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: NBjørn Mork <bjorn@mork.no>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/networking/cdc_mbim.txt

+     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