linux-kernel - Re: [PATCHv3 2/4] usb: gadget: replace "is_dualspeed" with "max

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Message-ID: <20110824085615.GD12549@legolas.emea.dhcp.ti.com>
Date:	Wed, 24 Aug 2011 11:56:16 +0300
From:	Felipe Balbi <balbi@...com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Michal Nazarewicz <mnazarewicz@...gle.com>,
	Felipe Balbi <balbi@...com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Yang Rui Rui <ruirui.r.yang@...to.com>,
	Dave Young <hidave.darkstar@...il.com>,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv3 2/4] usb: gadget: replace "is_dualspeed" with
 "max_speed"

Hi,

On Tue, Aug 23, 2011 at 04:49:37PM -0400, Alan Stern wrote:
> On Tue, 23 Aug 2011, Michal Nazarewicz wrote:
> 
> > On Tue, 23 Aug 2011 20:00:25 +0200, Alan Stern <stern@...land.harvard.edu>  
> > wrote:
> > > For the time being, perhaps the best answer is to use the _lowest_
> > > max_speed value among all the function drivers.  It's the simplest
> > > answer and most likely to be correct most of the time.
> > 
> > In theory, one could create a composite function with only SS/HS
> > descriptors.
> 
> The only reason for doing that would be if the function was incapable
> of carrying out its duties when running at full speed (insufficient
> bandwidth, for example).  Something like this is discussed briefly in 
> section 9.2.6.6 of the USB-2 spec.
> 
> >  So it would work for SS/HS but not for FS.  I'm not
> > saying that it would make sense (or that it's not a bug) but that
> > adds to the discussion.
> 
> If somebody writes a composite gadget containing two functions, one of 
> which supports only full speed and the other only high speed, they'll 
> get what they deserve.  :-)
> 
> Still, maybe I was wrong.  Maybe it would be better to use the fastest
> speed supported by at least one of the function drivers.  The user can
> always force a SuperSpeed-capable device to run at high speed by using
> a USB-2 cable to plug it in.  I'm not sure whether the user could force
> such a device to run at full speed, however.

plug it behind a full speed hub, like those that usually come on USB
keyboards.

-- 
balbi

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