[RFC,17/18] net: iosm: readme file

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+IOSM Driver for PCIe based Intel M.2 Modems
+================================================
+The IOSM (IPC over Shared Memory) driver is a PCIe host driver implemented
+for linux or chrome platform for data exchange over PCIe interface between
+Host platform & Intel M.2 Modem. The driver exposes interface conforming to the
+MBIM protocol [1]. Any front end application ( eg: Modem Manager) could easily
+manage the MBIM interface to enable data communication towards WWAN.
+
+Basic usage
+===========
+MBIM functions are inactive when unmanaged. The IOSM driver only
+provides a userspace interface of a character device representing
+MBIM control channel and does not play any role in managing the
+functionality. It is the job of a userspace application to enumerate
+the port appropriately and enable MBIM functionality.
+
+Examples of few such userspace application are:
+ - mbimcli (included with the libmbim [2] library), and
+ - ModemManager [3]
+
+For establishing an MBIM IP session at least these actions are required by the
+management application:
+ - open the control channel
+ - configure network connection settings
+ - connect to network
+ - configure IP interface
+
+Management application development
+----------------------------------
+The driver and userspace interfaces are described below. The MBIM
+control channel protocol is described in [1].
+
+MBIM control channel userspace ABI
+==================================
+
+/dev/wwanctrl character device
+------------------------------
+The driver exposes an interface to the MBIM function control channel using char
+driver as a subdriver. The userspace end of the control channel pipe is a
+/dev/wwanctrl character device.
+
+The /dev/wwanctrl device is created as a subordinate character device under
+IOSM driver. The character device associated with a specific MBIM function
+can be looked up using sysfs with matching the above device name.
+
+Control channel configuration
+-----------------------------
+The wMaxControlMessage field of the MBIM functional descriptor
+limits the maximum control message size. The management application needs to
+negotiate the control message size as per the requirements.
+See also the ioctl documentation below.
+
+Fragmentation
+-------------
+The userspace application is responsible for all control message
+fragmentation and defragmentation as per MBIM.
+
+/dev/wwanctrl write()
+---------------------
+The MBIM control messages from the management application must not
+exceed the negotiated control message size.
+
+/dev/wwanctrl read()
+--------------------
+The management application must accept control messages of up the
+negotiated control message size.
+
+/dev/wwanctrl ioctl()
+--------------------
+IOCTL_WDM_MAX_COMMAND: Get Maximum Command Size
+This IOCTL command could be used by applications to fetch the Maximum Command
+buffer length supported by the driver which is restricted to 4096 bytes.
+
+	#include <stdio.h>
+	#include <fcntl.h>
+	#include <sys/ioctl.h>
+	#include <linux/types.h>
+	int main()
+	{
+		__u16 max;
+		int fd = open("/dev/wwanctrl", O_RDWR);
+		if (!ioctl(fd, IOCTL_WDM_MAX_COMMAND, &max))
+			printf("wMaxControlMessage is %d\n", max);
+	}
+
+MBIM data channel userspace ABI
+===============================
+
+wwanY network device
+--------------------
+The IOSM driver represents the MBIM data channel as a single
+network device of the "wwan0" type. This network device is initially
+mapped to MBIM IP session 0.
+
+Multiplexed IP sessions (IPS)
+-----------------------------
+IOSM driver allows multiplexing of several IP sessions over the single network
+device of type wwan0. IOSM driver models such IP sessions as 802.1q VLAN
+subdevices of the master wwanY device, mapping MBIM IP session M to VLAN ID M
+for all values of M greater than 0.
+
+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.
+
+For example, adding a link for a MBIM IP session with SessionId 5:
+
+  ip link add link wwan0 name wwan0.<name> type vlan id 5
+
+The driver will automatically map the "wwan0.<name>" network device to MBIM
+IP session 5.
+
+References
+==========
+
+[1] "MBIM (Mobile Broadband Interface Model) Registry"
+       - http://compliance.usb.org/mbim/
+
+[2] 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/
+
+[3] ModemManager - "a DBus-activated daemon which controls mobile
+      broadband (2G/3G/4G) devices and connections"
+      - http://www.freedesktop.org/wiki/Software/ModemManager/
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