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

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Message-ID: <e9ab55d7-bd2b-4241-bd40-322c2beb97d9@oss.qualcomm.com>
Date: Mon, 10 Feb 2025 18:13:58 +0100
From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
To: Konrad Dybcio <konrad.dybcio@....qualcomm.com>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        Danila Tikhonov <danila@...xyga.com>
Cc: 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, dmitry.baryshkov@...aro.org,
        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 10.02.2025 6:10 PM, Konrad Dybcio wrote:
> On 8.02.2025 11:09 PM, 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).
> 
> It seems like the lowest bit should be set iff the input is 10bpc, the
> current situation where our '8' bleeds into the following (correctly named
> fields) is bad.

See also

https://github.com/Wikidepia/kernel_xiaomi_santoni-4.9/blob/master/drivers/gpu/drm/msm/sde/sde_hw_dsc.c#L67-L80

Konrad