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