linux-kernel - Re: RFC: Platform data for onboard USB assets

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Date:	Fri, 11 Mar 2011 14:12:10 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	andy.green@...aro.org
cc:	Greg KH <greg@...ah.com>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Arnd Bergmann <arnd@...db.de>,
	Linux USB list <linux-usb@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: RFC: Platform data for onboard USB assets

On Fri, 11 Mar 2011, Andy Green wrote:

> I don't believe I referred to class devices anywhere.  It does not 
> matter if the main chip function is class device or not.

It matters because the class specification for a USB device is never 
going to mention information sources that are outside the USB protocol, 
such as board definitions.  Consequently a class driver will never need 
to use such a thing.

> If there is any kind of "functional implementation" knowledge that is 
> outside the chip and driver itself, it has to be held somewhere, and 
> applied appropriately.  platform_data from the board definition file is 
> the established place for that knowledge that is specific to a board.

Since essentially all of the USB drivers currently in the kernel _are_
class drivers (at least, I'm not aware of any non-trivial exceptions),
this means none of the existing USB drivers should need to access any
platform data.

Of course, this doesn't rule out the possibility of platform-specific 
USB drivers that _do_ need this information.

> > Also, do you have a real example of a USB driver today that needs this?
> 
> I think you find without devpath -> platform_data mapping, the kind of 
> layout given above is made quite difficult to support in Linux.

What would be needed to support such a mapping?  It seems to me that we 
probably have all the necessary ingredients in place already.

Alan Stern

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