Re: [PATCH net,stable-3.8] net: cdc_ncm, cdc_mbim: allow user to prefer NCM for backwards compatibility

[Ben Hutchings <ben-/+tVBieCtBitmTQ+vhA3Yw@public.gmane.org>]

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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-AKjrjAf1O7qe8kRwQpwjMg@public.gmane.org>
> Cc: Alexey Orishko <alexey.orishko-0IS4wlFg1OjSUeElwK9/Pw@public.gmane.org>
> Reported-by: Geir Haatveit <nospam-br728iSPf5UsCylrc8G9yg@public.gmane.org>
> Reported-by: Tommi Kyntola <kynde-oSY6wnT0nbj1KXRcyAk9cg@public.gmane.org>
> Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=54791
> Signed-off-by: Bjørn Mork <bjorn-yOkvZcmFvRU@public.gmane.org>
> ---
> 
> 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.

Ben.

-- 
Ben Hutchings
Humans are not rational beings; they are rationalising beings.

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