netdev - Re: [PATCH net] net: cdc_ncm: workaround for missing CDC Union

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Date:	Fri, 18 Jan 2013 22:17:03 +0100
From:	Bjørn Mork <bjorn@...k.no>
To:	Oliver Neukum <oliver@...kum.org>
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] net: cdc_ncm: workaround for missing CDC Union

Oliver Neukum <oliver@...kum.org> writes:
> On Friday 18 January 2013 15:25:47 Bjørn Mork wrote:
>> Adding support for the MBIM mode in some Sierra Wireless devices.
>> 
>> Some Sierra Wireless firmwares support CDC MBIM but have no CDC
>> Union funtional descriptor. This violates the MBIM specification,
>> but we can easily work around the bug by looking at the Interface
>> Association Descriptor instead.  This is most likely what
>> Windows uses too, which explains how the firmware bug has gone
>> unnoticed until now.
>
> Should we do this for everything CDC?

I don't know.  There are workarounds for missing CDC Union descriptors
on RNDIS devices in cdc_ether.c, but I have no idea if those devices in
provide an IAD instead.  I don't think this bug is a very common.

MBIM devices are likely to have IADs because Microsoft let you skip some
of the more hairy vendor specific control message parts if you provide
one:
http://msdn.microsoft.com/en-us/library/windows/hardware/mbim-based-mobile-broadband-requirements-for-windows.aspx

But they should of course provide the CDC Union functional descriptor as
well, even if that is "just" a USB-IF requirement and not a Windows
requirement ;-)


Bjørn
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