Probing MBIM ports on devices with both MBIM and other cdc-wdmX devices
bjorn at mork.no
Mon May 6 01:19:04 PDT 2013
Aleksander Morgado <aleksander at lanedo.com> writes:
> By default cdc-wdm ports are never probed for AT capabilities in MM,
> only ttys are assumed to be either AT or QCDM. We could have a new more
> generic MMPortAt in addition to the MMPortAtSerial I guess. Question is,
> are these ports usually only in Ericsson devices?
I can't remember seeing them in any other device. But that doesn't mean
much these days...
> Or is it really more
> generic, like any cdc-wdm port found in modems out there may very well
> be an ÄT port?
Oliver recently pointed me to the CDC WMC spec ("Universal Serial Bus
CDC Subclass Specification for Wireless Mobile Communications Devices -
Revision 1.1 (Errata 1) - November 3, 2010"), which seems to indicate
that CDC WDM ports on mobile devices are supposed to use AT commands.
6.6.1 Functional Topology
The Device Management data exchange facility allows management
information to be exchanged between the logical handset and the USB
host, without interfering with any other activities that are in
process. The facility is conceptually similar to a Communications
Class/Abstract Control Model interface. However, Device Management
differs from ACM in two ways:
1. There is no data class interface; all information is exchanged
using commands over the control endpoint.
2. The AT command set is the only way of communicating with this
interface; there is no “data plane” associated with this kind of
7.1 Encapsulating AT Command Data
[USBPSTN1.2] defines SendEncapsulatedCommand and
GetEncapsulatedResponse as mechanisms for transporting control-plane
data. Experience has shown that the defined mechanisms are explained
with insufficient clarity to ensure interoperability between drivers
and firmware designed by different development groups.
For the purposes of this subclass specification, functions within
devices shall be implemented to transport AT command data using the
assumptions and algorithms outlined in this section.
So it is reasonable to expect that a port driven by the cdc_wdm driver
can be used as an additional AT port, with the extremely unfortunate
exception of the Huawei QMI devices in v3.4 and v3.5. That historical
misfortune is going to haunt me... I really wish I had never done that.
I could claim "young, stupid and needed the money", but I'm afraid only
"stupid" applies :)
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