Date:	Fri, 15 Mar 2013 08:02:43 +0100
From:	Bjørn Mork <bjorn@...k.no>
To:	Ben Hutchings <ben@...adent.org.uk>
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

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?

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.

(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...)


Bjørn
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