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

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Message-ID: <20110823171942.GC10996@legolas.emea.dhcp.ti.com>
Date:	Tue, 23 Aug 2011 20:19:43 +0300
From:	Felipe Balbi <balbi@...com>
To:	Michal Nazarewicz <mnazarewicz@...gle.com>
Cc:	Felipe Balbi <balbi@...com>,
	Alan Stern <stern@...land.harvard.edu>,
	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 05:26:24PM +0200, Michal Nazarewicz wrote:
> >On Tue, Aug 23, 2011 at 05:07:11PM +0200, Michal Nazarewicz wrote:
> >>If you look at config_buf() in composite.c, you'll notice that composite
> >>just skips functions that do not support current speed.  This
> >>means, that if a connection uses high speed but given function
> >>can handle only
> >>full speed, it won't be included in the descriptors.
> >>
> >>config_buf() is called from config_desc() which similarly, iterates over
> >>configurations skipping the ones that do not support given speed.
> >>
> >>Configuration is defined to support given speed if at least one function
> >>that is added to that configuration supports that speed (see
> >>usb_add_function()).
> 
> On Tue, 23 Aug 2011 17:11:06 +0200, Felipe Balbi <balbi@...com> wrote:
> >I'm trying to think if this is really what we want. This might mean
> >$current_config will be different depending if we connect it to SS, HS
> >or FS hubs.
> 
> What do you mean by $current_config?  The bConfigurationValue will be

the currently chosen configuration.

> the same for "the same" configuration regardless of speed but different
> configurations may be available for different speeds and "the same"
> configuration in various speeds might have different functions.
> 
> So just for the sake of having something concrete, let's say we have:
> 
> Configuration #1 with
>   function A supporting FS and HS and

my question is what happens with the HS support on function A
(descriptors, endpoints, etc) when you connect it to a HS hub or to a FS
hub.

>   function B supporting FS only.
> Configuration #2 with
>   function B supporting FS only.
> Configuration #3 with
>   function A supporting FS and HS and
>   function C supporting HS only (dunno if that makes sense).
> 
> When connected to FS hub, we will return the following:
> 
> Configuration #1: A, B
> Configuration #2: B
> Configuration #3: A
> 
> When connect to HS hub, we will return the following:
> 
> Configuration #1: A

See, I'm not sure this is allowed by USB spec. But I could be wrong. (I
mean having a function disappear)

-- 
balbi

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