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

[Krzysztof Opasiak]

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

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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