linux-kernel - Re: [linux-usb-devel] [PATCH] : Allow embedded developers USB options normally reserved for OTG

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Date:	Thu, 3 Jan 2008 11:52:27 +0800
From:	"Bryan Wu" <cooloney.lkml@...il.com>
To:	"Alan Stern" <stern@...land.harvard.edu>
Cc:	"David Brownell" <david-b@...bell.net>,
	"Mike Frysinger" <vapier.adi@...il.com>, gregkh@...e.de,
	linux-usb-devel@...ts.sourceforge.net,
	"Robin Getz" <rgetz@...ckfin.uclinux.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [linux-usb-devel] [PATCH] : Allow embedded developers USB options normally reserved for OTG

On Jan 3, 2008 4:58 AM, Alan Stern <stern@...land.harvard.edu> wrote:
> On Wed, 2 Jan 2008, David Brownell wrote:
>
> > On Wednesday 02 January 2008, Alan Stern wrote:
> > > On Wed, 2 Jan 2008, Mike Frysinger wrote:
> > >
> > > > perhaps the code size is arguable as to whether it really matters.
> > > > the reason we want it is that we have a USB host controller that will
> > > > not work with USB hubs, so we want to make sure the system does not
> > > > attempt such things.  (yes, such a USB host controller is retarded,
> > > > but the decision was out of our hands.)
> > >
> > > Just out of curiosity, how does a host controller manage to avoid
> > > working with external hubs?
> >
> > The transaction translators in external high speed hubs require
> > hosts to issue particular USB transactions.  If the host controller
> > doesn't implement the that split transaction support, then it won't
> > be supporting external hubs.
>
> So in theory one could connect a high-speed hub to such a host
> controller and expect it to communicate with high-speed devices.  So
> long as no full- or low-speed devices are added there wouldn't be any
> split transactions.  It wouldn't be USB-2.0 compliant but it should
> still work.
>

Hmmm, basically, I think the answer is yes.

But when you tell customers your devices support USB 2.0, they will
try to plug-in lots of USB devices that you can not even imagine.
If they plug-in a combo USB device including an external USB hub, the
whole embedded Linux system maybe crash or hang there.
So this patch is to refuse enumerate such unsupported USB devices.

-Bryan Wu
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