[PATCH v6 4/7] drm/msm/dpu: Fix slice_last_group_size calculation

[Marijn Suijten]

On 2023-04-12 16:25:18, Jessica Zhang wrote:
> Correct the math for slice_last_group_size so that it matches the
> calculations downstream.
> 
> Changes in v3:
> - Reworded slice_last_group_size calculation to
>   `(dsc->slice_width + 2) % 3`
> 
> Fixes: c110cfd1753e ("drm/msm/disp/dpu1: Add support for DSC")
> Signed-off-by: Jessica Zhang <quic_jesszhan at quicinc.com>
> Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov at linaro.org>
> ---
>  drivers/gpu/drm/msm/disp/dpu1/dpu_hw_dsc.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/msm/disp/dpu1/dpu_hw_dsc.c b/drivers/gpu/drm/msm/disp/dpu1/dpu_hw_dsc.c
> index b952f7d2b7f5..ff1c8f92fb20 100644
> --- a/drivers/gpu/drm/msm/disp/dpu1/dpu_hw_dsc.c
> +++ b/drivers/gpu/drm/msm/disp/dpu1/dpu_hw_dsc.c
> @@ -56,9 +56,10 @@ static void dpu_hw_dsc_config(struct dpu_hw_dsc *hw_dsc,
>  	if (is_cmd_mode)
>  		initial_lines += 1;
>  
> -	slice_last_group_size = 3 - (dsc->slice_width % 3);
> +	slice_last_group_size = (dsc->slice_width + 2) % 3;
> +
>  	data = (initial_lines << 20);
> -	data |= ((slice_last_group_size - 1) << 18);
> +	data |= (slice_last_group_size << 18);

Agreed, this matches the calculation found in newer downstream.  This
older calculation is only present in an older fbdev driver, and was
working for the only panel I was able to test because the new and the
old calculation result in the same value:

    3 - (720 % 3) - 1 = 2
    (720 + 2) % 3 = 2

The other two outcomes are flipped, and match the downstream switch-case
on slice_width % 3:

    0 -> 2
    1 -> 0
    2 -> 1

More importantly it is one of the fixes necessary to get DSC working on
my SM8150 and SM8250 devices [1].

Reviewed-by: Marijn Suijten <marijn.suijten at somainline.org>

Thanks!

- Marijn

[1]: https://gitlab.freedesktop.org/drm/msm/-/issues/24#note_1899310

>  	/* bpp is 6.4 format, 4 LSBs bits are for fractional part */
>  	data |= (dsc->bits_per_pixel << 8);
>  	data |= (dsc->block_pred_enable << 7);
> 
> -- 
> 2.40.0
>