[PATCH] drm/i915/dp_mst: Fix dsc mst bw overhead calculation

[Kandpal, Suraj]

> -----Original Message-----
> From: Jani Nikula <jani.nikula at linux.intel.com>
> Sent: Wednesday, October 9, 2024 3:30 PM
> To: Kandpal, Suraj <suraj.kandpal at intel.com>; intel-
> xe at lists.freedesktop.org; intel-gfx at lists.freedesktop.org
> Cc: Nautiyal, Ankit K <ankit.k.nautiyal at intel.com>; Kandpal, Suraj
> <suraj.kandpal at intel.com>
> Subject: Re: [PATCH] drm/i915/dp_mst: Fix dsc mst bw overhead calculation
> 
> On Wed, 09 Oct 2024, Suraj Kandpal <suraj.kandpal at intel.com> wrote:
> > Fix the DSC flag assignment based on the dsc_slice_count returned to
> > avoid divide by zero error.
> >
> > Fixes: 4e0837a8d00a ("drm/i915/dp_mst: Account for FEC and DSC
> > overhead during BW allocation")
> > Signed-off-by: Suraj Kandpal <suraj.kandpal at intel.com>
> > ---
> >  drivers/gpu/drm/i915/display/intel_dp_mst.c | 7 ++++++-
> >  1 file changed, 6 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/i915/display/intel_dp_mst.c
> > b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> > index 4765bda154c1..bacd294d6bb6 100644
> > --- a/drivers/gpu/drm/i915/display/intel_dp_mst.c
> > +++ b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> > @@ -105,11 +105,16 @@ static int intel_dp_mst_bw_overhead(const
> struct intel_crtc_state *crtc_state,
> >  	if (dsc) {
> >  		int num_joined_pipes =
> intel_crtc_num_joined_pipes(crtc_state);
> >
> > -		flags |= DRM_DP_BW_OVERHEAD_DSC;
> >  		dsc_slice_count = intel_dp_dsc_get_slice_count(connector,
> >  							       adjusted_mode-
> >clock,
> >  							       adjusted_mode-
> >hdisplay,
> >
> num_joined_pipes);
> > +		/*
> > +		 * Try with dsc only if dsc_slice_count has a sane value i.e
> value is no
> > +		 * 0
> > +		 */
> > +		if (dsc_slice_count)
> > +			flags |= DRM_DP_BW_OVERHEAD_DSC;
> 
> Do you think that's enough to handle the error?!

Well this will make sure that if dsc_slice_count ends up being zero we don't take dsc overhead into account.
Which should be enough to make sure we don't go and end up having a divide by zero error

Regards,
Suraj Kandpal
> 
> BR,
> Jani.
> 
> >  	}
> >
> >  	overhead = drm_dp_bw_overhead(crtc_state->lane_count,
> 
> --
> Jani Nikula, Intel