1.. SPDX-License-Identifier: GPL-2.0 2 3====================================================== 4cdc_mbim - Driver for CDC MBIM Mobile Broadband modems 5====================================================== 6 7The cdc_mbim driver supports USB devices conforming to the "Universal 8Serial Bus Communications Class Subclass Specification for Mobile 9Broadband Interface Model" [1], which is a further development of 10"Universal Serial Bus Communications Class Subclass Specifications for 11Network Control Model Devices" [2] optimized for Mobile Broadband 12devices, aka "3G/LTE modems". 13 14 15Command Line Parameters 16======================= 17 18The cdc_mbim driver has no parameters of its own. But the probing 19behaviour for NCM 1.0 backwards compatible MBIM functions (an 20"NCM/MBIM function" as defined in section 3.2 of [1]) is affected 21by a cdc_ncm driver parameter: 22 23prefer_mbim 24----------- 25:Type: Boolean 26:Valid Range: N/Y (0-1) 27:Default Value: Y (MBIM is preferred) 28 29This parameter sets the system policy for NCM/MBIM functions. Such 30functions will be handled by either the cdc_ncm driver or the cdc_mbim 31driver depending on the prefer_mbim setting. Setting prefer_mbim=N 32makes the cdc_mbim driver ignore these functions and lets the cdc_ncm 33driver handle them instead. 34 35The parameter is writable, and can be changed at any time. A manual 36unbind/bind is required to make the change effective for NCM/MBIM 37functions bound to the "wrong" driver 38 39 40Basic usage 41=========== 42 43MBIM functions are inactive when unmanaged. The cdc_mbim driver only 44provides a userspace interface to the MBIM control channel, and will 45not participate in the management of the function. This implies that a 46userspace MBIM management application always is required to enable a 47MBIM function. 48 49Such userspace applications includes, but are not limited to: 50 51 - mbimcli (included with the libmbim [3] library), and 52 - ModemManager [4] 53 54Establishing a MBIM IP session reequires at least these actions by the 55management application: 56 57 - open the control channel 58 - configure network connection settings 59 - connect to network 60 - configure IP interface 61 62Management application development 63---------------------------------- 64The driver <-> userspace interfaces are described below. The MBIM 65control channel protocol is described in [1]. 66 67 68MBIM control channel userspace ABI 69================================== 70 71/dev/cdc-wdmX character device 72------------------------------ 73The driver creates a two-way pipe to the MBIM function control channel 74using the cdc-wdm driver as a subdriver. The userspace end of the 75control channel pipe is a /dev/cdc-wdmX character device. 76 77The cdc_mbim driver does not process or police messages on the control 78channel. The channel is fully delegated to the userspace management 79application. It is therefore up to this application to ensure that it 80complies with all the control channel requirements in [1]. 81 82The cdc-wdmX device is created as a child of the MBIM control 83interface USB device. The character device associated with a specific 84MBIM function can be looked up using sysfs. For example:: 85 86 bjorn@nemi:~$ ls /sys/bus/usb/drivers/cdc_mbim/2-4:2.12/usbmisc 87 cdc-wdm0 88 89 bjorn@nemi:~$ grep . /sys/bus/usb/drivers/cdc_mbim/2-4:2.12/usbmisc/cdc-wdm0/dev 90 180:0 91 92 93USB configuration descriptors 94----------------------------- 95The wMaxControlMessage field of the CDC MBIM functional descriptor 96limits the maximum control message size. The management application is 97responsible for negotiating a control message size complying with the 98requirements in section 9.3.1 of [1], taking this descriptor field 99into consideration. 100 101The userspace application can access the CDC MBIM functional 102descriptor of a MBIM function using either of the two USB 103configuration descriptor kernel interfaces described in [6] or [7]. 104 105See also the ioctl documentation below. 106 107 108Fragmentation 109------------- 110The userspace application is responsible for all control message 111fragmentation and defragmentaion, as described in section 9.5 of [1]. 