linux-kernel - Re: [PATCH] USB: storage: fix Huawei mode switching regression

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Date:	Tue, 05 Mar 2013 12:52:39 +0100
From:	Oliver Neukum <oneukum@...e.de>
To:	Bjørn Mork <bjorn@...k.no>
Cc:	"Fangxiaozhi (Franko)" <fangxiaozhi@...wei.com>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Xueguiying (Zihan)" <zihan.xue@...wei.com>,
	"Linlei (Lei Lin)" <lei.lin@...wei.com>,
	Greg KH <gregkh@...uxfoundation.org>,
	"Yili (Neil)" <neil.yi@...wei.com>,
	"Wangyuhua (Roger, Credit)" <wangyuhua@...wei.com>,
	"Huqiao (C)" <huqiao36@...wei.com>, "balbi@...com" <balbi@...com>,
	"mdharm-usb@...-eyed-alien.net" <mdharm-usb@...-eyed-alien.net>,
	"sebastian@...akpoint.cc" <sebastian@...akpoint.cc>,
	stable <stable@...r.kernel.org>
Subject: Re: [PATCH] USB: storage: fix Huawei mode switching regression

On Tuesday 05 March 2013 11:07:05 Bjørn Mork wrote:
> "Fangxiaozhi (Franko)" <fangxiaozhi@...wei.com> writes:
> 
> > ------ commit 200e0d99 and commit cd060956, only put the switch
> > command into kernel, instead of userspace usb_modeswitch utility.
> 
> Yes. And that is the problem.  It was agreed years ago that this
> functionality belongs in userspace.  Ref e.g
> https://lkml.org/lkml/2009/12/15/332
> https://lkml.org/lkml/2010/4/19/216
> https://lkml.org/lkml/2012/2/28/569
> 
> Please note the re-occurrence of this discussion, despite the fact that
> Matthew's message from 2009 should be quite clear.

Since 2009 the facts have been changing. Basically we are encountering
two trends

1. Power save has become more important
2. We are seeing devices that are switchable, yet include interfaces other
    than communication and virtual storage, foremost real storage in the
    form of a microSD-reader

Whenever we cut power to a switchable device it reverts to the power-on
state. That involves not only the communication functionality (which we
could live with, as it cannot handle loss of power anyway) but also other
interfaces. In addition we need to deal with resets which may or may not
make the device revert to power-on

This is a basic problem of the design. If you want reset_resume() for power
loss to work you need to do it in kernel space for the same reason we restore
the configuration in kernel space before we rebind the drivers and before
we thaw user space.

However, this argues not for doing the switch simply in the storage driver
but to switch in the core.

	Regards
		Oliver

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