linux-kernel - Re: [Freedreno] [RFC PATCH 1/2] drm/msm/dpu: add dsc helper functions

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Message-ID: <04156713-3f8e-c29e-322f-259ae163a93a@linaro.org>
Date:   Thu, 16 Mar 2023 18:23:01 +0200
From:   Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To:     Abhinav Kumar <quic_abhinavk@...cinc.com>,
        Jessica Zhang <quic_jesszhan@...cinc.com>
Cc:     Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        quic_sbillaka@...cinc.com, sean@...rly.run, andersson@...nel.org,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        dianders@...omium.org, vkoul@...nel.org, agross@...nel.org,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        marijn.suijten@...ainline.org, swboyd@...omium.org,
        Kuogee Hsieh <quic_khsieh@...cinc.com>,
        freedreno@...ts.freedesktop.org,
        Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
        linux-arm-msm@...r.kernel.org
Subject: Re: [Freedreno] [RFC PATCH 1/2] drm/msm/dpu: add dsc helper functions

On 16/03/2023 18:13, Abhinav Kumar wrote:
> 
> 
> On 3/16/2023 9:03 AM, Dmitry Baryshkov wrote:
>> Hi,
>>
>> [removed previous conversation]
>>
>>>
>>> Hi Dmitry and Abhinav,
>>>
>>> Just wanted to follow up on this thread. I've gone over the MSM-specific
>>> DSC params for DP and DSI and have found a few shared calculations and
>>> variables between both DSI and DP paths:
>>>
>>> - (as mentioned earlier in the thread) almost all the calculations in
>>> dpu_dsc_populate_dsc_config() match dsi_populate_dsc_params() [1]. The
>>> only difference in the math I'm seeing is initial_scale_value.
>>
>> The value in dsi code is valid for initial_offset = 6144. Please use
>> the formula from the standard (= sde_dsc_populate_dsc_config) and add
>> it to drm_dsc_helper.c
>>
>> If I remember correctly the last remaining item in
>> dsi_populate_dsc_params() (except mentioned initial_offset) was
>> line_buf_depth, see [3]. I'm not sure about setting it to bpc+1.
>> According to the standard it should come from a DSC decoder spec,
>> which means it should be set by the DSI panel driver or via
>> drm_dp_dsc_sink_line_buf_depth() in the case of DP output.
>>
>>> - dsc_extra_pclk_cycle_cnt and dce_bytes_per_line, which were introduced
>>> in Kuogee's v1 DSC series [2], are used for DSI, DP, and the DPU timing
>>> engine. dsc_extra_pclk_cycle_cnt is calculated based on pclk_per_line
>>> (which is calculated differently between DP and DSI), but
>>> dce_bytes_per_line is calculated the same way between DP and DSI.
>>>
>>> To avoid having to duplicate math in 2 different places, I think it
>>> would help to have these calculations in some msm_dsc_helper.c file. Any
>>> thoughts on this?
>>
>> dsc_extra_pclk_cycle_cnt and dce_bytes_per_line are used only in DPU
>> code, so they can stay in DPU driver.
>>
> 
> They can stay in the dpu driver is fine but where?
> 
> Like Jessica wrote, this is computed and used in 3 places today :
> 
> 1) DSI video engine computation
> 2) DP controller computation
> 3) timing engine programming

Please excuse me if I'm wrong. I checked both vendor techpack and the 
Kuogee's patches. I see them being used only in the SDE / DPU driver 
code. Could you please point me to the code path that we are discussing?


> So either we have a helper in a common location somewhere so that these 
> 3 modules can call that helper and use it OR each module duplicates the 
> computation code.
> 
> What should be the common location is the discussion here.
> 
> It cannot be dpu_encoder.c as the DSI/DP dont call into the encoder 
> methods.
> 
>>>
>>> Thanks,
>>>
>>> Jessica Zhang
>>>
>>> [1]
>>> https://elixir.bootlin.com/linux/v6.3-rc2/source/drivers/gpu/drm/msm/dsi/dsi_host.c#L1756
>>>
>>> [2] https://patchwork.freedesktop.org/patch/519845/?series=113240&rev=1
>>
>> [3] https://patchwork.freedesktop.org/patch/525441/?series=114472&rev=2
>>
>>
>>

-- 
With best wishes
Dmitry