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

[Marijn Suijten]

On 2023-06-20 00:06:47, 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

The

> + * @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

And in v1 you explained that it is _not_ about 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

I feel like "For VIDEO mode" and "For CMD mode" reads more naturally, no
need for "the", but I don't know the grammar rule that states so.

> + *   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.

Nit: well, refresh rate is already part of this calculation (that's how
drm_display_mode's clock member comes to be, and how drm_mode_vrefresh()
figures out fps after the fact).  It's all about the per-CMD transfer
overhead in bytes that is currently not part of the calculation.

> + */
>  static unsigned long dsi_adjust_pclk_for_compression(const struct drm_display_mode *mode,
>  		const struct drm_dsc_config *dsc)
>  {
> @@ -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

Should this be pixels (1 pixel), not bytes, just like in the compressed
scenario?

> +		 * 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.
> +		 *
> +		 * 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.

Okay, so one can not send more than 6 pixels even if the bpp is less
than 8, is what this comes down to.

> +		 *
> +		 * 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().

Nit: note that this is only for VIDEO mode, h_total and ha_end are
accurately unused in the CMDmode path below.

- Marijn

> +		 *
> +		 * 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);
> -- 
> 2.39.2
>