Re: [RFC][PATCH 1/2] usb: gadget: configfs: add MTP function

From: Krzysztof Opasiak
Date: Thu Aug 13 2015 - 16:23:18 EST

On 08/13/2015 09:57 PM, Greg Kroah-Hartman wrote:
On Thu, Aug 13, 2015 at 09:34:46PM +0200, Krzysztof Opasiak wrote:

On 08/13/2015 09:12 PM, Amit Pundir wrote:
his MTP function is based on years of work originally done in the
Android kernel tree by:
Mike Lockwood<lockwood@xxxxxxxxxxx>
Benoit Goby<benoit@xxxxxxxxxxx>
Colin Cross<ccross@xxxxxxxxxxx>
Arve Hjønnevåg<arve@xxxxxxxxxxx>
Peter Oh<poh@xxxxxxxxxxxx>
Greg Hackmann<ghackmann@xxxxxxxxxx>
Badhri Jagan Sridharan<Badhri@xxxxxxxxxx>
I've folded the series up to make it easier to review, and to provide
a coherent patch description.

Post Gingerbread (Android v2.3), Android dropped USB Mass Storage
in favor of Media Transfer Protocal (MTP), which is widely used for
transferring media files to digital music players and similar
applications. This USB gadget function implements MTP functionalty.

Historically this function has been a part of Android composite
gadget driver. Android composite driver was Android's solution
for dynamic gadget function switching prior to the ConfigFS gadget
being merged. There were failed few attempts in past to upstream Android
composite driver as well. Now this Android MTP gadget function has been
re-implemented so as to be used as a generic ConfigFS function instead.

Again, many thanks to Mike, Benoit, Colin, Arve, Peter, Greg and Badhri,
as they are the real authors of this work. However, I've folded their
patches together and modified it enough that I don't want them to be
blamed for any mistakes I've made condensing their patches down.

Cc: Mike Lockwood<lockwood@xxxxxxxxxxx>
Cc: Benoit Goby<benoit@xxxxxxxxxxx>
Cc: Colin Cross<ccross@xxxxxxxxxxx>
Cc: Arve Hjønnevåg<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>

In my humble opinion adding such function to Linux kernel doesn't make any
sense. By design, MTP is a protocol which requires access to userspace
features esp. file system. It is very important to run MTP daemon with
suitable user and LSM label and many many other issues which should be
handled by userspace access policy.

Moreover this is not a fully functional USB function but only some interface
which can be used by mtp-responder (mtp-daemon - call it as you like) to
communicate with host. As we have FunctionFS which allows to implement any
USB function in as a userspace service. As MTP nature is more related to
userspace I think that porting MTP daemon to use this is a right way to go.
This should be much more reasonable than adding new function which also
requires daemon for proper working. So why add another interface while we
can use a generic one?

Isn't there already a userspace MTP daemon that uses the existing
functionfs for usb gadgets? I thought I remember seeing that

I know for sure that ADB and SDB has been ported to use functionfs. I can even see ADB with ffs backend working on my nexus 9 with 3.10 kernel.

I've seen such mtp-responder implementation but that time it has not been published to open source. I don't know what is the current state...

Best regards,

Krzysztof Opasiak
Samsung R&D Institute Poland
Samsung Electronics
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