linux-kernel - Re: [PATCH v2] drm/msm/dsi: Enable DATABUS

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Date:   Wed, 19 Jul 2023 01:00:16 +0300
From:   Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To:     Jessica Zhang <quic_jesszhan@...cinc.com>
Cc:     freedreno@...ts.freedesktop.org, quic_abhinavk@...cinc.com,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        Sean Paul <sean@...rly.run>
Subject: Re: [PATCH v2] drm/msm/dsi: Enable DATABUS_WIDEN for DSI command mode

On 18/07/2023 23:44, Jessica Zhang wrote:
> 
> 
> On 7/17/2023 11:04 PM, Dmitry Baryshkov wrote:
>> On 18/07/2023 03:30, Jessica Zhang wrote:
>>>
>>>
>>> In addition, *_is_widebus_enabled() would only work under the 
>>> assumption that DSC (if supported) will always be enabled during 
>>> bootup for DSI. If there ends up being support for a panel that 
>>> allows for DSC to be enabled on the fly after bootup, the proposed 
>>> *_is_widebus_enabled() check would break since the assumption that 
>>> DSC is always enabled on bootup would not be true anymore.
>>
>> Why is that going to break?
> 
> 
> Currently, the msm_host->dsc is initialized in attach() [1] and we do 
> widebus setup during power_on() [2]. So we can assume that if the panel 
> supports DSC, msm_host->dsc will be not NULL during power_on() and the 
> widebus setup will work as expected.
> 
> However, if a panel supports enabling DSC after bootup, then 
> msm_host->dsc will be set later within the commit enable() path meaning 
> the necessary widebus setup during power_on() would *not* happen (as 
> *_is_widebus_enabled() would return false on account of msm_host->dsc == 
> NULL during power_on()).


Minor corrections:
- power_on() happens during pre_enable() stage,
- the dynamic msm_host->dsc can also be set during pre_enable() stage.

Note: there still exists dsi_mgr_bridge_mode_set(), which can also be 
used to set msm_host->dsc (however I would prefer to drop mode_set() 
completely and set the mode from atomic_enable().

I'd say, this is the minor issue. The dsi_timing_setup() is plagued with 
if (dsc) checks, so if we make DSC dynamic, it will have to be audited 
anyway.

> 
> Thanks,
> 
> Jessica Zhang
> 
> [1] 
> https://elixir.bootlin.com/linux/v6.5-rc2/source/drivers/gpu/drm/msm/dsi/dsi_host.c#L1587
> 
> [2] 
> https://elixir.bootlin.com/linux/v6.5-rc2/source/drivers/gpu/drm/msm/dsi/dsi_host.c#L2359
> 
>>
>> -- 
>> With best wishes
>> Dmitry
>>

-- 
With best wishes
Dmitry