[PATCH 5/5] drm/dsc: Prevent negative BPG offsets from shadowing adjacent bitfields

[Marijn Suijten]

On 2022-10-04 13:22:25, Abhinav Kumar wrote:
> 
> On 10/1/2022 12:08 PM, Marijn Suijten wrote:
> > msm's dsi_host specifies negative BPG offsets which fill the full 8 bits
> > of a char thanks to two's complement: this however results in those bits
> > bleeding into the next parameter when the field is only expected to
> > contain 6-bit wide values.
> > As a consequence random slices appear corrupted on-screen (tested on a
> > Sony Tama Akatsuki device with sdm845).
> > 
> > Use AND operators to limit all values that constitute the RC Range
> > parameter fields to their expected size.
> > 
> > Fixes: b9080324d6ca ("drm/msm/dsi: add support for dsc data")
> > Signed-off-by: Marijn Suijten <marijn.suijten at somainline.org>
> > ---
> >   drivers/gpu/drm/display/drm_dsc_helper.c | 6 +++---
> >   1 file changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/display/drm_dsc_helper.c b/drivers/gpu/drm/display/drm_dsc_helper.c
> > index c869c6e51e2b..2e7ef242685d 100644
> > --- a/drivers/gpu/drm/display/drm_dsc_helper.c
> > +++ b/drivers/gpu/drm/display/drm_dsc_helper.c
> > @@ -243,11 +243,11 @@ void drm_dsc_pps_payload_pack(struct drm_dsc_picture_parameter_set *pps_payload,
> >   	 */
> >   	for (i = 0; i < DSC_NUM_BUF_RANGES; i++) {
> >   		pps_payload->rc_range_parameters[i] =
> > -			cpu_to_be16((dsc_cfg->rc_range_params[i].range_min_qp <<
> > +			cpu_to_be16(((dsc_cfg->rc_range_params[i].range_min_qp & 0x1f) <<
> >   				     DSC_PPS_RC_RANGE_MINQP_SHIFT) |
> > -				    (dsc_cfg->rc_range_params[i].range_max_qp <<
> > +				    ((dsc_cfg->rc_range_params[i].range_max_qp & 0x1f) <<
> >   				     DSC_PPS_RC_RANGE_MAXQP_SHIFT) |
> > -				    (dsc_cfg->rc_range_params[i].range_bpg_offset));
> > +				    (dsc_cfg->rc_range_params[i].range_bpg_offset & 0x3f));
> >   	}
> >   
> 
> Looking at some examples of this for other vendors, they have managed to 
> limit the value to 6 bits in their drivers:
> 
> https://gitlab.freedesktop.org/drm/msm/-/blob/msm-next/drivers/gpu/drm/i915/display/intel_vdsc.c#L532
> 
> https://gitlab.freedesktop.org/drm/msm/-/blob/msm-next/drivers/gpu/drm/amd/display/dc/dsc/rc_calc_dpi.c#L87
> 
> Perhaps, msm should do the same thing instead of the helper change.

Thanks, I should have done my due-diligence and look up how other
drivers dealt with this, but wasn't immediately expecting negative
values elsewhere.

Alas, as explained in the cover letter I opted to perform the masking in
the PPS packing code as the DSC block code also reads these values, and
would suddenly write 6-bit intead of 8-bit values to the
DSC_RANGE_BPG_OFFSET registers.  Quick testing on the mentioned sdm845
platform shows no regressions, but I'm not sure if that's safe to rely
on?

> If you want to move to helper, other drivers need to be changed too to 
> remove duplicate & 0x3f.

Sure, we only have to confirm whether those drivers also read back the
value(s) in rc_range_params, and expect / allow this to be 8 instead of
6 bits.

> FWIW, this too has already been fixed in the latest downstream driver too.

What is this supposed to mean?  Is there a downstream DPU project that
has pending patches needing to be upstreamed?  Or is the downstream SDE,
techpack/display, or whatever it is called nowadays, slowly using more
DRM structs like drm_dsc_config and this drm_dsc_pps_payload_pack()
helper function as pointed out in an earlier mail?

Offtopic: are SDE and DPU growing closer together, hopefully achieving
feature parity allowing the SDE project to be dropped in favour of a
fully upstreamed DPU driver for day-one out-of-the-box mainline support
for new SoCs (as long as work is published and on its way upstream)?

- Marijn