340 lines
12 KiB
Text
340 lines
12 KiB
Text
|
cdc_mbim - Driver for CDC MBIM Mobile Broadband modems
|
||
|
|
========================================================
|
||
|
|
|
||
|
|
The cdc_mbim driver supports USB devices conforming to the "Universal
|
||
|
|
Serial Bus Communications Class Subclass Specification for Mobile
|
||
|
|
Broadband Interface Model" [1], which is a further development of
|
||
|
|
"Universal Serial Bus Communications Class Subclass Specifications for
|
||
|
|
Network Control Model Devices" [2] optimized for Mobile Broadband
|
||
|
|
devices, aka "3G/LTE modems".
|
||
|
|
|
||
|
|
|
||
|
|
Command Line Parameters
|
||
|
|
=======================
|
||
|
|
|
||
|
|
The cdc_mbim driver has no parameters of its own. But the probing
|
||
|
|
behaviour for NCM 1.0 backwards compatible MBIM functions (an
|
||
|
|
"NCM/MBIM function" as defined in section 3.2 of [1]) is affected
|
||
|
|
by a cdc_ncm driver parameter:
|
||
|
|
|
||
|
|
prefer_mbim
|
||
|
|
-----------
|
||
|
|
Type: Boolean
|
||
|
|
Valid Range: N/Y (0-1)
|
||
|
|
Default Value: Y (MBIM is preferred)
|
||
|
|
|
||
|
|
This parameter sets the system policy for NCM/MBIM functions. Such
|
||
|
|
functions will be handled by either the cdc_ncm driver or the cdc_mbim
|
||
|
|
driver depending on the prefer_mbim setting. Setting prefer_mbim=N
|
||
|
|
makes the cdc_mbim driver ignore these functions and lets the cdc_ncm
|
||
|
|
driver handle them instead.
|
||
|
|
|
||
|
|
The parameter is writable, and can be changed at any time. A manual
|
||
|
|
unbind/bind is required to make the change effective for NCM/MBIM
|
||
|
|
functions bound to the "wrong" driver
|
||
|
|
|
||
|
|
|
||
|
|
Basic usage
|
||
|
|
===========
|
||
|
|
|
||
|
|
MBIM functions are inactive when unmanaged. The cdc_mbim driver only
|
||
|
|
provides an userspace interface to the MBIM control channel, and will
|
||
|
|
not participate in the management of the function. This implies that a
|
||
|
|
userspace MBIM management application always is required to enable a
|
||
|
|
MBIM function.
|
||
|
|
|
||
|
|
Such userspace applications includes, but are not limited to:
|
||
|
|
- mbimcli (included with the libmbim [3] library), and
|
||
|
|
- ModemManager [4]
|
||
|
|
|
||
|
|
Establishing a MBIM IP session reequires at least these actions by the
|
||
|
|
management application:
|
||
|
|
- open the control channel
|
||
|
|
- configure network connection settings
|
||
|
|
- connect to network
|
||
|
|
- configure IP interface
|
||
|
|
|
||
|
|
Management application development
|
||
|
|
----------------------------------
|
||
|
|
The driver <-> userspace interfaces are described below. The MBIM
|
||
|
|
control channel protocol is described in [1].
|
||
|
|
|
||
|
|
|
||
|
|
MBIM control channel userspace ABI
|
||
|
|
==================================
|
||
|
|
|
||
|
|
/dev/cdc-wdmX character device
|
||
|
|
------------------------------
|
||
|
|
The driver creates a two-way pipe to the MBIM function control channel
|
||
|
|
using the cdc-wdm driver as a subdriver. The userspace end of the
|
||
|
|
control channel pipe is a /dev/cdc-wdmX character device.
|
||
|
|
|
||
|
|
The cdc_mbim driver does not process or police messages on the control
|
||
|
|
channel. The channel is fully delegated to the userspace management
|
||
|
|
application. It is therefore up to this application to ensure that it
|
||
|
|
complies with all the control channel requirements in [1].
|
||
|
|
|
||
|
|
The cdc-wdmX device is created as a child of the MBIM control
|
||
|
|
interface USB device. The character device associated with a specific
|
||
|
|
MBIM function can be looked up using sysfs. For example:
|
||
|
|
|
||
|
|
bjorn@nemi:~$ ls /sys/bus/usb/drivers/cdc_mbim/2-4:2.12/usbmisc
|
||
|
|
cdc-wdm0
|
||
|
|
|
||
|
|
bjorn@nemi:~$ grep . /sys/bus/usb/drivers/cdc_mbim/2-4:2.12/usbmisc/cdc-wdm0/dev
|
||
|
|
180:0
|
||
|
|
|
||
|
|
|
||
|
|
USB configuration descriptors
|
||
|
|
-----------------------------
|
||
|
|
The wMaxControlMessage field of the CDC MBIM functional descriptor
|
||
|
|
limits the maximum control message size. The managament application is
|
||
|
|
responsible for negotiating a control message size complying with the
|
||
|
|
requirements in section 9.3.1 of [1], taking this descriptor field
|
||
|
|
into consideration.
