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

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Message-ID: <d7aa140d-ea0d-4288-8e31-29aab9a49077@jiaxyga.com>
Date: Tue, 11 Feb 2025 21:06:19 +0300
From: Danila Tikhonov <danila@...xyga.com>
To: Marijn Suijten <marijn.suijten@...ainline.org>
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 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.
> 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
>>