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

[Krzysztof Opasiak <k.opasiak-Sze3O3UU22JBDgjK7y7TUQ@public.gmane.org>]

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-z5hGa2qSFaRBDgjK7y7TUQ@public.gmane.org>
>>>      Benoit Goby<benoit-z5hGa2qSFaRBDgjK7y7TUQ@public.gmane.org>
>>>      Colin Cross<ccross-z5hGa2qSFaRBDgjK7y7TUQ@public.gmane.org>
>>>      Arve Hjønnevåg<arve-z5hGa2qSFaRBDgjK7y7TUQ@public.gmane.org>
>>>      Peter Oh<poh-dY08KVG/lbpWk0Htik3J/w@public.gmane.org>
>>>      Greg Hackmann<ghackmann-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
>>>      Badhri Jagan Sridharan<Badhri-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
>>> 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-z5hGa2qSFaRBDgjK7y7TUQ@public.gmane.org>
>>> Cc: Benoit Goby<benoit-z5hGa2qSFaRBDgjK7y7TUQ@public.gmane.org>
>>> Cc: Colin Cross<ccross-z5hGa2qSFaRBDgjK7y7TUQ@public.gmane.org>
>>> Cc: Arve Hjønnevåg<arve-z5hGa2qSFaRBDgjK7y7TUQ@public.gmane.org>
>>> Cc: Peter Oh<poh-dY08KVG/lbpWk0Htik3J/w@public.gmane.org>
>>> Cc: Greg Hackmann<ghackmann-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
>>> Cc: Badhri Jagan Sridharan<Badhri-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
>>> Cc: Android Kernel Team<kernel-team-z5hGa2qSFaRBDgjK7y7TUQ@public.gmane.org>
>>> Cc: Greg Kroah-Hartman<gregkh-hQyY1W1yCW8ekmWlsbkhG0B+6BGkLq7r@public.gmane.org>
>>> Cc: Jonathan Corbet<corbet-T1hC0tSOHrs@public.gmane.org>
>>> Cc: Felipe Balbi<balbi-l0cyMroinI0@public.gmane.org>
>>> Cc: Andrzej Pietrasiewicz<andrzej.p-Sze3O3UU22JBDgjK7y7TUQ@public.gmane.org>
>>> Cc: Laurent Pinchart<laurent.pinchart-ryLnwIuWjnjg/C1BVhZhaw@public.gmane.org>
>>> Cc: Yegor Yefremov<yegorslists-gM/Ye1E23mwN+BqQ9rBEUg@public.gmane.org>
>>> Cc: Philippe Reynes<tremyfr-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
>>> Cc: John Stultz<john.stultz-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
>>> Cc: Sumit Semwal<sumit.semwal-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
>>> Signed-off-by: Amit Pundir<amit.pundir-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
>>
>> 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