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

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Message-ID: <9e9bbe10-8646-9443-06b2-fa6737d0c267@linaro.org>
Date:   Tue, 9 May 2023 03:47:50 +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 09/05/2023 03:45, Abhinav Kumar wrote:
> 
> 
> On 5/8/2023 4:27 PM, Dmitry Baryshkov wrote:
>> 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?
>>
> 
> The difference is that in fig 40(b) there is only one packet of data 
> (check closely, there is only one header).
> 
> In fig 41, there are multiple headers so its showing multiple packets.

Yes, this is the description of the pictures. I mean what is the 
functional difference?

> 
>> Are there are known panels that require slice_per_packet != 1? If so, 
>> we will have to implement support for such configurations.
>>
> 
> Unless explicitly requested by the panel, we can use 1. From the device 
> tree files of the panels we support downstream, I do see 
> qcom,mdss-dsc-slice-per-pkt set to 2 for some panels. I dont know 
> whether those panels will not work with 1. I really don't think any of 
> the DSC panels working with MSM were upstreamed.
> 
> I think the one jessica will be posting (and merging) will be the first 
> and that works with 1.
> 
> If there are other panels in the works which require 2 slice_per_pkt, I 
> would wait to first see them on the list and if they cannot work with 1 
> slice_per_pkt, add support for that.

If slice_per_pkt change is localized - touching only few lines in DSI or 
in msm_helpers. Otherwise we should consider having that from the 
beginning (but hardcoded to 1 for now).

> 
>>> 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