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

On Tue, 23 Oct 2012, Stephen Warren wrote:

quoted

Nothing intrinsically distinguishes this class of hardware.  The only
thing these devices have in common is that they can be managed by
Linux's ehci-platform driver.

I don't agree. They're all EHCI USB controllers (or all EHCI USB
controllers that don't require custom drivers due to quirks), which is a
much more general statement than anything related to which driver Linux
uses for them.

That's true, but it doesn't distinguish these devices.  There are other
EHCI controllers with no quirks that nevertheless can't be handled by
ehci-platform because they have requirements (_not_ quirks) that
ehci-platform is unable to deal with currently -- clocks or power
supplies or special register settings or PHYs or etc.

quoted

In essense, we're trying to say that the HW is compatible with a 
particular driver rather than with some other type of HW.

Using a compatible value in order to pick a specific driver in Linux is
explicitly against the device tree design principles; DT is supposed to
represent just the hardware.

quoted

Maybe DT
wasn't intended to provide this kind of information, but it seems like 
a logical thing to do.  Unless DT already offers some other way to 
accomplish the same thing?

The typical way is for the Linux driver to declare that is supports a
variety of different HW models.

Admittedly this is annoying when there are many HW models that otherwise
wouldn't need any driver changes, hence the desire to (additionally)
include some generic value in the compatible field, but again, what that
generic value is should be driven by the HW itself, not the SW that's
using it.

Okay, fine.  But then what should the binding documentation specify for
the compatible value?  A list of all the HW models that the driver
currently supports?

Alan Stern

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