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

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Message-ID: <vlni5ophwkwfbvh6zj6aqqdhyt3wwjjkv77d6fublj22zidv2d@hr4qlot72vqi>
Date: Wed, 12 Feb 2025 01:52:42 +0200
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Danila Tikhonov <danila@...xyga.com>
Cc: Marijn Suijten <marijn.suijten@...ainline.org>, 
	neil.armstrong@...aro.org, quic_jesszhan@...cinc.com, maarten.lankhorst@...ux.intel.com, 
	mripard@...nel.org, tzimmermann@...e.de, airlied@...il.com, simona@...ll.ch, 
	robh@...nel.org, krzk+dt@...nel.org, conor+dt@...nel.org, andersson@...nel.org, 
	konradybcio@...nel.org, robdclark@...il.com, quic_abhinavk@...cinc.com, sean@...rly.run, 
	jonathan@...ek.ca, jun.nie@...aro.org, fekz115@...il.com, 
	dri-devel@...ts.freedesktop.org, devicetree@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-arm-msm@...r.kernel.org, freedreno@...ts.freedesktop.org, linux@...nlining.org, 
	~postmarketos/upstreaming@...ts.sr.ht
Subject: Re: [PATCH 3/4] drm/msm/dsi: Allow all bpc values

On Tue, Feb 11, 2025 at 09:06:19PM +0300, Danila Tikhonov wrote:
> On 2/9/25 01:09, Marijn Suijten wrote:
> > On 2025-02-03 21:14:26, Danila Tikhonov wrote:
> > > From: Eugene Lepshy <fekz115@...il.com>
> > > 
> > > 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@...il.com>
> > > Signed-off-by: Danila Tikhonov <danila@...xyga.com>
> > 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.

Yes, please. Add a caselist and also a note that only 8, 10 and 12 are
valid for DSC 1.1 block. Then whoever stomps upon other bpc value will
have to extend the check, verifying DSC version.

> > 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.

This is now being fixed by a separate patch.

> 
> > > @@ -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?

No, the pre-SCR comment is fine.

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

-- 
With best wishes
Dmitry