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

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Message-ID: <nx5zeueyifytvxpr5mm3njztt2npfbp7oczdc7zk2i7vulnoon@u2c5dfim5xci>
Date: Sun, 9 Feb 2025 00:32:15 +0200
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Marijn Suijten <marijn.suijten@...ainline.org>
Cc: Danila Tikhonov <danila@...xyga.com>, 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 Sat, Feb 08, 2025 at 11:09:56PM +0100, 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.
> 
> > (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?
> 
> And your title might have space to spell out "Bits Per Component" entirely.
> 
> > ---
> >  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).

Indeed. msm-4.14 explicitly names that single-bit field as
'input_10_bits'. The block is supposed to support bpc of 8, 10 and 12.
This bit should only be set for bpc=10.

Marijn, thanks for catching it!

We should start rewriting DPU register accessors to use generated
accessors. At least it will clearly show if the field is a flag or a
field which has some values. With the current code it is impossible to
notice the difference.

> 
> > @@ -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.

I think the =8 part is a leftover of the old, pre-helper code.

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

-- 
With best wishes
Dmitry