea174dc5425d4e85e25d0c4ba961a2e494392a.
> >
> 
> Please explain why. What's going on, why we need the revert.
> 
> It's fine to reply here, the commit message can be amended by whoever
> applies the patch.

Yes, 

Unfortunately according to our recent findings there is still some
unidentified factor, requiring CDCLK to be set higher - otherwise we 
still get underruns on some multipipe configurations, despite CDCLK being set
according to BSpec formula. So getting again back into debug mode to
indentify the cause, meanwhile setting CDCLK=Pixel rate back in order
to remove regression in 10% of the cases due to FIFO underruns.

Stan 

> 
> BR,
> Jani.
> 
> 
> > Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy at intel.com>
> > Fixes: cd1915460861 ("drm/i915: Adjust CDCLK accordingly to our DBuf bw needs")
> > ---
> >  drivers/gpu/drm/i915/display/intel_cdclk.c | 12 ++++++++++++
> >  1 file changed, 12 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/i915/display/intel_cdclk.c b/drivers/gpu/drm/i915/display/intel_cdclk.c
> > index 08468b121d02..45f7f33d1144 100644
> > --- a/drivers/gpu/drm/i915/display/intel_cdclk.c
> > +++ b/drivers/gpu/drm/i915/display/intel_cdclk.c
> > @@ -2071,6 +2071,18 @@ int intel_crtc_compute_min_cdclk(const struct intel_crtc_state *crtc_state)
> >  	/* Account for additional needs from the planes */
> >  	min_cdclk = max(intel_planes_min_cdclk(crtc_state), min_cdclk);
> >  
> > +	/*
> > +	 * HACK. Currently for TGL platforms we calculate
> > +	 * min_cdclk initially based on pixel_rate divided
> > +	 * by 2, accounting for also plane requirements,
> > +	 * however in some cases the lowest possible CDCLK
> > +	 * doesn't work and causing the underruns.
> > +	 * Explicitly stating here that this seems to be currently
> > +	 * rather a Hack, than final solution.
> > +	 */
> > +	if (IS_TIGERLAKE(dev_priv))
> > +		min_cdclk = max(min_cdclk, (int)crtc_state->pixel_rate);
> > +
> >  	if (min_cdclk > dev_priv->max_cdclk_freq) {
> >  		drm_dbg_kms(&dev_priv->drm,
> >  			    "required cdclk (%d kHz) exceeds max (%d kHz)\n",
> 
> -- 
> Jani Nikula, Intel Open Source Graphics Center