On 2023-06-16 12:41:52, 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@xxxxxxxxxx>
> ---
> drivers/gpu/drm/msm/dsi/dsi_host.c | 35 ++++++++++++++++++++++++++++--
> 1 file changed, 33 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..72c377c9c7be 100644
> --- a/drivers/gpu/drm/msm/dsi/dsi_host.c
> +++ b/drivers/gpu/drm/msm/dsi/dsi_host.c
> @@ -528,6 +528,21 @@ void dsi_link_clk_disable_v2(struct msm_dsi_host *msm_host)
> clk_disable_unprepare(msm_host->byte_clk);
> }
>
> +/*
> + * Adjust the pclk rate by calculating a new hdisplay proportional to
Make this a kerneldoc with:
/**
* dsi_adjust_pclk_for_compression() - Adjust ...
> + * 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.
as-is
Though this was never tested nor confirmed by QUIC, but we can assume it
is the case for now?
> + * - For the CMD mode the are no actual porches. Instead they represent the
the are no -> these are not
they currently* represent. Let's make sure that folks read the FIXME
below by perhaps rewording it right into this entry?
> + * 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)
> {
> @@ -926,8 +941,24 @@ 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
3 compressed bytes*, not pixels.
> + * cycle.
> + *
> + * If widebus is enabled, bus width is extended to 6 bytes.
> + * This way the DPU can transfer 6 compressed pixels with bpp
pixels -> bytes?
> + * less or equal to 8 or 3 compressed pyxels in case bpp is
pixels*... bytes?
And I will ask this **again**: does this mean we can halve pclk?
> + * greater than 8.
> + *
> + * The back/font porch and pulse width are kept intact. They
> + * represent timing parameters rather than actual data
> + * transfer.
See FIXME above on dsi_adjust_pclk_for_compression()?
Thanks so much for finally putting some of this to paper.
- 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
>
