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

[Marijn Suijten]

On 2022-10-04 17:41:07, Dmitry Baryshkov wrote:
> On Sat, 1 Oct 2022 at 23:23, Marijn Suijten
> <marijn.suijten at somainline.org> wrote:
> [..]
> > 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.

Anything is fine by me, I have no strong opinion on this and rather
leave it up to the maintainers.  However, FIELD_PREP gives us concise
`#define`s through a single `GENMASK()` carrying both the shift and
mask/field-width.
At the same time these genmask definitions map more clearly to the
layout comment right above:

	/*
	 * For DSC sink programming the RC Range parameter fields
	 * are as follows: Min_qp[15:11], max_qp[10:6], offset[5:0]
	 */

If switching to `data |=` however, I've been recommended to not use
FIELD_PREP but fulyl write out `data |= (value & MASK) << SHIFT`
instead.

Perhaps a more important question is how to apply this consistently
throughout the function?

- Marijn