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

[Alan Stern]

On Wed, 24 Oct 2012, Stephen Warren wrote:

> On 10/24/2012 09:26 AM, Sebastian Andrzej Siewior wrote:
> > On Wed, Oct 24, 2012 at 10:57:00AM -0400, Alan Stern wrote:
> >> Under the circumstances, do we really need a new binding document for 
> >> the ehci-platform driver?
> 
> It seems reasonable to add the new properties to usb-ehci.txt, since
> they do describe the HW.
> 
> >> We should be able to use the existing 
> >> usb-ehci binding, perhaps with some new properties added:
> >>
> >> 	has-synopsys-hc-bug
> >> 	no-io-watchdog
> >> 	has-tt
> 
> That sounds fine to me.
> 
> However, there is an implementation issue here. I believe the way Linux
> searches for a driver for a particular node is:
> 
> for every driver that's registered
>     if the driver's supported compatible list matches the device
>         use the driver
> 
> (See drivers/base/platform.c:platform_match() which implements the if
> statement above, and I assume the driver core implements the outer for
> loop above)

Yes, it does.

> That means that if the generic driver supports compatible="usb-ehci", it
> may "steal" device nodes that have
> compatible="something-custom","usb-ehci", even if there's a driver
> specifically for "something-custom". We would need to re-arrange the
> driver matching code to:
> 
> for each compatible value in the node:
>     for each driver that's registered:
>         if the driver supports the compatible value:
>             use the driver.

Which might be difficult since the inner loop would be controlled by
the outer code in the driver core.

How do we determine which existing drivers claim to support usb-ehci?  
A quick search under arch/ and drivers/ turns up nothing but
drivers/usb/host/ehci-ppc-of.c.  Changing it to a more HW-specific
match should be easy enough, and then "usb-ehci" would be safe to use
in ehci-platform.c.

Alan Stern