Re: [PATCH v2 02/12] drm/i915/dp_mst: Simplify handling the single-bpp case during state computation

[Imre Deak <imre.deak@intel.com>]

On Tue, May 06, 2025 at 03:21:13PM +0530, Nautiyal, Ankit K wrote:
> 
> On 4/28/2025 7:01 PM, Imre Deak wrote:
> > A follow-up change wants to skip invalid bpps in the bpp select loop of
> > a stream state computation. To allow for that, using the usual
> > 'continue' statement in the loop, change the way the sinlge-bpp range is
> 
> 
> s/sinlge/single
> 
> 
> > handled.
> > 
> > Cc: Jani Nikula <jani.nikula@intel.com>
> > Signed-off-by: Imre Deak <imre.deak@intel.com>
> > ---
> >   drivers/gpu/drm/i915/display/intel_dp_mst.c | 10 ++++++----
> >   1 file changed, 6 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/display/intel_dp_mst.c b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> > index 49b836cd8816c..d8033e55dc093 100644
> > --- a/drivers/gpu/drm/i915/display/intel_dp_mst.c
> > +++ b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> > @@ -283,6 +283,12 @@ int intel_dp_mtp_tu_compute_config(struct intel_dp *intel_dp,
> >   					   fxp_q4_to_frac(max_bpp_x16) ||
> >   					   fxp_q4_to_frac(bpp_step_x16)));
> > +	if (!bpp_step_x16) {
> > +		/* Allow using zero step only to indicate single try for a given bpp. */
> > +		drm_WARN_ON(display->drm, min_bpp_x16 != max_bpp_x16);
> > +		bpp_step_x16 = 1;
> > +	}
> 
> I can see this function is called for non-dsc uhbr case with bpp_step_x16 =0
> and dsc false.
> 
> So we just want to get the loop run once and get the slots with min_bpp_x16
> == max_bpp_x16 == fxp_q4_from_int(pipe_bpp) right?
> 
> Similarly for dsc uhbr case check with a given compressed bpp dsc_bpp_x16 if
> its valid.
> 
> bpp_step_x16 = 1 will mean 1/16th fraction. The value itself won't have any
> effect for the case where min_bpp_x16 == max_bpp_x16.
> 
> Is my understanding correct here?

Yes, correct. On SST UHBR for each bpp a minimal link config is
selected, so this function - checking if the link config is valid - is
called for a single bpp only, hence min_bpp_x16 == max_bpp_x16.

> Regards,
> 
> Ankit
> 
> 
> 
> 
> 
> > +
> >   	if (is_mst) {
> >   		mst_state = drm_atomic_get_mst_topology_state(state, &intel_dp->mst.mgr);
> >   		if (IS_ERR(mst_state))
> > @@ -408,10 +414,6 @@ int intel_dp_mtp_tu_compute_config(struct intel_dp *intel_dp,
> >   			break;
> >   		}
> > -
> > -		/* Allow using zero step to indicate one try */
> > -		if (!bpp_step_x16)
> > -			break;
> >   	}
> >   	if (slots < 0) {