Subject: How to encode being an I2C slave in DT?

From: [email-masked] (Stephen Warren)
Date: Wed, 06 May 2015 09:57:37 -0600

On 05/06/2015 02:09 AM, Uwe Kleine-K?nig wrote:
> Hello Marc,
>
> On Wed, May 06, 2015 at 09:53:55AM +0200, Marc Dietrich wrote:
>> Am Mittwoch, 6. Mai 2015, 08:59:28 schrieb Uwe Kleine-K?nig:
>>> On Tue, May 05, 2015 at 12:55:13PM +0200, Wolfram Sang wrote:
>>>> So what about adding a new property "i2c-slave-reg"? This does not only
>>>> prevent the confusion above, but also makes it very clear that this node
>>>> is an I2C slave without the need to encode that somehow in the
>>>> compatible property (although it probably should be described there as
>>>> well, still).
>>>
>>> I admit I didn't follow the discussions referenced in the footnotes, but
>>> I wonder if the slave part should be added to the device tree at all.
>>> AFAICT it could (and so should) be completely userspace-defined which
>>> slave driver is used on which address. I imagine that for most
>>> controllers the bus addresses to use can be chosen more or less freely.
>>> So what am I missing?
>>
>> if you had read the footnotes you would know :-) Our usecase is connect an
>> embeedded controller via i2c to the host soc, similar to cros-ec, but here the
>> ec is the i2c master. The ec connects keyboard, mouse, pwrmngt, and other
>> stuff, for which the drivers are best implemented in kernel code AFAIK.
 >
> Right, the driver might sensibly be implemented in kernel space. But I'd
> vote that you still need to do the binding of these drivers to your
> slave controller from userspace. Then there is no need to specify
> anything in your dtb.

I think the set of I2C slave devices that are implemented by the Linux 
system can reasonably be considered part of the HW definition. Most DT 
content to date has been a definition of the HW that's available to SW, 
but at least in this case, this I2C slave device is something that must 
be implemented (admittedly in conjunction with SW) using the I2C slave 
HW on the main SoC, in order for the overall HW to work as intended.

BTW, I believe devicetree-spec at vger.kernel.org was created to address 
subsystem-level DT schema questions like this. It's much lower volume, 
so perhaps the thread would get noticed by the DT maintainers if posted 
there (or perhaps just CC them)?