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

[Marijn Suijten <marijn.suijten@somainline.org>]

On 2022-10-05 22:31:43, Dmitry Baryshkov wrote:
> On Wed, 5 Oct 2022 at 21:17, Marijn Suijten
> <marijn.suijten@somainline.org> 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@somainline.org>
> > ---
> >  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));

Fwiw this discussion happened in dsi_update_dsc_timing() where a similar
calculation was the sole user of bits_per_pixel.  This function,
dsi_populate_dsc_params(), has more uses of bits_per_pixel so it made
more sense to compute and document this "discrepancy" up front.
drm_dsc_compute_rc_parameters() doesn't document where this "16" comes
from, unfortunately (requiring knowledge of the contents of the struct).

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

There's alread a:

	/* FIXME: need to call drm_dsc_compute_rc_parameters() so that rest of
	 * params are calculated
	 */

And it was trivial to replace everything below that comment with this
function call; I have not checked the math in detail but it assigns
_every_ field that was also assigned here, and the resulting values
provide an identical DCS PPS (which I happened to be printing to compare
to downstream, leading to find all the issues solved in this series) and
working phone screen.

Makes me wonder why this wasn't caught in review and replaced from the
get-go...

Do you want me to do this in a v3, before applying this fractional-bits
fix?  At that point this becomes the only user of bits_per_pixel:

	dsc->initial_offset = 6144; /* Not bpp 12 */
	if (bpp != 8)
		dsc->initial_offset = 2048;	/* bpp = 12 */

Note that intel_vdsc.c has the exact same code right where they fill
vdsc_cfg->initial_offset:

	int bpp = vdsc_cfg->bits_per_pixel >> 4;

I'm inclined to leave this as it is.

- Marijn