Manual Pages  — IICMUX

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.