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

On 14 August 2015 at 02:11, Krzysztof Opasiak [off-list ref] wrote:

On 08/13/2015 09:57 PM, Greg Kroah-Hartman wrote:

quoted

On Thu, Aug 13, 2015 at 09:34:46PM +0200, Krzysztof Opasiak wrote:

quoted

Hello,

On 08/13/2015 09:12 PM, Amit Pundir wrote:

quoted

his MTP function is based on years of work originally done in the
Android kernel tree by:
     Mike Lockwood[off-list ref]
     Benoit Goby[off-list ref]
     Colin Cross[off-list ref]
     Arve Hjønnevåg[off-list ref]
     Peter Oh[off-list ref]
     Greg Hackmann[off-list ref]
     Badhri Jagan Sridharan[off-list ref]
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
http://marc.info/?l=linux-usb&m=132451695808552  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<redacted>
Cc: Benoit Goby<redacted>
Cc: Colin Cross<redacted>
Cc: Arve Hjønnevåg<redacted>
Cc: Peter Oh<poh-dY08KVG/lbpWk0Htik3J/w@public.gmane.org>
Cc: Greg Hackmann<redacted>
Cc: Badhri Jagan Sridharan<redacted>
Cc: Android Kernel Team<redacted>
Cc: Greg Kroah-Hartman<redacted>
Cc: Jonathan Corbet<redacted>
Cc: Felipe Balbi<redacted>
Cc: Andrzej Pietrasiewicz<redacted>
Cc: Laurent Pinchart<laurent.pinchart-ryLnwIuWjnjg/C1BVhZhaw@public.gmane.org>
Cc: Yegor Yefremov<yegorslists-gM/Ye1E23mwN+BqQ9rBEUg@public.gmane.org>
Cc: Philippe Reynes<redacted>
Cc: John Stultz<redacted>
Cc: Sumit Semwal<redacted>
Signed-off-by: Amit Pundir<redacted>

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?

Fairly valid point. I did see MTP mentioned in FunctionFS context in
Documentation/usb/functionfs.txt but I could not find an open
userpsace MTP daemon implementation based on F_FS to try.

quoted

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

I've found some interesting link[2] which may mean that Sailfish OS guys has
some mtp implementation with functionfs backend:

<<<<< cite

- /dev/mtp
mtp functionfs rw,relatime

quoted

quoted

quoted

quoted

quoted

cite

Started digging and got it!

This looks like mtp with ffs backend:

https://github.com/nemomobile/buteo-mtp

Thanks for the pointer. This "buteo-mtp" looks interesting.

Regards,
Amit Pundir

Didn't tested, even didn't try to compile, no guarantee;)

Footnotes:
1 -
http://reviewjolla.blogspot.com/2014/06/techspecs-android-on-jolla-phone.html


Best regards,

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