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

[Dmitry Baryshkov]

On Sat, 1 Oct 2022 at 23:23, Marijn Suijten
<marijn.suijten at somainline.org> wrote:
>
> On 2022-10-01 21:08:07, 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));
>
> Pre-empting the reviews: I was contemplating whether to use FIELD_PREP
> here, given that it's not yet used anywhere else in this file.  For that
> I'd remove the existing _SHIFT definitions and replace them with:
>
>         #define DSC_PPS_RC_RANGE_MINQP_MASK             GENMASK(15, 11)
>         #define DSC_PPS_RC_RANGE_MAXQP_MASK             GENMASK(10, 6)
>         #define DSC_PPS_RC_RANGE_BPG_OFFSET_MASK        GENMASK(5, 0)
>
> And turn this section of code into:
>
>         cpu_to_be16(FIELD_PREP(DSC_PPS_RC_RANGE_MINQP_MASK,
>                                dsc_cfg->rc_range_params[i].range_min_qp) |
>                     FIELD_PREP(DSC_PPS_RC_RANGE_MAXQP_MASK,
>                                dsc_cfg->rc_range_params[i].range_max_qp) |
>                     FIELD_PREP(DSC_PPS_RC_RANGE_BPG_OFFSET_MASK,
>                                dsc_cfg->rc_range_params[i].range_bpg_offset));
>
> Is that okay/recommended?

This is definitely easier to review. However if you do not want to use
FIELD_PREP, it would be better to split this into a series of `data |=
something` assignments terminated with the rc_range_parameters[i]
assignment.

>
> - Marijn
>
> >       }
> >
> >       /* PPS 88 */
> > --
> > 2.37.3
> >



-- 
With best wishes
Dmitry