Re: [PATCH v6 6/6] drm/msm/dsi: Document DSC related pclk_rate and hdisplay calculations

[Dmitry Baryshkov <dmitry.baryshkov@linaro.org>]

On 10/06/2023 01:57, Jessica Zhang wrote:
> Add documentation comments explaining the pclk_rate and hdisplay math
> related to DSC.
> 
> Signed-off-by: Jessica Zhang <quic_jesszhan@quicinc.com>
> ---
>   drivers/gpu/drm/msm/dsi/dsi_host.c | 10 ++++++++++
>   1 file changed, 10 insertions(+)
> 
> diff --git a/drivers/gpu/drm/msm/dsi/dsi_host.c b/drivers/gpu/drm/msm/dsi/dsi_host.c
> index fb1d3a25765f..aeaadc18bc7b 100644
> --- a/drivers/gpu/drm/msm/dsi/dsi_host.c
> +++ b/drivers/gpu/drm/msm/dsi/dsi_host.c
> @@ -564,6 +564,13 @@ void dsi_link_clk_disable_v2(struct msm_dsi_host *msm_host)
>   static unsigned long dsi_adjust_pclk_for_compression(const struct drm_display_mode *mode,
>   		const struct drm_dsc_config *dsc)
>   {
> +	/*
> +	 * Adjust the pclk rate by calculating a new hdisplay proportional to
> +	 * the compression ratio such that:
> +	 *     new_hdisplay = old_hdisplay * target_bpp / source_bpp

I'd say `* compressed_bpp / uncompressed_bpp'. This is easier to follow 
than source and target.

> +	 *
> +	 * Porches need not be adjusted during compression.
> +	 */
>   	int new_hdisplay = DIV_ROUND_UP(mode->hdisplay * drm_dsc_get_bpp_int(dsc),
>   			dsc->bits_per_component * 3);
>   
> @@ -961,6 +968,9 @@ static void dsi_timing_setup(struct msm_dsi_host *msm_host, bool is_bonded_dsi)
>   
>   		/* Divide the display by 3 but keep back/font porch and
>   		 * pulse width same
> +		 *
> +		 * hdisplay will be divided by 3 here to account for the fact
> +		 * that DPU sends 3 bytes per pclk cycle to DSI.
>   		 */
>   		h_total -= hdisplay;
>   		hdisplay = DIV_ROUND_UP(msm_dsc_get_bytes_per_line(msm_host->dsc), 3);
> 

-- 
With best wishes
Dmitry