Ben Hutchings <ben@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx>
>> Cc: Alexey Orishko <alexey.orishko@xxxxxxxxxxxxxx>
>> Reported-by: Geir Haatveit <nospam@xxxxxxxxxxx>
>> Reported-by: Tommi Kyntola <kynde@xxxxxxxxx>
>> Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=54791
>> Signed-off-by: Bjørn Mork <bjorn@xxxxxxx>
>> ---
>>
>> 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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