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

From: Krzysztof Opasiak
Date: Thu Aug 13 2015 - 15:42:29 EST


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@xxxxxxxxxx> 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@xxxxxxxxxxx>
Cc: Benoit Goby<benoit@xxxxxxxxxxx>
Cc: Colin Cross<ccross@xxxxxxxxxxx>
Cc: Arve HjÃnnevÃg<arve@xxxxxxxxxxx>
Cc: Peter Oh<poh@xxxxxxxxxxxx>
Cc: Greg Hackmann<ghackmann@xxxxxxxxxx>
Cc: Badhri Jagan Sridharan<Badhri@xxxxxxxxxx>
Cc: Android Kernel Team<kernel-team@xxxxxxxxxxx>
Cc: Greg Kroah-Hartman<gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Jonathan Corbet<corbet@xxxxxxx>
Cc: Felipe Balbi<balbi@xxxxxx>
Cc: Andrzej Pietrasiewicz<andrzej.p@xxxxxxxxxxx>
Cc: Laurent Pinchart<laurent.pinchart@xxxxxxxxxxxxxxxx>
Cc: Yegor Yefremov<yegorslists@xxxxxxxxxxxxxx>
Cc: Philippe Reynes<tremyfr@xxxxxxxxx>
Cc: John Stultz<john.stultz@xxxxxxxxxx>
Cc: Sumit Semwal<sumit.semwal@xxxxxxxxxx>
Signed-off-by: Amit Pundir<amit.pundir@xxxxxxxxxx>

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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