linux-kernel - Re: [RFC][PATCH 2/2] usb: gadget: configfs: notify userspace of usb state changes

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Date:	Thu, 13 Aug 2015 21:42:17 +0200
From:	Krzysztof Opasiak <k.opasiak@...sung.com>
To:	Amit Pundir <amit.pundir@...aro.org>, linux-usb@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-api@...r.kernel.org
Cc:	Mike Lockwood <lockwood@...roid.com>,
	Benoit Goby <benoit@...roid.com>,
	Colin Cross <ccross@...roid.com>,
	Arve Hjønnevåg <arve@...roid.com>,
	Peter Oh <poh@...adcom.com>,
	Greg Hackmann <ghackmann@...gle.com>,
	Badhri Jagan Sridharan <Badhri@...gle.com>,
	Android Kernel Team <kernel-team@...roid.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jonathan Corbet <corbet@....net>, Felipe Balbi <balbi@...com>,
	Andrzej Pietrasiewicz <andrzej.p@...sung.com>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Yegor Yefremov <yegorslists@...glemail.com>,
	Philippe Reynes <tremyfr@...il.com>,
	John Stultz <john.stultz@...aro.org>,
	Sumit Semwal <sumit.semwal@...aro.org>
Subject: Re: [RFC][PATCH 2/2] usb: gadget: configfs: notify userspace of usb
 state changes

Hello,

On 08/13/2015 09:12 PM, Amit Pundir wrote:
> This is more of an RFC than an actual submission. There are few
> scattered #ifdefs..#endifs here and there which still need to be
> taken care of before going for actual submission.
>
> Currently there is no way with the upstream ConfigFS gadget to
> communicate state changes (connected, disconnected, configured), at
> the gadget level. Instead such state changes are handled function by
> function independently I presume. This is problematic, because some
> coordination between the functions, across the state changes, may be
> desired at the userspace level. Thus to address this issue, this
> patch send uevents to allow userspace to be notified of these usb
> state changes, allowing userspace to respond and configure the
> configfs gadget appropriately.
>
> This patch is based on an Android patchset originaly authored by
> Badhri Jagan Sridharan<Badhri@...gle.com>  to send uevent notifications
> to Android userpace for USB state changes. I've folded his patches
> together and modified it enough that I don't want him to be blamed for
> any mistakes I've made condensing his patches down.
>
> This patch introduces USB_CONFIGFS_UEVENT Kconfig to handle userspace
> notifications of usb state changes, and add setup and disconnect
> functions to intercept the setup requests from the usb_core. It also
> creates a sysfs device class entry and a device attribute (state) to
> read and respond to gadget's current state from userspace. As of now
> this sysfs device class (/sys/class/android_usb) and gadget device
> (/sys/class/android_usb/android0) with state attribute
> (/sys/class/android_usb/android0/state) are strictly tied up to
> facilitate Android userspace requests. But going forward we may want
> to bring all function devices (hid, printer etc) under a unified usb
> gadget device class e.g. /sys/class/usb_gadget/g_{func0,func1} etc..
>
> Also I think it make sense to add this state attribute to the configfs
> usb gadget itself i.e. have something like /config/usb_gadget/g1/state
> to read USB gadget's current state. Since it is going to be consistent
> throughout all the functions tied up to that gadget.
>
> Again this is just an initial RFC, thoughts and feedback would be
> greatly appreciated.
>
> Cc: Mike Lockwood<lockwood@...roid.com>
> Cc: Benoit Goby<benoit@...roid.com>
> Cc: Colin Cross<ccross@...roid.com>
> Cc: Arve Hjønnevåg<arve@...roid.com>
> Cc: Peter Oh<poh@...adcom.com>
> Cc: Greg Hackmann<ghackmann@...gle.com>
> Cc: Badhri Jagan Sridharan<Badhri@...gle.com>
> Cc: Android Kernel Team<kernel-team@...roid.com>
> Cc: Greg Kroah-Hartman<gregkh@...uxfoundation.org>
> Cc: Jonathan Corbet<corbet@....net>
> Cc: Felipe Balbi<balbi@...com>
> Cc: Andrzej Pietrasiewicz<andrzej.p@...sung.com>
> Cc: Laurent Pinchart<laurent.pinchart@...asonboard.com>
> Cc: Yegor Yefremov<yegorslists@...glemail.com>
> Cc: Philippe Reynes<tremyfr@...il.com>
> Cc: John Stultz<john.stultz@...aro.org>
> Cc: Sumit Semwal<sumit.semwal@...aro.org>
> Signed-off-by: Amit Pundir<amit.pundir@...aro.org>

Generally I agree that there should be some way of notifying userspace 
about gadget state but I'm not sure if this is proper way to go. In my 
opinion gadget-bus which has been discussed some time ago on linux-usb 
makes much more sense than this.

Maybe I will be wrong but I guess that you are adding this feature to 
make android mtp-responder working properly in Linux (not android). 
Again if we use FFS instead of adding mtp function we don't need this 
notification as mtp-responder can get all the required informations 
about gadget (function) via ep0 using functionfs events.

Best regards,

-- 
Krzysztof Opasiak
Samsung R&D Institute Poland
Samsung Electronics
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