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

[Abhinav Kumar]

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.

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

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


Thanks

Abhinav

>   	/* PPS 88 */