Ubuntu Manpage: iicmux — I2C bus mulitiplexer framework

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NAME

     iicmux — I2C bus mulitiplexer framework

SYNOPSIS

     To compile this driver into the kernel, place the following line in your kernel
     configuration file:

           device iicmux

     Alternatively, to load the driver as a module at boot time, place the following line in
     loader.conf(5):

           iicmux_load="YES"

     Note that it is usually not necessary to explicitly load the driver module, as it will be
     loaded automatically along with the driver for the specific mux hardware in use.

DESCRIPTION

The iicmux framework provides support code to help implement drivers for various I2C bus
multiplexer (mux) hardware. iicmux is not a standalone driver, it is a collection of
support functions and driver methods which are used by individual mux hardware drivers. It
will be loaded automatically when needed by a mux hardware driver. This manual page
provides an overview of the I2C mux framework and its behavior.

Generally speaking, an I2C mux is connected to an upstream I2C bus, and to one or more
downstream I2C buses, and it can be commanded to connect any one of the downstream buses to
the upstream bus. Some hardware may be able to connect multiple downstream buses at the
same time, but that concept is not supported by iicmux.

The iicmux framework operates automatically when I2C slave devices initiate I/O. It does
not require (or even allow for) any external control to select the active downstream bus.

When there is no I/O in progress, the mux is said to be in the “idle” state. Some mux
hardware has the ability to disconnect all downstream buses when in an idle state. Other
hardware must always have one of the downstream buses connected. Individual mux hardware
drivers typically provide a way to select which downstream bus (if any) should be connected
while in the idle state. In the absence of such configuration, whichever downstream bus was
last used remains connected to the upstream bus.

When an I2C slave device on a bus downstream of a mux initiates I/O, it first requests
exclusive use of the bus by calling iicbus_request_bus(). This request is communicated to
the bus's parent, which is the iicmux framework mux driver. Once exclusive bus ownership is
obtained, the mux driver connects the upstream I2C bus to the downstream bus which hosts the
slave device that requested bus ownership. The mux hardware maintains that upstream-to-
downstream connection until the slave device calls iicbus_release_bus(). Before releasing
ownership, the mux driver returns the mux hardware to the idle state.

FDT CONFIGURATION

     On an fdt(4) based system, an I2C mux device node is defined as a child node of its upstream
     I2C bus when the mux device is an I2C slave itself.  It may be defined as a child node of
     any other bus or device in the system when it is not an I2C slave, in which case the
     i2c-parent property indicates which upstream bus the mux is attached to.  In either case,
     the children of the mux node are additional I2C buses, which will have one or more I2C slave
     devices described in their child nodes.

     Drivers using the iicmux framework conform to the standard i2c/i2c-mux.txt bindings
     document.

HINTS CONFIGURATION

     On a device.hints(5) based system, these values are configurable for iicmux framework
     drivers :

     hint.<driver>.<unit>.at
             The upstream iicbus(4) the iicmux instance is attached to.

     When configured via hints, the driver automatically adds an iicbus instance for every
     downstream bus supported by the chip.  There is currently no way to indicate used versus
     unused downstream buses.

HISTORY