[PATCH] drm/i915/dp_mst: Fix DSC input BPP computation

[Imre Deak]

On Wed, Jun 12, 2024 at 04:35:36PM +0300, Luca Coelho wrote:
> Hi Imre,
> 
> On Tue, 2024-06-11 at 18:33 +0300, Imre Deak wrote:
> > The branch or sink device decompressing a stream may have a limitation
> > on the input/uncompressed BPP, which is lower than the base line BPP
> > (determined by the sink's EDID). In some cases a stream with an input
> > BPP higher than this limit will be converted automatically by the device
> > decompressing the stream, by truncating the BPP, however in some cases
> > - seen at least in Dell dock's DP->HDMI converters - the decompression
> > will fail.
> > 
> > Fix the above by limiting the input BPP correctly. This is done already
> > correctly for SST outputs.
> > 
> > Signed-off-by: Imre Deak <imre.deak at intel.com>
> > ---
> >  drivers/gpu/drm/i915/display/intel_dp_mst.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/i915/display/intel_dp_mst.c b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> > index 00fdcbc28e9b7..15c20bedea091 100644
> > --- a/drivers/gpu/drm/i915/display/intel_dp_mst.c
> > +++ b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> > @@ -349,6 +349,8 @@ static int intel_dp_dsc_mst_compute_link_config(struct intel_encoder *encoder,
> >  	if (max_bpp > sink_max_bpp)
> >  		max_bpp = sink_max_bpp;
> >  
> > +	crtc_state->pipe_bpp = max_bpp;
> > +
> >  	max_compressed_bpp = intel_dp_dsc_sink_max_compressed_bpp(connector,
> >  								  crtc_state,
> >  								  max_bpp / 3);
> 
> Wouldn't it be better to make the assignment in
> intel_dp_dsc_sink_max_compressed_bpp(), since that function is already
> making modifications to crtc_state?

Agreed, this is less than ideal atm. The whole MST DSC state computation
would need a refactor, at least to sanitize it wrt. the MST/non-DSC and
the SST DSC counterparts (also planned by others). Hence, it made sense
to keep this fix simple.

Also, the rational I saw for this way was that the input/uncompressed
bpp (max_bpp above), which isn't relavant in the non-DSC case, is
computed in intel_dp_dsc_mst_compute_link_config() and
intel_dp_mst_find_vcpi_slots_for_bpp() would only compute the
output/compressed bpp (relevant to both DSC and non-DSC case). Imo in
the end the computation for these two cases should be separated into
their own functions, instead of passing a 'bool dsc' param to a common
function handling both cases.

> There is another caller, but I think it may benefit from the same check.

For the non-DSC case the above limit doesn't apply (at least I'm not
aware of it).

> But this is just a nitpick.  Either way, you have:
> 
> Reviewed-by: Luca Coelho <luciano.coelho at intel.com>

Thanks!

> 
> --
> Cheers,
> Luca.