linux-kernel - Re: [PATCH 2/3] arm64: dts: ti: k3-j721e-main.dtsi: Add DSS node

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Date:   Mon, 27 Apr 2020 14:41:37 +0300
From:   Tero Kristo <t-kristo@...com>
To:     Tomi Valkeinen <tomi.valkeinen@...com>, Jyri Sarha <jsarha@...com>,
        Nishanth Menon <nm@...com>, Rob Herring <robh+dt@...nel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] arm64: dts: ti: k3-j721e-main.dtsi: Add DSS node

On 27/04/2020 14:37, Tomi Valkeinen wrote:
> On 27/04/2020 14:15, Tomi Valkeinen wrote:
>> On 27/04/2020 14:10, Jyri Sarha wrote:
>>> On 27/04/2020 13:51, Tomi Valkeinen wrote:
>>>> On 27/04/2020 13:37, Jyri Sarha wrote:
>>>>> On 27/04/2020 13:09, Tero Kristo wrote:
>>>>>>> +        status = "disabled";
>>>>>>
>>>>>> Again, why disabled by default?
>>>>>>
>>>>>
>>>>> tidss device is not functional without a defined video-port. The 
>>>>> driver
>>>>> is not implemented in a way that it would handle a broken 
>>>>> configuration
>>>>> gracefully.
>>>>
>>>> Then we need to fix it. The driver should handle the case where there
>>>> are no ports defined just fine.
>>>>
>>>
>>> Just by reading the code, I would say that currently the probe would
>>> fail with returned -ENOMEM after calling drm_vblank_init() with zero 
>>> CRTCs.
>>>
>>> So should the probe fail gracefully and silently, or should we try to
>>> register a DRM device with no CRTCs? Is that even possible?
>>
>> My first thought is that the driver should exit probe silently with 
>> ENODEV if there are no outputs defined (but, of course, with 
>> EPROBE_DEFER if there are outputs which haven't been probed yet).
>>
>> It gets a bit more complex if we ever support writeback, as that can 
>> be used as mem-to-mem without any displays, but I think we can ignore 
>> that for now.
> 
> In any case, that's not the reason for status = "disabled", so that 
> discussion is not related to these patches as such.
> 
> The reason to have DSS disabled is just to prevent pointless driver 
> probing. When a board dts or a DT overlay adds a display, the DSS DT 
> node has to be modified anyway to add the DT graph and the panel/bridge 
> data. So one can as well add the single line of "status = enabled" there.

Ok, thanks for the explanation, queued all three patches towards 5.8 
based on that.

-Tero
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