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