|
||
|
|
|
||
|
|
The userspace application can access the CDC MBIM functional
|
||
|
|
descriptor of a MBIM function using either of the two USB
|
||
|
|
configuration descriptor kernel interfaces described in [6] or [7].
|
||
|
|
|
||
|
|
See also the ioctl documentation below.
|
||
|
|
|
||
|
|
|
||
|
|
Fragmentation
|
||
|
|
-------------
|
||
|
|
The userspace application is responsible for all control message
|
||
|
|
fragmentation and defragmentaion, as described in section 9.5 of [1].
|
||
|
|
|
||
|
|
|
||
|
|
/dev/cdc-wdmX write()
|
||
|
|
---------------------
|
||
|
|
The MBIM control messages from the management application *must not*
|
||
|
|
exceed the negotiated control message size.
|
||
|
|
|
||
|
|
|
||
|
|
/dev/cdc-wdmX read()
|
||
|
|
--------------------
|
||
|
|
The management application *must* accept control messages of up the
|
||
|
|
negotiated control message size.
|
||
|
|
|
||
|
|
|
||
|
|
/dev/cdc-wdmX ioctl()
|
||
|
|
--------------------
|
||
|
|
IOCTL_WDM_MAX_COMMAND: Get Maximum Command Size
|
||
|
|
This ioctl returns the wMaxControlMessage field of the CDC MBIM
|
||
|
|
functional descriptor for MBIM devices. This is intended as a
|
||
|
|
convenience, eliminating the need to parse the USB descriptors from
|
||
|
|
userspace.
|
||
|
|
|
||
|
|
#include <stdio.h>
|
||
|
|
#include <fcntl.h>
|
||
|
|
#include <sys/ioctl.h>
|
||
|
|
#include <linux/types.h>
|
||
|
|
#include <linux/usb/cdc-wdm.h>
|
||
|
|
int main()
|
||
|
|
{
|
||
|
|
__u16 max;
|
||
|
|
int fd = open("/dev/cdc-wdm0", O_RDWR);
|
||
|
|
if (!ioctl(fd, IOCTL_WDM_MAX_COMMAND, &max))
|
||
|
|
printf("wMaxControlMessage is %d\n", max);
|
||
|
|
}
|
||
|
|
|
||
|
|
|
||
|
|
Custom device services
|
||
|
|
----------------------
|
||
|
|
The MBIM specification allows vendors to freely define additional
|
||
|
|
services. This is fully supported by the cdc_mbim driver.
|
||
|
|
|
||
|
|
Support for new MBIM services, including vendor specified services, is
|
||
|
|
implemented entirely in userspace, like the rest of the MBIM control
|
||
|
|
protocol
|
||
|
|
|
||
|
|
New services should be registered in the MBIM Registry [5].
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
MBIM data channel userspace ABI
|
||
|
|
===============================
|
||
|
|
|
||
|
|
wwanY network device
|
||
|
|
--------------------
|
||
|
|
The cdc_mbim driver represents the MBIM data channel as a single
|
||
|
|
network device of the "wwan" type. This network device is initially
|
||
|
|
mapped to MBIM IP session 0.
|
||
|
|
|
||
|
|
|
||
|
|
Multiplexed IP sessions (IPS)
|
||
|
|
-----------------------------
|
||
|
|
MBIM allows multiplexing up to 256 IP sessions over a single USB data
|
||
|
|
channel. The cdc_mbim driver models such IP sessions as 802.1q VLAN
|
||
|
|
subdevices of the master wwanY device, mapping MBIM IP session Z to
|
||
|
|
VLAN ID Z for all values of Z greater than 0.
|
||
|
|
|
||
|
|
The device maximum Z is given in the MBIM_DEVICE_CAPS_INFO structure
|
||
|
|
described in section 10.5.1 of [1].
|
||
|
|
|
||
|
|
The userspace management application is responsible for adding new
|
||
|
|
VLAN links prior to establishing MBIM IP sessions where the SessionId
|
||
|
|
is greater than 0. These links can be added by using the normal VLAN
|
||
|
|
kernel interfaces, either ioctl or netlink.
|
||
|
|
|
||
|
|
For example, adding a link for a MBIM IP session with SessionId 3:
|
||
|
|
|
||
|
|
ip link add link wwan0 name wwan0.3 type vlan id 3
|
||
|
|
|
||
|
|
The driver will automatically map the "wwan0.3" network device to MBIM
|
||
|
|
IP session 3.
