[PATCH v2] drm/msm/dsi: Document DSC related pclk_rate and hdisplay calculations

[Dmitry Baryshkov]

On 23/06/2023 03:14, Abhinav Kumar wrote:
> 
> 
> On 6/19/2023 2:06 PM, Dmitry Baryshkov wrote:
>> Provide actual documentation for the pclk and hdisplay calculations in
>> the case of DSC compression being used.
>>
>> Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov at linaro.org>
>> ---
>>
>> Changes since v1:
>> - Converted dsi_adjust_pclk_for_compression() into kerneldoc (Marijn)
>> - Added a pointer from dsi_timing_setup() docs to
>>    dsi_adjust_pclk_for_compression() (Marijn)
>> - Fixed two typo (Marijn)
>>
>> ---
>>   drivers/gpu/drm/msm/dsi/dsi_host.c | 40 ++++++++++++++++++++++++++++--
>>   1 file changed, 38 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/msm/dsi/dsi_host.c 
>> b/drivers/gpu/drm/msm/dsi/dsi_host.c
>> index 3f6dfb4f9d5a..a8a31c3dd168 100644
>> --- a/drivers/gpu/drm/msm/dsi/dsi_host.c
>> +++ b/drivers/gpu/drm/msm/dsi/dsi_host.c
>> @@ -528,6 +528,25 @@ void dsi_link_clk_disable_v2(struct msm_dsi_host 
>> *msm_host)
>>       clk_disable_unprepare(msm_host->byte_clk);
>>   }
>> +/**
>> + * dsi_adjust_pclk_for_compression() - Adjust the pclk rate for 
>> compression case
>> + * @mode: the selected mode for the DSI output
>> + * @dsc: DRM DSC configuration for this DSI output
>> + *
>> + * Adjust the pclk rate by calculating a new hdisplay proportional to
>> + * the compression ratio such that:
>> + *     new_hdisplay = old_hdisplay * compressed_bpp / uncompressed_bpp
>> + *
>> + * Porches do not need to be adjusted:
>> + * - For the VIDEO mode they are not compressed by DSC and are passed 
>> as is.
>> + * - For the CMD mode there are no actual porches. Instead these fields
>> + *   currently represent the overhead to the image data transfer. As 
>> such, they
>> + *   are calculated for the final mode parameters (after the 
>> compression) and
>> + *   are not to be adjusted too.
>> + *
>> + *  FIXME: Reconsider this if/when CMD mode handling is rewritten to use
>> + *  refresh rate and data overhead as a starting point of the 
>> calculations.
>> + */
>>   static unsigned long dsi_adjust_pclk_for_compression(const struct 
>> drm_display_mode *mode,
>>           const struct drm_dsc_config *dsc)
> 
> I am fine with this part of the doc.
> 
>>   {
>> @@ -926,8 +945,25 @@ static void dsi_timing_setup(struct msm_dsi_host 
>> *msm_host, bool is_bonded_dsi)
>>           if (ret)
>>               return;
>> -        /* Divide the display by 3 but keep back/font porch and
>> -         * pulse width same
>> +        /*
>> +         * DPU sends 3 bytes per pclk cycle to DSI. If compression is
>> +         * not used, a single pixel is transferred at each pulse, no
>> +         * matter what bpp or pixel format is used. In case of DSC
>> +         * compression this results (due to data alignment
>> +         * requirements) in a transfer of 3 compressed pixel per pclk
>> +         * cycle.
>> +         *
> 
> I dont want to talk about data alignment nor formats and I will not ack 
> any references to those.
> 
> I would like to keep this simple and say that DPU sends 3 bytes of 
> compressed data / pclk (6 with widebus enabled) and all this math is 
> doing is that its calculating number of bytes and diving it by 3 OR 6 
> with widebus to calculate the pclk cycles. Thats it.

This makes it unclear, why do we simply by 3 rather than doing * dsc_bpp 
/ 24.  My description might be inaccurate as I don't have hw docs at 
hand, but simple description is not enough.

> 
>> +         * If widebus is enabled, bus width is extended to 6 bytes.
>> +         * This way the DPU can transfer 6 compressed pixels with bpp
>> +         * less or equal to 8 or 3 compressed pixels in case bpp is
>> +         * greater than 8.
>> +         *
>> +         * The back/font porch and pulse width are kept intact.  They
>> +         * represent timing parameters rather than actual data
>> +         * transfer. See the documentation of
>> +         * dsi_adjust_pclk_for_compression().
> 
> this part is fine.
> 
>> +         *
>> +         * XXX: widebus is not supported by the driver (yet).
>>            */
>>           h_total -= hdisplay;
>>           hdisplay = 
>> DIV_ROUND_UP(msm_dsc_get_bytes_per_line(msm_host->dsc), 3);

-- 
With best wishes
Dmitry