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

[Jessica Zhang]

On 4/2/2023 4:27 AM, Dmitry Baryshkov wrote:
> 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 at quicinc.com>
>> Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov at 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.

Hi Dmitry,

The calculation should match the techpack calculation (I kept the `data 
|= ((slice_last_group_size - 1) << 18);` a few lines down).

Thanks,

Jessica Zhang

> 
>> +
>>       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
>