|
||
|
|
|
||
|
|
|
||
|
|
Device Service Streams (DSS)
|
||
|
|
----------------------------
|
||
|
|
MBIM also allows up to 256 non-IP data streams to be multiplexed over
|
||
|
|
the same shared USB data channel. The cdc_mbim driver models these
|
||
|
|
sessions as another set of 802.1q VLAN subdevices of the master wwanY
|
||
|
|
device, mapping MBIM DSS session A to VLAN ID (256 + A) for all values
|
||
|
|
of A.
|
||
|
|
|
||
|
|
The device maximum A is given in the MBIM_DEVICE_SERVICES_INFO
|
||
|
|
structure described in section 10.5.29 of [1].
|
||
|
|
|
||
|
|
The DSS VLAN subdevices are used as a practical interface between the
|
||
|
|
shared MBIM data channel and a MBIM DSS aware userspace application.
|
||
|
|
It is not intended to be presented as-is to an end user. The
|
||
|
|
assumption is that an userspace application initiating a DSS session
|
||
|
|
also takes care of the necessary framing of the DSS data, presenting
|
||
|
|
the stream to the end user in an appropriate way for the stream type.
|
||
|
|
|
||
|
|
The network device ABI requires a dummy ethernet header for every DSS
|
||
|
|
data frame being transported. The contents of this header is
|
||
|
|
arbitrary, with the following exceptions:
|
||
|
|
- TX frames using an IP protocol (0x0800 or 0x86dd) will be dropped
|
||
|
|
- RX frames will have the protocol field set to ETH_P_802_3 (but will
|
||
|
|
not be properly formatted 802.3 frames)
|
||
|
|
- RX frames will have the destination address set to the hardware
|
||
|
|
address of the master device
|
||
|
|
|
||
|
|
The DSS supporting userspace management application is responsible for
|
||
|
|
adding the dummy ethernet header on TX and stripping it on RX.
|
||
|
|
|
||
|
|
This is a simple example using tools commonly available, exporting
|
||
|
|
DssSessionId 5 as a pty character device pointed to by a /dev/nmea
|
||
|
|
symlink:
|
||
|
|
|
||
|
|
ip link add link wwan0 name wwan0.dss5 type vlan id 261
|
||
|
|
ip link set dev wwan0.dss5 up
|
||
|
|
socat INTERFACE:wwan0.dss5,type=2 PTY:,echo=0,link=/dev/nmea
|
||
|
|
|
||
|
|
This is only an example, most suitable for testing out a DSS
|
||
|
|
service. Userspace applications supporting specific MBIM DSS services
|
||
|
|
are expected to use the tools and programming interfaces required by
|
||
|
|
that service.
|
||
|
|
|
||
|
|
Note that adding VLAN links for DSS sessions is entirely optional. A
|
||
|
|
management application may instead choose to bind a packet socket
|
||
|
|
directly to the master network device, using the received VLAN tags to
|
||
|
|
map frames to the correct DSS session and adding 18 byte VLAN ethernet
|
||
|
|
headers with the appropriate tag on TX. In this case using a socket
|
||
|
|
filter is recommended, matching only the DSS VLAN subset. This avoid
|
||
|
|
unnecessary copying of unrelated IP session data to userspace. For
|
||
|
|
example:
|
||
|
|
|
||
|
|
static struct sock_filter dssfilter[] = {
|
||
|
|
/* use special negative offsets to get VLAN tag */
|
||
|
|
BPF_STMT(BPF_LD|BPF_B|BPF_ABS, SKF_AD_OFF + SKF_AD_VLAN_TAG_PRESENT),
|
||
|
|
BPF_JUMP(BPF_JMP|BPF_JEQ|BPF_K, 1, 0, 6), /* true */
|
||
|
|
|
||
|
|
/* verify DSS VLAN range */
|
||
|
|
BPF_STMT(BPF_LD|BPF_H|BPF_ABS, SKF_AD_OFF + SKF_AD_VLAN_TAG),
|
||
|
|
BPF_JUMP(BPF_JMP|BPF_JGE|BPF_K, 256, 0, 4), /* 256 is first DSS VLAN */
|
||
|
|
BPF_JUMP(BPF_JMP|BPF_JGE|BPF_K, 512, 3, 0), /* 511 is last DSS VLAN */
|
||
|
|
|
||
|
|
/* verify ethertype */
|
||
|
|
BPF_STMT(BPF_LD|BPF_H|BPF_ABS, 2 * ETH_ALEN),
|
||
|
|
BPF_JUMP(BPF_JMP|BPF_JEQ|BPF_K, ETH_P_802_3, 0, 1),
|
||
|
|
|
||
|
|
BPF_STMT(BPF_RET|BPF_K, (u_int)-1), /* accept */
|
||
|
|
BPF_STMT(BPF_RET|BPF_K, 0), /* ignore */
|
||
|
|
};
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
Tagged IP session 0 VLAN
|
||
|
|
------------------------
|
||
|
|
As described above, MBIM IP session 0 is treated as special by the
|
||
|
|
driver. It is initially mapped to untagged frames on the wwanY
|
||
|
|
network device.
