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 [off-list ref] wrote:

...

quoted

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