Re: [PATCH 3/4] drm/msm/dsi: Allow all bpc values

[Danila Tikhonov]

On 2/9/25 01:09, Marijn Suijten wrote:
> On 2025-02-03 21:14:26, Danila Tikhonov wrote:
> > From: Eugene Lepshy <fekz115@xxxxxxxxx>
> >
> > DRM DSC helper has parameters for various bpc values ​​other than 8:
> Weird zero-width \u200b spaces here between "values" and "other", please delete
> those.
Thanks, I will fix it in the next version.
> > (8/10/12/14/16).
> >
> > Remove this guard.
> >
> > Signed-off-by: Eugene Lepshy <fekz115@xxxxxxxxx>
> > Signed-off-by: Danila Tikhonov <danila@xxxxxxxxxxx>
> Should this patch elaborate that those "DRM DSC helper" don't have any
> additional guarding for the values you mention either, i.e. passing 9 or 11 or
> > 16 don't seem to be checked anywhere else either?
There are no other bpc checks, you are right. But to be honest I don't
really see any sense in this. Anyway, if you still want us to leave the
current guard and just extend it with new values ​​(for example via
switch case) - let me know.
> And your title might have space to spell out "Bits Per Component" entirely.
I'll fix that too.
> > ---
> >   drivers/gpu/drm/msm/dsi/dsi_host.c | 7 +------
> >   1 file changed, 1 insertion(+), 6 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/msm/dsi/dsi_host.c b/drivers/gpu/drm/msm/dsi/dsi_host.c
> > index 007311c21fda..d182af7bbb81 100644
> > --- a/drivers/gpu/drm/msm/dsi/dsi_host.c
> > +++ b/drivers/gpu/drm/msm/dsi/dsi_host.c
> > @@ -1767,11 +1767,6 @@ static int dsi_populate_dsc_params(struct msm_dsi_host *msm_host, struct drm_dsc
> >   		return -EINVAL;
> >   	}
> >   
> > -	if (dsc->bits_per_component != 8) {
> > -		DRM_DEV_ERROR(&msm_host->pdev->dev, "DSI does not support bits_per_component != 8 yet\n");
> > -		return -EOPNOTSUPP;
> > -	}
> > -
> >   	dsc->simple_422 = 0;
> >   	dsc->convert_rgb = 1;
> >   	dsc->vbr_enable = 0;
> This seems supicous on the dpu1 side, in the original DSC 1.1 (not 1.2) block in
> dpu_hw_dsc_config(), which has:
>
> 	data |= (dsc->line_buf_depth << 3);
> 	data |= (dsc->simple_422 << 2);
> 	data |= (dsc->convert_rgb << 1);
> 	data |= dsc->bits_per_component;
>
> The original value of `8` would overlap with the lowest bit of line_buf_depth
> (4th bit in `data`).  Now, the 2nd bit which will take the value from
> convert_rgb, which is already set to 1 above, will overlap with the 2nd bit in
> your new bpc value of 10.
>
> Can you double-check that this code in DPU1 is actually valid?  I assume you
> have tested this panel at least and it is working (worthy mention in the cover
> letter?), this just seems like yet another mistake in the original bindings
> (though the register always had a matching value with downstream on 8 BPC panels
> for me).

Of course I have tested the panel and it works, I just thought it would
be obvious. We also have tested sm7150-xiaomi-courbet, sm8450-xiaomi-cupid
and sm8475-nothing-pong, which already have bpp = bpc = 10 panels and
with some hack it also work without any changes to the DRM.

> > @@ -1779,7 +1774,7 @@ static int dsi_populate_dsc_params(struct msm_dsi_host *msm_host, struct drm_dsc
> >   	drm_dsc_set_const_params(dsc);
> >   	drm_dsc_set_rc_buf_thresh(dsc);
> >   
> > -	/* handle only bpp = bpc = 8, pre-SCR panels */
> > +	/* handle only pre-SCR panels */
> >   	ret = drm_dsc_setup_rc_params(dsc, DRM_DSC_1_1_PRE_SCR);
> Good catch - this comment sounds like it's documenting a limitation of
> this helper function, but the function does not have such limitations...
> rc_parameters_pre_scr has values for all these combinations.
Maybe we should remove this comment entirely?

Regards,
Danila
> - Marijn
>
> >   	if (ret) {
> >   		DRM_DEV_ERROR(&msm_host->pdev->dev, "could not find DSC RC parameters\n");
> > --
> > 2.48.1