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

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Message-ID: <3eb05f6d-523f-f7e5-a0e9-d47b852fe079@ti.com>
Date:   Mon, 27 Apr 2020 14:37:37 +0300
From:   Tomi Valkeinen <tomi.valkeinen@...com>
To:     Jyri Sarha <jsarha@...com>, Tero Kristo <t-kristo@...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 14:15, 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.

  Tomi

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