linux-kernel - Re: [PATCH 2/4] drm/msm/dsi: Fix compressed word count calculation

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Date:   Wed, 3 May 2023 10:34:42 -0700
From:   Jessica Zhang <quic_jesszhan@...cinc.com>
To:     Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Rob Clark <robdclark@...il.com>,
        Abhinav Kumar <quic_abhinavk@...cinc.com>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Marijn Suijten <marijn.suijten@...ainline.org>
CC:     Konrad Dybcio <konrad.dybcio@...aro.org>,
        <linux-arm-msm@...r.kernel.org>, <dri-devel@...ts.freedesktop.org>,
        <freedreno@...ts.freedesktop.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/4] drm/msm/dsi: Fix compressed word count calculation



On 5/3/2023 1:26 AM, Dmitry Baryshkov wrote:
> On 03/05/2023 04:19, Jessica Zhang wrote:
>> Currently, word count is calculated using slice_count. This is incorrect
>> as downstream uses slice per packet, which is different from
>> slice_count.
>>
>> Slice count represents the number of soft slices per interface, and its
>> value will not always match that of slice per packet. For example, it is
>> possible to have cases where there are multiple soft slices per interface
>> but the panel specifies only one slice per packet.
>>
>> Thus, use the default value of one slice per packet and remove 
>> slice_count
>> from the word count calculation.
>>
>> Fixes: bc6b6ff8135c ("drm/msm/dsi: Use DSC slice(s) packet size to 
>> compute word count")
>> Signed-off-by: Jessica Zhang <quic_jesszhan@...cinc.com>
>> ---
>>   drivers/gpu/drm/msm/dsi/dsi_host.c | 9 ++++++++-
>>   1 file changed, 8 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/msm/dsi/dsi_host.c 
>> b/drivers/gpu/drm/msm/dsi/dsi_host.c
>> index 35c69dbe5f6f..b0d448ffb078 100644
>> --- a/drivers/gpu/drm/msm/dsi/dsi_host.c
>> +++ b/drivers/gpu/drm/msm/dsi/dsi_host.c
>> @@ -996,7 +996,14 @@ static void dsi_timing_setup(struct msm_dsi_host 
>> *msm_host, bool is_bonded_dsi)
>>           if (!msm_host->dsc)
>>               wc = hdisplay * dsi_get_bpp(msm_host->format) / 8 + 1;
>>           else
>> -            wc = msm_host->dsc->slice_chunk_size * 
>> msm_host->dsc->slice_count + 1;
>> +            /*
>> +             * When DSC is enabled, WC = slice_chunk_size * 
>> slice_per_packet + 1.
>> +             * Currently, the driver only supports default value of 
>> slice_per_packet = 1
>> +             *
>> +             * TODO: Expand drm_panel struct to hold slice_per_packet 
>> info
>> +             *       and adjust DSC math to account for 
>> slice_per_packet.
> 
> slice_per_packet is not a part of the standard DSC, so I'm not sure how 
> that can be implemented. And definitely we should not care about the 
> drm_panel here. It should be either a part of drm_dsc_config, or 
> mipi_dsi_device.

Hi Dmitry,

IIRC slice per packet is given by the panel specs with the default value 
being 1 if no value is specified, so it might be better to have it as 
part of mipi_dsi_device.

Will update the TODO comment accordingly.

Thanks,

Jessica Zhang

> 
>> +             */
>> +            wc = msm_host->dsc->slice_chunk_size + 1;
>>           dsi_write(msm_host, REG_DSI_CMD_MDP_STREAM0_CTRL,
>>               DSI_CMD_MDP_STREAM0_CTRL_WORD_COUNT(wc) |
>>
> 
> -- 
> With best wishes
> Dmitry
>