112 113 114/dev/cdc-wdmX write() 115--------------------- 116The MBIM control messages from the management application *must not* 117exceed the negotiated control message size. 118 119 120/dev/cdc-wdmX read() 121-------------------- 122The management application *must* accept control messages of up the 123negotiated control message size. 124 125 126/dev/cdc-wdmX ioctl() 127--------------------- 128IOCTL_WDM_MAX_COMMAND: Get Maximum Command Size 129This ioctl returns the wMaxControlMessage field of the CDC MBIM 130functional descriptor for MBIM devices. This is intended as a 131convenience, eliminating the need to parse the USB descriptors from 132userspace. 133 134:: 135 136 #include <stdio.h> 137 #include <fcntl.h> 138 #include <sys/ioctl.h> 139 #include <linux/types.h> 140 #include <linux/usb/cdc-wdm.h> 141 int main() 142 { 143 __u16 max; 144 int fd = open("/dev/cdc-wdm0", O_RDWR); 145 if (!ioctl(fd, IOCTL_WDM_MAX_COMMAND, &max)) 146 printf("wMaxControlMessage is %d\n", max); 147 } 148 149 150Custom device services 151---------------------- 152The MBIM specification allows vendors to freely define additional 153services. This is fully supported by the cdc_mbim driver. 154 155Support for new MBIM services, including vendor specified services, is 156implemented entirely in userspace, like the rest of the MBIM control 157protocol 158 159New services should be registered in the MBIM Registry [5]. 160 161 162 163MBIM data channel userspace ABI 164=============================== 165 166wwanY network device 167-------------------- 168The cdc_mbim driver represents the MBIM data channel as a single 169network device of the "wwan" type. This network device is initially 170mapped to MBIM IP session 0. 171 172 173Multiplexed IP sessions (IPS) 174----------------------------- 175MBIM allows multiplexing up to 256 IP sessions over a single USB data 176channel. The cdc_mbim driver models such IP sessions as 802.1q VLAN 177subdevices of the master wwanY device, mapping MBIM IP session Z to 178VLAN ID Z for all values of Z greater than 0. 179 180The device maximum Z is given in the MBIM_DEVICE_CAPS_INFO structure 181described in section 10.5.1 of [1]. 182 183The userspace management application is responsible for adding new 184VLAN links prior to establishing MBIM IP sessions where the SessionId 185is greater than 0. These links can be added by using the normal VLAN 186kernel interfaces, either ioctl or netlink. 187 188For example, adding a link for a MBIM IP session with SessionId 3:: 189 190 ip link add link wwan0 name wwan0.3 type vlan id 3 191 192The driver will automatically map the "wwan0.3" network device to MBIM 193IP session 3. 194 195 196Device Service Streams (DSS) 197---------------------------- 198MBIM also allows up to 256 non-IP data streams to be multiplexed over 199the same shared USB data channel. The cdc_mbim driver models these 200sessions as another set of 802.1q VLAN subdevices of the master wwanY 201device, mapping MBIM DSS session A to VLAN ID (256 + A) for all values 202of A. 203 204The device maximum A is given in the MBIM_DEVICE_SERVICES_INFO 205structure described in section 10.5.29 of [1]. 206 207The DSS VLAN subdevices are used as a practical interface between the 208shared MBIM data channel and a MBIM DSS aware userspace application. 209It is not intended to be presented as-is to an end user. The 210assumption is that a userspace application initiating a DSS session 211also takes care of the necessary framing of the DSS data, presenting 212the stream to the end user in an appropriate way for the stream type. 213 214The network device ABI requires a dummy ethernet header for every DSS 215data frame being transported. The contents of this header is 216arbitrary, with the following exceptions: 217 218 - TX frames using an IP protocol (0x0800 or 0x86dd) will be dropped 219 - RX frames will have the protocol field set to ETH_P_802_3 (but will 220 not be properly formatted 802.3 frames) 221 - RX frames will have the destination address set to the hardware 222 address of the master device 223 224The DSS supporting userspace management application is responsible for 225adding the dummy ethernet header on TX and stripping it on RX. 226 227This is a simple example using tools commonly available, exporting 228DssSessionId 5 as a pty character device pointed to by a /dev/nmea 229symlink:: 230 231 ip link add link wwan0 name wwan0.dss5 type vlan id 261 232 ip link set dev wwan0.dss5 up 233 socat INTERFACE:wwan0.dss5,type=2 PTY:,echo=0,link=/dev/nmea 234 235This is only an example, most suitable for testing out a DSS 236service. Userspace applications supporting specific MBIM DSS services 237are expected to use the tools and programming interfaces required by 238that service. 