Date:	Sat, 16 Mar 2013 17:51:21 +0000
From:	Ben Hutchings <ben@...adent.org.uk>
To:	Bjørn Mork <bjorn@...k.no>
Cc:	netdev@...r.kernel.org, linux-usb@...r.kernel.org,
	Greg Suarez <gsuarez@...thmicro.com>,
	Alexey Orishko <alexey.orishko@...ricsson.com>
Subject: Re: [PATCH net,stable-3.8] net: cdc_ncm, cdc_mbim: allow user to
 prefer NCM for backwards compatibility

On Fri, 2013-03-15 at 08:02 +0100, Bjørn Mork wrote:
> Ben Hutchings <ben@...adent.org.uk> writes:
> 
> > On Thu, 2013-03-14 at 12:05 +0100, Bjørn Mork wrote:
> >> commit bd329e1 ("net: cdc_ncm: do not bind to NCM compatible MBIM devices")
> >> introduced a new policy, preferring MBIM for dual NCM/MBIM functions if
> >> the cdc_mbim driver was enabled.  This caused a regression for users
> >> wanting to use NCM.
> >> 
> >> Devices implementing NCM backwards compatibility according to section
> >> 3.2 of the MBIM v1.0 specification allow either NCM or MBIM on a single
> >> USB function, using different altsettings.  The cdc_ncm and cdc_mbim
> >> drivers will both probe such functions, and must agree on a common
> >> policy for selecting either MBIM or NCM.  Until now, this policy has
> >> been set at build time based on CONFIG_USB_NET_CDC_MBIM.
> >> 
> >> Use a module parameter to set the system policy at runtime, allowing the
> >> user to prefer NCM on systems with the cdc_mbim driver.
> >> 
> >> Cc: Greg Suarez <gsuarez@...thmicro.com>
> >> Cc: Alexey Orishko <alexey.orishko@...ricsson.com>
> >> Reported-by: Geir Haatveit <nospam@...tveit.nu>
> >> Reported-by: Tommi Kyntola <kynde@...ray.fi>
> >> Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=54791
> >> Signed-off-by: Bjørn Mork <bjorn@...k.no>
> >> ---
> >> 
> >> We now have two users independently reporting this as a 3.8 regression,
> >> so something needs to be done.  I am not sure if adding a new module
> >> parameter is acceptable for stable, but this problem is definitely a
> >> regression and no other solutions came up in response to my RFC.
> >> 
> >> The only real alternative I see for stable, is disabling MBIM support
> >> on any dual NCM/MBIM function.  Which of course will be a regression
> >> for any user wanting MBIM, making it unacceptable.
> > [...]
> >
> > It definitely makes sense for this to be a run-time parameter.  And the
> > default seems correct for custom kernels.
> >
> > For a distribution kernel - at least for Debian, where we can't assume
> > kernel and userland are always updated together - I think the
> > compile-time default should be false, and the userland package
> > (presumably ModemManager?) can install a modprobe.conf file to override
> > that once it can handle MBIM.  We handled KMS transitions in a similar
> > way.  I don't know that it's worth having a Kconfig option for that,
> > though.
> 
> We could consider always defaulting to NCM.  That would remove the
> ugliest parts of the code as well.  Should I send a new version with
> such a change?

We could, but I fear that would annoy people building custom kernels.

> Bundling a modprobe.conf file to enable MBIM with userland tools
> supporting it is a great idea, and sounds feasible for custom built
> systems as well.

Using unpatched Linux & ModemManager (or other userland) needs to just
work.  So if Linux is going to have MBIM disabled by default then
upstream ModemManager needs to ship and install that modprobe.conf file.

> (BTW, if there is any doubt about it, I feel really bad about my recent
> request to enable MBIM in Debian, where I wrote "enabling the driver
> will not prevent any existing solution from working.".  At the time, I
> had not seen any of these dual NCM/MBIM functions and I just didn't
> think about the problems such devices would run into. Sorry about
> that. I guess I owe you one more...)

It's called experimental for a reason. :-)

Ben.

-- 
Ben Hutchings
It is easier to change the specification to fit the program than vice versa.

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