[Intel-gfx] [PATCH v1] drm/i915: Clamp min_cdclk to max_cdclk_freq to unblock 8K

[Lisovskiy, Stanislav]

On Tue, Jun 30, 2020 at 07:29:09PM +0300, Ville Syrjälä wrote:
> On Tue, Jun 30, 2020 at 02:26:09PM +0300, Stanislav Lisovskiy wrote:
> > We still need "Bump up CDCLK" workaround otherwise getting
> > underruns - however currently it blocks 8K as CDCLK = Pixel rate,
> > in 8K case would require CDCLK to be around 1 Ghz which is not
> > possible.
> > 
> > Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy at intel.com>
> > ---
> >  drivers/gpu/drm/i915/display/intel_cdclk.c | 14 +++++++++++++-
> >  1 file changed, 13 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/display/intel_cdclk.c b/drivers/gpu/drm/i915/display/intel_cdclk.c
> > index 45f7f33d1144..01a5bc6b08c4 100644
> > --- a/drivers/gpu/drm/i915/display/intel_cdclk.c
> > +++ b/drivers/gpu/drm/i915/display/intel_cdclk.c
> > @@ -2080,9 +2080,21 @@ int intel_crtc_compute_min_cdclk(const struct intel_crtc_state *crtc_state)
> >  	 * Explicitly stating here that this seems to be currently
> >  	 * rather a Hack, than final solution.
> >  	 */
> > -	if (IS_TIGERLAKE(dev_priv))
> > +	if (IS_TIGERLAKE(dev_priv)) {
> >  		min_cdclk = max(min_cdclk, (int)crtc_state->pixel_rate);
> >  
> > +		/*
> > +		 * Clamp to max_cdclk_freq in order not to break an 8K,
> > +		 * but still leave W/A at place.
> > +		 */
> > +		min_cdclk = min(min_cdclk, (int)dev_priv->max_cdclk_freq);
> > +
> > +		/*
> > +		 * max_cdclk_freq check obviously not needed - just return.
> > +		 */
> > +		return min_cdclk;
> 
> Pointless return. But I think we should actually keep the max_cdclk
> check. Something like:
> 
> min_cdclk = max(min_cdclk,
> 		min(max_cdclk, pixel_rate));
> 
> Also what's with the (int) casts? There is min_t() if you
> actually need casts. But not sure why we need them though.

Don't like the casts either. Aware of min_t. Just remember that smth
prevented me from using it in initial patch. Will check once again
to figure out why I did so last time.

Regarding the check we actually you are proposing to do the same thing
as initially I check min_cdclk = max(min_cdclk, pixel_rate) and then
it gets to min_cdclk = min(min_cdclk, max_cdclk_freq). 

Just do it kind of in 2 steps in order not to change the 
original Bump CDCLK patch.

I.e min_cdclk still has to be more or equal than pixel rate, but always less
than max_cdclk_freq.

Stan

> 
> > +	}
> > +
> >  	if (min_cdclk > dev_priv->max_cdclk_freq) {
> >  		drm_dbg_kms(&dev_priv->drm,
> >  			    "required cdclk (%d kHz) exceeds max (%d kHz)\n",
> > -- 
> > 2.24.1.485.gad05a3d8e5
> 
> -- 
> Ville Syrjälä
> Intel