[PATCH] drm: Fix DSC BPP increment decoding

[Jani Nikula]

On Wed, 12 Feb 2025, Imre Deak <imre.deak at intel.com> wrote:
> Starting with DPCD version 2.0 bits 6:3 of the DP_DSC_BITS_PER_PIXEL_INC
> DPCD register contains the NativeYCbCr422_MAX_bpp_DELTA field, which can
> be non-zero as opposed to earlier DPCD versions, hence decoding the
> bit_per_pixel increment value at bits 2:0 in the same register requires
> applying a mask, do so.
>
> Cc: Ankit Nautiyal <ankit.k.nautiyal at intel.com>
> Fixes: 0c2287c96521 ("drm/display/dp: Add helper function to get DSC bpp precision")
> Signed-off-by: Imre Deak <imre.deak at intel.com>

Reviewed-by: Jani Nikula <jani.nikula at intel.com>

But we should really clean up the macros:

#define DP_DSC_BITS_PER_PIXEL_INC           0x06F
# define DP_DSC_RGB_YCbCr444_MAX_BPP_DELTA_MASK 0x1f
# define DP_DSC_RGB_YCbCr420_MAX_BPP_DELTA_MASK 0xe0

These are both for DPCD 0x6e, not 0x6f. They're misleading here. And
they should contain the /* DP 2.0 */ comment.

And a similar macro for 0x6f bits 6:3 could be added.

BR,
Jani.


> ---
>  drivers/gpu/drm/display/drm_dp_helper.c | 2 +-
>  include/drm/display/drm_dp.h            | 1 +
>  2 files changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/display/drm_dp_helper.c b/drivers/gpu/drm/display/drm_dp_helper.c
> index c488d160a3c1f..f5c596234729d 100644
> --- a/drivers/gpu/drm/display/drm_dp_helper.c
> +++ b/drivers/gpu/drm/display/drm_dp_helper.c
> @@ -2602,7 +2602,7 @@ u8 drm_dp_dsc_sink_bpp_incr(const u8 dsc_dpcd[DP_DSC_RECEIVER_CAP_SIZE])
>  {
>  	u8 bpp_increment_dpcd = dsc_dpcd[DP_DSC_BITS_PER_PIXEL_INC - DP_DSC_SUPPORT];
>  
> -	switch (bpp_increment_dpcd) {
> +	switch (bpp_increment_dpcd & DP_DSC_BITS_PER_PIXEL_MASK) {
>  	case DP_DSC_BITS_PER_PIXEL_1_16:
>  		return 16;
>  	case DP_DSC_BITS_PER_PIXEL_1_8:
> diff --git a/include/drm/display/drm_dp.h b/include/drm/display/drm_dp.h
> index 784a32bfbad8f..c413ef68f9a30 100644
> --- a/include/drm/display/drm_dp.h
> +++ b/include/drm/display/drm_dp.h
> @@ -359,6 +359,7 @@
>  # define DP_DSC_BITS_PER_PIXEL_1_4          0x2
>  # define DP_DSC_BITS_PER_PIXEL_1_2          0x3
>  # define DP_DSC_BITS_PER_PIXEL_1_1          0x4
> +# define DP_DSC_BITS_PER_PIXEL_MASK         0x7
>  
>  #define DP_PSR_SUPPORT                      0x070   /* XXX 1.2? */
>  # define DP_PSR_IS_SUPPORTED                1

-- 
Jani Nikula, Intel