[PATCH] drm: don't link DP aux i2c adapter to the hardware device node

[Andrzej Hajda]

On 20.11.2017 08:54, Daniel Vetter wrote:
> On Tue, Nov 14, 2017 at 03:34:19PM +0100, Andrzej Hajda wrote:
>> On 05.04.2017 14:04, Thierry Reding wrote:
>>> On Wed, Apr 05, 2017 at 10:52:32AM +0200, Lucas Stach wrote:
>>>> Hi Rob,
>>>>
>>>> Am Mittwoch, den 29.03.2017, 08:56 -0500 schrieb Rob Herring:
>>>>> On Mon, Jan 23, 2017 at 10:33 AM, Thierry Reding
>>>>> <thierry.reding at gmail.com> wrote:
>>>>>> On Fri, Jan 13, 2017 at 06:36:30PM +0100, Lucas Stach wrote:
>>>>>>> The i2c adapter on DP AUX is purely a software construct. Linking
>>>>>>> it to the device node of the parent device is wrong, as it leads to
>>>>>>> 2 devices sharing the same device node, which is bad practice, as
>>>>>> Who says that two devices can't share the same device node? It's done
>>>>>> all the time.
>>>>> It's done *some of the time* and I would not consider it best practice.
>>>>>
>>>>>>> well as the i2c trying to populate children of the i2c adapter by
>>>>>>> looking at the child device nodes of the parent device.
>>>>>> A set of patches landed in v4.9 to work around this issue in a better
>>>>>> way. See:
>>>>>>
>>>>>>         98b00488459e dt-bindings: i2c: Add support for 'i2c-bus' subnode
>>>>>>         7e4c224abfe8 i2c: core: Add support for 'i2c-bus' subnode
>>>>> What does this buy us? I don't see why this needs to be in DT either.
>>>>> Contrary to popular belief, DT is not the only way to instantiate
>>>>> devices, C code can still do it.
>>>>>
>>>>> Also, if this one line removal has no side effects, then how was it
>>>>> even needed? We can always add it back if there's some argument for
>>>>> why it is needed.
>>>> Okay, so I take this as you mostly agreeing with the rationale of this
>>>> patch.
>>> For some general background on this: I was originally using this for DP
>>> support on Tegra (though that ended up never getting merged because of a
>>> particularily frustrating episode of trying to get better link training
>>> support into the core helpers) and use it as a means to obtain the I2C
>>> controller used for DDC. On Tegra, and I suspect other devices as well,
>>> the DP AUX controller is separate from the encoder, so the idea was to
>>> link them together using a standard ddc-i2c-bus phandle.
>>>
>>> I ended up not needing that because the encoder and DP AUX controller
>>> are so tightly linked on Tegra that I need direct access to the DP AUX
>>> anyway and can therefore directly get the I2C controller from that.
>>>
>>> If there aren't any other users of this, I suppose we could simply
>>> remove the line. Should someone turn up in the future and require the
>>> I2C controller to be looked up from a phandle we could add it again,
>>> at which point we'd have to investigate again how to get rid of the
>>> errors.
>>>
>>> Acked-by: Thierry Reding <treding at nvidia.com>
>> It looks like the patch has been accepted but not merged. Who can pick
>> it up? or is it OK if I put it int drm-misc?
> I don't have the patch itself, but sounds good. Just go ahead and push.
> -Daniel

OK, done.


Regards

Andrzej