On Tue, 17 Nov 2009, Adrian Taylor wrote:
> Hi,
>
> Nokia S60 phones expose two CDC ACM channels. The first is a modem and
> is properly detected by the current cdc-acm driver. The second has a
> protocol ID of 0xff ('vendor specific') and /dev/ttyACM* is NOT
> currently created for it.
>
> This second channel is useful, since it's freely usable by third-party
> S60 application developers. It's sometimes used for debuggers or, in our
> case, remote control tools. At the S60 end, it just acts as a serial
> port (as it does on Windows and MacOS X). We should expose it as
> a /dev/ttyACM* device so that user-space tools can use it on Linux, too.
>
> I attach a simple patch which does just that.
>
> However! It's not that simple.
>
> The reason is that some Windows Mobile/Windows CE devices support a
> remote ethernet protocol called RNDIS, which is build atop CDC ACM. This
> sadly uses the same class, subclass and protocol IDs.
>
> So currently drivers/net/usb/rndis_host.c tries to register for exactly
> the same devices. It eventually fails (when usbnet_generic_cdc_bind
> determines that it's not a suitable ethernet interface after all). But,
> if we create a serial port in cdc-acm, rndis_host doesn't even get a
> chance to try.
>
> What's the best way to allow each driver to have a crack at this
> particular interface?
The best way is to have each driver probe the interface and check
whether it is the right driver. If it isn't the probe should fail
with -ENODEV; then the driver core will continue on to probe other
drivers.
Alan Stern
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