Blacklisting a broken AT port

[Dan Williams]

On Wed, 2014-01-29 at 00:04 -0800, Ben Chan wrote:
> Hi Aleksander,
> 
> I notice that some dongles have broken AT ports, e.g. ttyUSB1 appears to be
> AT-capable but fails to handle most AT commands, while ttyUSB3 is
> completely functional. ModemManager may pick ttyUSB1 as primary and ttyUSB3
> as secondary.
> 
> What's the recommended way to blacklist ttyUSB1 in such case?  I guess I
> could add a udev rule to avoid setting MM_CANDIDATE=1 on ttyUSB1.

We try to avoid per-port blacklisting because it's got a lot of
possibility for abuse for reasons kinda like this.  Not that your case
is a problem, but that in general per-port blacklisting is a
sledgehammer that often causes collateral damage.  I'd rather use
affirmative hints to mark some ports as primary and others as secondary
in these cases, instead of blacklisting.

But this is actually SOP for ZTE and other devices.  Usually we have to
scrape the windows INF files to find the modem and control ports, which
then get encoded into the udev rules.  So if you have a device where the
wrong port is being picked automatically, then we should certainly add
some port hints for it.  See 77-mm-zte-port-types.rules for example.

Dan