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

[Abhinav Kumar <quic_abhinavk@quicinc.com>]

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

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
> 
> 
>