|
||
|
|
|
||
|
|
This mapping implies a few restrictions on multiplexed IPS and DSS
|
||
|
|
sessions, which may not always be practical:
|
||
|
|
- no IPS or DSS session can use a frame size greater than the MTU on
|
||
|
|
IP session 0
|
||
|
|
- no IPS or DSS session can be in the up state unless the network
|
||
|
|
device representing IP session 0 also is up
|
||
|
|
|
||
|
|
These problems can be avoided by optionally making the driver map IP
|
||
|
|
session 0 to a VLAN subdevice, similar to all other IP sessions. This
|
||
|
|
behaviour is triggered by adding a VLAN link for the magic VLAN ID
|
||
|
|
4094. The driver will then immediately start mapping MBIM IP session
|
||
|
|
0 to this VLAN, and will drop untagged frames on the master wwanY
|
||
|
|
device.
|
||
|
|
|
||
|
|
Tip: It might be less confusing to the end user to name this VLAN
|
||
|
|
subdevice after the MBIM SessionID instead of the VLAN ID. For
|
||
|
|
example:
|
||
|
|
|
||
|
|
ip link add link wwan0 name wwan0.0 type vlan id 4094
|
||
|
|
|
||
|
|
|
||
|
|
VLAN mapping
|
||
|
|
------------
|
||
|
|
|
||
|
|
Summarizing the cdc_mbim driver mapping described above, we have this
|
||
|
|
relationship between VLAN tags on the wwanY network device and MBIM
|
||
|
|
sessions on the shared USB data channel:
|
||
|
|
|
||
|
|
VLAN ID MBIM type MBIM SessionID Notes
|
||
|
|
---------------------------------------------------------
|
||
|
|
untagged IPS 0 a)
|
||
|
|
1 - 255 IPS 1 - 255 <VLANID>
|
||
|
|
256 - 511 DSS 0 - 255 <VLANID - 256>
|
||
|
|
512 - 4093 b)
|
||
|
|
4094 IPS 0 c)
|
||
|
|
|
||
|
|
a) if no VLAN ID 4094 link exists, else dropped
|
||
|
|
b) unsupported VLAN range, unconditionally dropped
|
||
|
|
c) if a VLAN ID 4094 link exists, else dropped
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
References
|
||
|
|
==========
|
||
|
|
|
||
|
|
[1] USB Implementers Forum, Inc. - "Universal Serial Bus
|
||
|
|
Communications Class Subclass Specification for Mobile Broadband
|
||
|
|
Interface Model", Revision 1.0 (Errata 1), May 1, 2013
|
||
|
|
- http://www.usb.org/developers/docs/devclass_docs/
|
||
|
|
|
||
|
|
[2] USB Implementers Forum, Inc. - "Universal Serial Bus
|
||
|
|
Communications Class Subclass Specifications for Network Control
|
||
|
|
Model Devices", Revision 1.0 (Errata 1), November 24, 2010
|
||
|
|
- http://www.usb.org/developers/docs/devclass_docs/
|
||
|
|
|
||
|
|
[3] libmbim - "a glib-based library for talking to WWAN modems and
|
||
|
|
devices which speak the Mobile Interface Broadband Model (MBIM)
|
||
|
|
protocol"
|
||
|
|
- http://www.freedesktop.org/wiki/Software/libmbim/
|
||
|
|
|
||
|
|
[4] ModemManager - "a DBus-activated daemon which controls mobile
|
||
|
|
broadband (2G/3G/4G) devices and connections"
|
||
|
|
- http://www.freedesktop.org/wiki/Software/ModemManager/
|
||
|
|
|
||
|
|
[5] "MBIM (Mobile Broadband Interface Model) Registry"
|
||
|
|
- http://compliance.usb.org/mbim/
|
||
|
|
|
||
|
|
[6] "/proc/bus/usb filesystem output"
|
||
|
|
- Documentation/usb/proc_usb_info.txt
|
||
|
|
|
||
|
|
[7] "/sys/bus/usb/devices/.../descriptors"
|
||
|
|
- Documentation/ABI/stable/sysfs-bus-usb
|