Re: [PATCH v2 5/7] drm/msm/dsi: Account for DSC's bits_per_pixel having 4 fractional bits

[Dmitry Baryshkov]

On Wed, 5 Oct 2022 at 21:17, Marijn Suijten
<marijn.suijten@xxxxxxxxxxxxxx> wrote:
>
> drm_dsc_config's bits_per_pixel field holds a fractional value with 4
> bits, which all panel drivers should adhere to for
> drm_dsc_pps_payload_pack() to generate a valid payload.  All code in the
> DSI driver here seems to assume that this field doesn't contain any
> fractional bits, hence resulting in the wrong values being computed.
> Since none of the calculations leave any room for fractional bits or
> seem to indicate any possible area of support, disallow such values
> altogether.
>
> Fixes: b9080324d6ca ("drm/msm/dsi: add support for dsc data")
> Signed-off-by: Marijn Suijten <marijn.suijten@xxxxxxxxxxxxxx>
> ---
>  drivers/gpu/drm/msm/dsi/dsi_host.c | 25 +++++++++++++++++--------
>  1 file changed, 17 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/gpu/drm/msm/dsi/dsi_host.c b/drivers/gpu/drm/msm/dsi/dsi_host.c
> index f42794cdd4c1..4717d49d76be 100644
> --- a/drivers/gpu/drm/msm/dsi/dsi_host.c
> +++ b/drivers/gpu/drm/msm/dsi/dsi_host.c
> @@ -33,7 +33,7 @@
>
>  #define DSI_RESET_TOGGLE_DELAY_MS 20
>
> -static int dsi_populate_dsc_params(struct drm_dsc_config *dsc);
> +static int dsi_populate_dsc_params(struct msm_dsi_host *msm_host, struct drm_dsc_config *dsc);
>
>  static int dsi_get_version(const void __iomem *base, u32 *major, u32 *minor)
>  {
> @@ -908,6 +908,7 @@ static void dsi_timing_setup(struct msm_dsi_host *msm_host, bool is_bonded_dsi)
>         u32 va_end = va_start + mode->vdisplay;
>         u32 hdisplay = mode->hdisplay;
>         u32 wc;
> +       int ret;
>
>         DBG("");
>
> @@ -943,7 +944,9 @@ static void dsi_timing_setup(struct msm_dsi_host *msm_host, bool is_bonded_dsi)
>                 /* we do the calculations for dsc parameters here so that
>                  * panel can use these parameters
>                  */
> -               dsi_populate_dsc_params(dsc);
> +               ret = dsi_populate_dsc_params(msm_host, dsc);
> +               if (ret)
> +                       return;
>
>                 /* Divide the display by 3 but keep back/font porch and
>                  * pulse width same
> @@ -1769,7 +1772,7 @@ static char bpg_offset[DSC_NUM_BUF_RANGES] = {
>         2, 0, 0, -2, -4, -6, -8, -8, -8, -10, -10, -12, -12, -12, -12
>  };
>
> -static int dsi_populate_dsc_params(struct drm_dsc_config *dsc)
> +static int dsi_populate_dsc_params(struct msm_dsi_host *msm_host, struct drm_dsc_config *dsc)
>  {
>         int mux_words_size;
>         int groups_per_line, groups_total;
> @@ -1780,6 +1783,12 @@ static int dsi_populate_dsc_params(struct drm_dsc_config *dsc)
>         int data;
>         int final_value, final_scale;
>         int i;
> +       u16 bpp = dsc->bits_per_pixel >> 4;
> +
> +       if (dsc->bits_per_pixel & 0xf) {
> +               DRM_DEV_ERROR(&msm_host->pdev->dev, "DSI does not support fractional bits_per_pixel\n");
> +               return -EINVAL;
> +       }
>
>         dsc->rc_model_size = 8192;
>         dsc->first_line_bpg_offset = 12;
> @@ -1801,7 +1810,7 @@ static int dsi_populate_dsc_params(struct drm_dsc_config *dsc)
>         }
>
>         dsc->initial_offset = 6144; /* Not bpp 12 */
> -       if (dsc->bits_per_pixel != 8)
> +       if (bpp != 8)
>                 dsc->initial_offset = 2048;     /* bpp = 12 */
>
>         mux_words_size = 48;            /* bpc == 8/10 */
> @@ -1824,14 +1833,14 @@ static int dsi_populate_dsc_params(struct drm_dsc_config *dsc)
>          * params are calculated
>          */
>         groups_per_line = DIV_ROUND_UP(dsc->slice_width, 3);
> -       dsc->slice_chunk_size = DIV_ROUND_UP(dsc->slice_width * dsc->bits_per_pixel, 8);
> +       dsc->slice_chunk_size = DIV_ROUND_UP(dsc->slice_width * bpp, 8);

I'd still prefer if we can get closer to drm_dsc_compute_rc_parameters().
The mentioned function has the following code:

vdsc_cfg->slice_chunk_size = DIV_ROUND_UP(vdsc_cfg->slice_width *

vdsc_cfg->bits_per_pixel,
                                                          (8 * 16));

In fact, could you please take a look if we can switch to using this
function and drop our code?

>
>         /* rbs-min */
>         min_rate_buffer_size =  dsc->rc_model_size - dsc->initial_offset +
> -                               dsc->initial_xmit_delay * dsc->bits_per_pixel +
> +                               dsc->initial_xmit_delay * bpp +
>                                 groups_per_line * dsc->first_line_bpg_offset;
>
> -       hrd_delay = DIV_ROUND_UP(min_rate_buffer_size, dsc->bits_per_pixel);
> +       hrd_delay = DIV_ROUND_UP(min_rate_buffer_size, bpp);
>
>         dsc->initial_dec_delay = hrd_delay - dsc->initial_xmit_delay;
>
> @@ -1854,7 +1863,7 @@ static int dsi_populate_dsc_params(struct drm_dsc_config *dsc)
>         data = 2048 * (dsc->rc_model_size - dsc->initial_offset + num_extra_mux_bits);
>         dsc->slice_bpg_offset = DIV_ROUND_UP(data, groups_total);
>
> -       data = dsc->initial_xmit_delay * dsc->bits_per_pixel;
> +       data = dsc->initial_xmit_delay * bpp;
>         final_value =  dsc->rc_model_size - data + num_extra_mux_bits;
>         dsc->final_offset = final_value;
>
> --
> 2.38.0
>


-- 
With best wishes
Dmitry