[Intel-gfx] [PATCH] drm/i915: Don't rely that 2 VDSC engines are always enough for pixel rate

[Lisovskiy, Stanislav]

On Mon, Jul 03, 2023 at 10:23:00AM +0530, Nautiyal, Ankit K wrote:
> 
> On 6/28/2023 3:38 PM, Stanislav Lisovskiy wrote:
> > We are currently having FIFO underruns happening for kms_dsc test case,
> > problem is that, we check if curreny cdclk is >= pixel rate only if
> > there is a single VDSC engine enabled(i.e dsc_split=false) however if
> > we happen to have 2 VDSC engines enabled, we just kinda rely that this
> > would be automatically enough.
> > However pixel rate can be even >= than VDSC clock(cdclk) * 2, so in that
> > case even with 2 VDSC engines enabled, we still need to tweak it up.
> > So lets compare pixel rate with cdclk * slice count(VDSC engine count) and
> 
> Is it not that we use slice count for the number of DSC slices in which the
> horizontal scanline count is divided. So this can be 1,2, 4.
> 
> Whereas VDSC engine count is the number of VDSC engines the stream is
> splitted.
> 
> IIUC for a case where number of horizontal DSC slices is 4 and we use 2 VDSC
> engines, each VDSC engine will get two slices and slice width will be
> HACTIVE/4.
> 
> Perhaps what we want to do is to compare pixel rate with cdclk * (number of
> vdsc engine count = dsc_split ? 2 : 1)

Yes, we of course need amount of DSC engines here, however I was wondering is there
any other way to get amount of VDSC engines used more precisely, except just assuming
"2" if dsc_split is set to true?

As I understand amount of slices will always be >= amount of VDSC engines, however of course
if we will have 2 slices for each VDSC engines - that would be too optimistic.
However I just really don't want to hardcode "2" here.
Need to check if there is any other way..

Stan

> 
> Regards,
> 
> Ankit
> 
> 
> > check if it still requires bumping up.
> > Previously we had to bump up CDCLK many times for similar reasons.
> > 
> > Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy at intel.com>
> > ---
> >   drivers/gpu/drm/i915/display/intel_cdclk.c | 9 +++++++--
> >   1 file changed, 7 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/display/intel_cdclk.c b/drivers/gpu/drm/i915/display/intel_cdclk.c
> > index 4207863b7b2a..5880dcb11588 100644
> > --- a/drivers/gpu/drm/i915/display/intel_cdclk.c
> > +++ b/drivers/gpu/drm/i915/display/intel_cdclk.c
> > @@ -2607,9 +2607,14 @@ int intel_crtc_compute_min_cdclk(const struct intel_crtc_state *crtc_state)
> >   	 * When we decide to use only one VDSC engine, since
> >   	 * each VDSC operates with 1 ppc throughput, pixel clock
> >   	 * cannot be higher than the VDSC clock (cdclk)
> > +	 * If there 2 VDSC engines, then pixel clock can't be higher than
> > +	 * VDSC clock(cdclk) * 2. However even that can still be not enough.
> > +	 * Slice count reflects amount of VDSC engines,
> > +	 * so lets use that to determine, if need still need to tweak CDCLK higher.
> >   	 */
> > -	if (crtc_state->dsc.compression_enable && !crtc_state->dsc.dsc_split)
> > -		min_cdclk = max(min_cdclk, (int)crtc_state->pixel_rate);
> > +	if (crtc_state->dsc.compression_enable)
> > +		min_cdclk = max_t(int, min_cdclk * crtc_state->dsc.slice_count,
> > +			          crtc_state->pixel_rate);
> >   	/*
> >   	 * HACK. Currently for TGL/DG2 platforms we calculate