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

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Message-ID: <20110824225751.GC19689@legolas.emea.dhcp.ti.com>
Date:	Thu, 25 Aug 2011 01:57:52 +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"

On Wed, Aug 24, 2011 at 10:31:27AM -0400, Alan Stern wrote:
> On Wed, 24 Aug 2011, Michal Nazarewicz wrote:
> 
> > On Tue, 23 Aug 2011 22:49:37 +0200, Alan Stern <stern@...land.harvard.edu>  
> > wrote:
> > > 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.
> > 
> > I'm actually wondering whether we really need to worry about it.
> > composite_driver has the “max_speed” field and I would just leave it as
> > composite driver's author responsibility to put correct value there.
> > 
> > One thing that could be worth doing is iterate over all the configurations
> > and figure out if at least one supports the speed declared by “max_speed”
> > and if not, lower it.
> 
> That's the same as what I said: Use the highest speed supported by any 
> function driver.  There really isn't any point in allowing connections 
> faster than that.  And as Felipe pointed out, the user always has the 
> capability to force the connection to be slower, by using the right 
> sorts of cables and hubs.
> 
> > Like I've said before, as additional functionality, composite.c could
> > check if it's USB_SPEED_UNKWONW in which case it would look for the lowest
> > speed that all the functions support.
> > 
> > Such configuration would allow composite driver authors set the speed to
> > USB_SPEED_SUPER when they mean “choose maximum speed at least one function
> > supports” and to “USB_SPEED_UNKWONW” when they mean “choose maximum speed
> > all the functions support”.
> > 
> > Does that make sense?
> 
> It's reasonable.  Just be sure to document it properly; otherwise 
> nobody will understand or remember it!

good point. I agree with this approach.

-- 
balbi

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