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

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Date:	Fri, 11 Mar 2011 08:56:42 -0800
From:	Greg KH <greg@...ah.com>
To:	Mark Brown <broonie@...nsource.wolfsonmicro.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 04:48:50PM +0000, Mark Brown wrote:
> 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.

Then you use the existing platform data for your USB host controller
driver.  Doesn't that work today just fine?

> > > 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.

Then the bus is not really USB, sorry.  USB is discoverable, _and_ can
support enumeration in non-deterministic ways.  If people are using it
in other ways then it is not USB and is something else.

thanks,

greg k-h
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