Re: [PATCH v6 09/12] i2c: of-prober: Add regulator support

[Andy Shevchenko]

On Tue, Sep 10, 2024 at 05:30:07PM -0700, Doug Anderson wrote:
> On Thu, Sep 5, 2024 at 8:45 PM Chen-Yu Tsai <wenst@xxxxxxxxxxxx> wrote:

...

> > At least for the stuff that we have (touchscreens and trackpads) such
> > registers typically don't exist, unless it's an HID-over-I2C device,
> > in which case there's the standard HID descriptor at some address.
> > But, yeah, reading the HID descriptor was the use case I had in mind.
> >
> > At least for one Chromebooks it's a bit more tricky because that one
> > HID-over-I2C component shares the same address as a non-HID one. We
> > currently have different SKU IDs and thus different device trees for
> > them, but we could make the prober work with this. It just has be able
> > to tell if the component it's currently probing needs the special
> > prober and is it responding correctly. This bit I need to think about.
> 
> I guess Mark Brown also thought that there wouldn't be some magic
> register, but my gut still tells me that most i2c devices have some
> way to confirm that they are what you expect even if it's not an
> official "vendor" or "version" register. Some type of predictable
> register at a predictable location that you could use, at least if you
> knew all of the options that someone might stuff.

"most" is way too optimistic to say, I believe that not even close to majority
of I²C target devices they are not reliably discoverable.

That's the downside of non-discoverable busses like I²C. Maybe I³C has
a mechanism for that, but I am not an expert, just wondering.

> For instance, in elan trackpads you can see elan_i2c_get_product_id().
> That just reads a location (ETP_I2C_UNIQUEID_CMD = 0x0101) that could
> theoretically be used to figure out (maybe in conjunction with other
> registers) that it's an elan trackpad instead of an i2c-hid one. You'd
> have to (of course) confirm that an i2c-hid device wouldn't somehow
> return back data from this read that made it look like an elan
> trackpad, but it feels like there ought to be some way to figure it
> out with a few i2c register reads.
> 
> ...that being said, I guess my original assertion that you might be
> able to figure out with a simple register read was naive and you'd
> actually need a function (maybe as a callback) to figure this out.

-- 
With Best Regards,
Andy Shevchenko