239 240Note that adding VLAN links for DSS sessions is entirely optional. A 241management application may instead choose to bind a packet socket 242directly to the master network device, using the received VLAN tags to 243map frames to the correct DSS session and adding 18 byte VLAN ethernet 244headers with the appropriate tag on TX. In this case using a socket 245filter is recommended, matching only the DSS VLAN subset. This avoid 246unnecessary copying of unrelated IP session data to userspace. For 247example:: 248 249 static struct sock_filter dssfilter[] = { 250 /* use special negative offsets to get VLAN tag */ 251 BPF_STMT(BPF_LD|BPF_B|BPF_ABS, SKF_AD_OFF + SKF_AD_VLAN_TAG_PRESENT), 252 BPF_JUMP(BPF_JMP|BPF_JEQ|BPF_K, 1, 0, 6), /* true */ 253 254 /* verify DSS VLAN range */ 255 BPF_STMT(BPF_LD|BPF_H|BPF_ABS, SKF_AD_OFF + SKF_AD_VLAN_TAG), 256 BPF_JUMP(BPF_JMP|BPF_JGE|BPF_K, 256, 0, 4), /* 256 is first DSS VLAN */ 257 BPF_JUMP(BPF_JMP|BPF_JGE|BPF_K, 512, 3, 0), /* 511 is last DSS VLAN */ 258 259 /* verify ethertype */ 260 BPF_STMT(BPF_LD|BPF_H|BPF_ABS, 2 * ETH_ALEN), 261 BPF_JUMP(BPF_JMP|BPF_JEQ|BPF_K, ETH_P_802_3, 0, 1), 262 263 BPF_STMT(BPF_RET|BPF_K, (u_int)-1), /* accept */ 264 BPF_STMT(BPF_RET|BPF_K, 0), /* ignore */ 265 }; 266 267 268 269Tagged IP session 0 VLAN 270------------------------ 271As described above, MBIM IP session 0 is treated as special by the 272driver. It is initially mapped to untagged frames on the wwanY 273network device. 274 275This mapping implies a few restrictions on multiplexed IPS and DSS 276sessions, which may not always be practical: 277 278 - no IPS or DSS session can use a frame size greater than the MTU on 279 IP session 0 280 - no IPS or DSS session can be in the up state unless the network 281 device representing IP session 0 also is up 282 283These problems can be avoided by optionally making the driver map IP 284session 0 to a VLAN subdevice, similar to all other IP sessions. This 285behaviour is triggered by adding a VLAN link for the magic VLAN ID 2864094. The driver will then immediately start mapping MBIM IP session 2870 to this VLAN, and will drop untagged frames on the master wwanY 288device. 289 290Tip: It might be less confusing to the end user to name this VLAN 291subdevice after the MBIM SessionID instead of the VLAN ID. For 292example:: 293 294 ip link add link wwan0 name wwan0.0 type vlan id 4094 295 296 297VLAN mapping 298------------ 299 300Summarizing the cdc_mbim driver mapping described above, we have this 301relationship between VLAN tags on the wwanY network device and MBIM 302sessions on the shared USB data channel:: 303 304 VLAN ID MBIM type MBIM SessionID Notes 305 --------------------------------------------------------- 306 untagged IPS 0 a) 307 1 - 255 IPS 1 - 255 <VLANID> 308 256 - 511 DSS 0 - 255 <VLANID - 256> 309 512 - 4093 b) 310 4094 IPS 0 c) 311 312 a) if no VLAN ID 4094 link exists, else dropped 313 b) unsupported VLAN range, unconditionally dropped 314 c) if a VLAN ID 4094 link exists, else dropped 315 316 317 318 319References 320========== 321 322 1) USB Implementers Forum, Inc. - "Universal Serial Bus 323 Communications Class Subclass Specification for Mobile Broadband 324 Interface Model", Revision 1.0 (Errata 1), May 1, 2013 325 326 - http://www.usb.org/developers/docs/devclass_docs/ 327 328 2) USB Implementers Forum, Inc. - "Universal Serial Bus 329 Communications Class Subclass Specifications for Network Control 330 Model Devices", Revision 1.0 (Errata 1), November 24, 2010 331 332 - http://www.usb.org/developers/docs/devclass_docs/ 333 334 3) libmbim - "a glib-based library for talking to WWAN modems and 335 devices which speak the Mobile Interface Broadband Model (MBIM) 336 protocol" 337 338 - http://www.freedesktop.org/wiki/Software/libmbim/ 339 340 4) ModemManager - "a DBus-activated daemon which controls mobile 341 broadband (2G/3G/4G) devices and connections" 342 343 - http://www.freedesktop.org/wiki/Software/ModemManager/ 344 345 5) "MBIM (Mobile Broadband Interface Model) Registry" 346 347 - http://compliance.usb.org/mbim/ 348 349 6) "/sys/kernel/debug/usb/devices output format" 350 351 - Documentation/driver-api/usb/usb.rst 352 353 7) "/sys/bus/usb/devices/.../descriptors" 354 355 - Documentation/ABI/stable/sysfs-bus-usb 356