Re: [PATCH RFC v2 4/6] drm/msm/dpu: Fix slice_last_group_size calculation

[Dmitry Baryshkov <dmitry.baryshkov@linaro.org>]

On 31/03/2023 21:49, Jessica Zhang wrote:
> Correct the math for slice_last_group_size so that it matches the
> calculations downstream.
> 
> Fixes: c110cfd1753e ("drm/msm/disp/dpu1: Add support for DSC")
> Signed-off-by: Jessica Zhang <quic_jesszhan@quicinc.com>
> Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
> ---
>   drivers/gpu/drm/msm/disp/dpu1/dpu_hw_dsc.c | 6 +++++-
>   1 file changed, 5 insertions(+), 1 deletion(-)
> 
> 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..9312a8d7fbd9 100644
> --- a/drivers/gpu/drm/msm/disp/dpu1/dpu_hw_dsc.c
> +++ b/drivers/gpu/drm/msm/disp/dpu1/dpu_hw_dsc.c
> @@ -56,7 +56,11 @@ 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 % 3;
> +
> +	if (slice_last_group_size == 0)
> +		slice_last_group_size = 3;

Hmm. As I went on checking this against techpack:

mod = dsc->slice_width % 3

mod | techpack | old | your_patch
0   | 2        | 3   | 3
1   | 0        | 2   | 1
2   | 1        | 1   | 2

So, obviously neither old nor new code match the calculations of the 
techpack. If we assume that sde_dsc_helper code is correct (which I have 
no reasons to doubt), then the proper code should be:

slice_last_group_size = (dsc->slice_width + 2) % 3;

Could you please doublecheck and adjust.

> +
>   	data = (initial_lines << 20);
>   	data |= ((slice_last_group_size - 1) << 18);
>   	/* bpp is 6.4 format, 4 LSBs bits are for fractional part */
> 

-- 
With best wishes
Dmitry