Re: [RFC] i2c: device-tree: Handling child nodes which are not i2c devices

[Jon Hunter <jonathanh@nvidia.com>]

Hi Wolfram,

Any thoughts on this? Would you accept option #1 below?

Cheers
Jon

On 25/04/16 16:51, Stephen Warren wrote:
> On 04/25/2016 08:51 AM, Jon Hunter wrote:
>> Hi all,
>>
>> Any thoughts on this? Let me know if this is not clear in anyway.
> 
> I'm not sure if you're expecting comment from me since I suggested
> option (1) below. For the record though I do think that's the better
> option; it's very general/explicit, should apply well to any OS
> environment, and relying on OF_POPULATED implies an order that the
> driver must initialize things in, so that it ensures OF_POPULATED is set
> before any I2C children are handled.
> 
>> On 15/04/16 16:35, Jon Hunter wrote:
>>> Hi all,
>>>
>>> For Tegra we have an i2c device for display port, namely the display
>>> port auxiliary channel (or dpaux) as specified by the display port
>>> standard. If an design using Tegra does not utilise the display port
>>> interface, then the pads assigned to the dpaux can be re-assigned to
>>> another generic i2c controller (i2c6 for Tegra124/210). In other words,
>>> the pads can be re-used for a generic i2c interface.
>>>
>>> The registers that control whether the pads are mapped to the dpaux or
>>> i2c6 are located in the dpaux register space. Therefore, I am looking at
>>> adding pin controller support for dpaux so that i2c6 can request these
>>> pads if it is enabled and I was hoping to add a pinmux node the to dpaux
>>> device in device-tree to do this. For example, something like ...
>>>
>>>     dpaux@0,545c0000 {
>>>         ...
>>>
>>>         /* pinctrl node */
>>>         pinmux {
>>>             ...
>>>         };
>>>     };
>>>
>>> Although the above works, when doing this I noticed that when the device
>>> booted, I would seeing the following error messages on boot ...
>>>
>>>   i2c i2c-5: of_i2c: modalias failure on ...
>>>
>>> These error messages being caused by the new pinmux node because it is
>>> not recognised as an i2c device. To avoid this error messages we have
>>> come up with a couple solutions but wanted to get some feedback on the
>>> best approach.
>>>
>>> 1. Add a i2c-bus sub-node to the dpaux binding (suggested by Stephen
>>>     Warren), so we would have something like the below. Then i2c devices
>>>     for dpaux would be place in the i2c-bus sub-node.
>>>
>>>     dpaux@0,545c0000 {
>>>         ...
>>>
>>>         /* pinctrl node */
>>>         pinmux {
>>>             ...
>>>         };
>>>
>>>         /* place-holder for i2c devices */
>>>         i2c-bus {
>>>             ...
>>>         };
>>>     };   
>>>
>>>     To make the above work ideally we would like to make the 'i2c-bus'
>>>     node a generic solution for all i2c devices, so the i2c core would
>>>     check for the presence of this node and if it is found then would
>>>     default to this node for looking for i2c-devices.
>>>
>>> 2. When registering i2c devices via device-tree, the function
>>>     of_i2c_register_devices() checks to see if OF_POPULATED flag is set
>>>     for a given node. If it is set, then the node is skipped. I believe
>>>     this was added for device-tree overlays (commit 4f001fd30145 i2c:
>>>     Mark instantiated device nodes with OF_POPULATE). Another option is
>>>     for the dpaux driver to mark the pinmux node as populated before
>>>     registering the i2c adapter and this will prevent the i2c core from
>>>     trying to parse the pinmux node. I am not sure if this would be
>>>     frowned upon in anyway or if we can guarantee that no future changes
>>>     to DT overlays would change this in a way where it would not work.
>>>
>>> Let me know your thoughts.
>