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