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

On Wed, 24 Oct 2012, Stephen Warren wrote:

On 10/24/2012 09:26 AM, Sebastian Andrzej Siewior wrote:

quoted

On Wed, Oct 24, 2012 at 10:57:00AM -0400, Alan Stern wrote:

quoted

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.

quoted

quoted

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

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