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

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Date:	Thu, 13 Aug 2015 12:57:48 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Krzysztof Opasiak <k.opasiak@...sung.com>
Cc:	Amit Pundir <amit.pundir@...aro.org>, linux-usb@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-api@...r.kernel.org, Mike Lockwood <lockwood@...roid.com>,
	Benoit Goby <benoit@...roid.com>,
	Colin Cross <ccross@...roid.com>,
	Arve Hjønnevåg <arve@...roid.com>,
	Peter Oh <poh@...adcom.com>,
	Greg Hackmann <ghackmann@...gle.com>,
	Badhri Jagan Sridharan <Badhri@...gle.com>,
	Android Kernel Team <kernel-team@...roid.com>,
	Jonathan Corbet <corbet@....net>, Felipe Balbi <balbi@...com>,
	Andrzej Pietrasiewicz <andrzej.p@...sung.com>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Yegor Yefremov <yegorslists@...glemail.com>,
	Philippe Reynes <tremyfr@...il.com>,
	John Stultz <john.stultz@...aro.org>,
	Sumit Semwal <sumit.semwal@...aro.org>
Subject: Re: [RFC][PATCH 1/2] usb: gadget: configfs: add MTP function

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@...roid.com>
> >     Benoit Goby<benoit@...roid.com>
> >     Colin Cross<ccross@...roid.com>
> >     Arve Hjønnevåg<arve@...roid.com>
> >     Peter Oh<poh@...adcom.com>
> >     Greg Hackmann<ghackmann@...gle.com>
> >     Badhri Jagan Sridharan<Badhri@...gle.com>
> >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@...roid.com>
> >Cc: Benoit Goby<benoit@...roid.com>
> >Cc: Colin Cross<ccross@...roid.com>
> >Cc: Arve Hjønnevåg<arve@...roid.com>
> >Cc: Peter Oh<poh@...adcom.com>
> >Cc: Greg Hackmann<ghackmann@...gle.com>
> >Cc: Badhri Jagan Sridharan<Badhri@...gle.com>
> >Cc: Android Kernel Team<kernel-team@...roid.com>
> >Cc: Greg Kroah-Hartman<gregkh@...uxfoundation.org>
> >Cc: Jonathan Corbet<corbet@....net>
> >Cc: Felipe Balbi<balbi@...com>
> >Cc: Andrzej Pietrasiewicz<andrzej.p@...sung.com>
> >Cc: Laurent Pinchart<laurent.pinchart@...asonboard.com>
> >Cc: Yegor Yefremov<yegorslists@...glemail.com>
> >Cc: Philippe Reynes<tremyfr@...il.com>
> >Cc: John Stultz<john.stultz@...aro.org>
> >Cc: Sumit Semwal<sumit.semwal@...aro.org>
> >Signed-off-by: Amit Pundir<amit.pundir@...aro.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...

thanks,

greg k-h
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