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

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Message-ID: <4cf2e9ab-7e08-fb26-d924-8ea8141d9f58@linaro.org>
Date:   Tue, 9 May 2023 02:27:17 +0300
From:   Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To:     Abhinav Kumar <quic_abhinavk@...cinc.com>,
        Jessica Zhang <quic_jesszhan@...cinc.com>,
        Rob Clark <robdclark@...il.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 08/05/2023 23:09, Abhinav Kumar wrote:
> 
> 
> 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.
>>
> 
> This is not correct.
> 
> It is part of the DSI standard (not DSC standard). Please refer to 
> Figure 40 "One Line Containing One Packet with Data from One or More 
> Compressed Slices" and Figure 41 "One Line Containing More than One 
> Compressed Pixel Stream Packet".

I have reviewed section 8.8.24 and Annex D of the DSI standard.

It is not clear to me, if we can get away with always using 
slice_per_packet = 1. What is the DSI sink's difference between Fig. 
40.(b) and Fig 41?

Are there are known panels that require slice_per_packet != 1? If so, we 
will have to implement support for such configurations.

> This has details about this. So I still stand by my point that this 
> should be in the drm_panel.

Note, the driver doesn't use drm_panel directly. So slices_per_packet 
should go to mipi_dsi_device instead (which in turn can be filled from 
e.g. drm_panel or from any other source).

> 
>>> +             */
>>> +            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