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

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Message-ID: <20110311164850.GT1760@opensource.wolfsonmicro.com>
Date:	Fri, 11 Mar 2011 16:48:50 +0000
From:	Mark Brown <broonie@...nsource.wolfsonmicro.com>
To:	Greg KH <greg@...ah.com>
Cc:	Arnd Bergmann <arnd@...db.de>, andy.green@...aro.org,
	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, Mar 11, 2011 at 08:35:22AM -0800, Greg KH wrote:
> On Fri, Mar 11, 2011 at 04:27:59PM +0000, Mark Brown wrote:

> > I'm actually not that fussed about the MAC address use case itself and
> > do tend to agree with you that that's handlable in userspace but for the
> > other things that might need to be configured there's a lot of things
> > that for non-discoverable buses we're currently passing through platform
> > data so switching to bouncing things through userspace would itself be a
> > substantial change from other systems.

> But USB _is_ discoverable, that's my point.

USB itself is discoverable but the when the USB bus you're looking at is
one that's soldered down onto a board in a specific design all bets are
off regarding how complete the information you get will be.  On a basic
level the designers may have done things like omit the configuration
EEPROMs that would set the device IDs that the driver should be relying
on to identify the hardware configuration.  There may be other, nastier,
things going on.

> > You can't in general rely on the system being neatly abstracted and
> > while discoverable buses do avoid many problems in this sort of area
> > there are still things which can only be discovered through reference to
> > the schematics or similar, especially if you care about the microamps.

> Again, that's not USB, so it isn't relevant here.

You really can't make this assumption about discoverable buses on
embedded devices.  The discoverability will get you most of the way
there but not always all of the way there.
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