[PATCH v2 2/2] USB: doc: Binding document for ehci-platform driver

On 10/25/2012 04:23 AM, Sebastian Andrzej Siewior wrote:

On Wed, Oct 24, 2012 at 08:55:27AM -1000, Mitch Bradley wrote:

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So by listing "usb-ehci" in its device ID table, a driver would
essentially be saying that it can handle _all_ USB EHCI controllers.  

Actually, it means that the driver can handle at least USB EHCI
controllers that are 100% compatible with the EHCI spec (requiring
nothing extra).  The driver might be able to handle almost-compatible
controllers, possibly with the help of additional properties.

If a DT node lists "usb-ehci" as a "fallback", it's not guaranteed that
a given version of that driver will work, but it's worth a try in the
event that no more-specific driver is matched.

Not sure fallback is a good term here. The of parses the compatible from left
to right. If the device specific entry is not found (in the driver) then end
up with usb-ehci. If we need a quirk later on we add the device specific entry
to the driver (which will match before usb-ehci is found) and we could use
this entry to apply the quirk. That way you can apply quirks without touch the
firmware / device tree.

That's the theory on how the compatible values are used, but in
practice, each driver is tested against all the values in a node before
moving on to the next driver, so the (current kernel's implementation of
the) matching order is not what's needed to make that work.