316e68ee6e
An i2c bus can be divided into segments which can be selectively connected and disconnected from the main bus. This is usually done to enable using multiple slave devices having the same address, by isolating the devices onto separate bus segments, only one of which is connected to the main bus at once. There are several types of i2c bus muxes, which break down into two general categories... - Muxes which are themselves i2c slaves. These devices respond to i2c commands on their upstream bus, and based on those commands, connect various downstream buses to the upstream. In newbus terms, they are both a child of an iicbus and the parent of one or more iicbus instances. - Muxes which are not i2c devices themselves. Such devices are part of the i2c bus electrically, but in newbus terms their parent is some other bus. The association with the upstream bus must be established by separate metadata (such as FDT data). In both cases, the mux driver has one or more iicbus child instances representing the downstream buses. The mux driver implements the iicbus_if interface, as if it were an iichb host bridge/i2c controller driver. It services the IO requests sent to it by forwarding them to the iicbus instance representing the upstream bus, after electrically connecting the upstream bus to the downstream bus that hosts the i2c slave device which made the IO request. The net effect is automatic mux switching which is transparent to slaves on the downstream buses. They just do i2c IO they way they normally do, and the bus is electrically connected for the duration of the IO and then idled when it is complete. The existing iicbus_if callback() method is enhanced so that the parameter passed to it can be a struct which contains a device_t for the requesting bus and slave devices. This change is done by adding a flag that indicates the extra values are present, and making the flags field the first field of a new args struct. If the flag is set, the iichb or mux driver can recast the pointer-to-flags into a pointer-to-struct and access the extra fields. Thus abi compatibility with older drivers is retained (but a mux cannot exist on the bus with the older iicbus driver in use.) A new set of core support routines exists in iicbus.c. This code will help implement mux drivers for any type of mux hardware by supplying all the boilerplate code that forwards IO requests upstream. It also has code for parsing metadata and instantiating the child iicbus instances based on it. Two new hardware mux drivers are added. The ltc430x driver supports the LTC4305/4306 mux chips which are controlled via i2c commands. The iic_gpiomux driver supports any mux hardware which is controlled by manipulating the state of one or more gpio pins. Test Plan Tested locally using a variety of mux'd bus configurations involving both ltc4305 and a homebrew gpio-controlled mux. Tested configurations included cascaded muxes (unlikely in the real world, but useful to prove that 'it all just works' in terms of the automatic switching and upstream forwarding of IO requests).
149 lines
5.1 KiB
Groff
149 lines
5.1 KiB
Groff
.\"-
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.\" SPDX-License-Identifier: BSD-2-Clause
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.\"
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.\" Copyright (c) 2019 Ian Lepore <ian@freebsd.org>
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd January 1, 2020
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.Dt IICMUX 4
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.Os
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.Sh NAME
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.Nm iicmux
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.Nd I2C bus mulitiplexer framework
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.Sh SYNOPSIS
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To compile this driver into the kernel,
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place the following line in your
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kernel configuration file:
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.Bd -ragged -offset indent
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.Cd "device iicmux"
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.Ed
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.Pp
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Alternatively, to load the driver as a
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module at boot time, place the following line in
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.Xr loader.conf 5 :
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.Bd -literal -offset indent
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iicmux_load="YES"
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.Ed
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.Pp
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Note that it is usually not necessary to explicitly load the
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driver module, as it will be loaded automatically along with
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the driver for the specific mux hardware in use.
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.Sh DESCRIPTION
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The
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.Nm
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framework provides support code to help implement drivers for various
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I2C bus multiplexer (mux) hardware.
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.Nm
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is not a standalone driver,
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it is a collection of support functions and driver methods which are
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used by individual mux hardware drivers.
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It will be loaded automatically when needed by a mux hardware driver.
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This manual page provides an overview of the I2C mux framework and its
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behavior.
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.Pp
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Generally speaking, an I2C mux is connected to an upstream I2C bus, and to
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one or more downstream I2C buses, and it can be commanded to connect
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any one of the downstream buses to the upstream bus.
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Some hardware may be able to connect multiple downstream buses at the
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same time, but that concept is not supported by
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.Nm .
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.Pp
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The
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.Nm
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framework operates automatically when I2C slave devices initiate I/O.
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It does not require (or even allow for) any external control to select
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the active downstream bus.
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.Pp
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When there is no I/O in progress, the mux is said to be in the
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.Dq idle
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state.
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Some mux hardware has the ability to disconnect all downstream buses
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when in an idle state.
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Other hardware must always have one of the downstream buses connected.
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Individual mux hardware drivers typically provide a way to select which
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downstream bus (if any) should be connected while in the idle state.
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In the absence of such configuration, whichever downstream bus was
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last used remains connected to the upstream bus.
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.Pp
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When an I2C slave device on a bus downstream of a mux initiates I/O,
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it first requests exclusive use of the bus by calling
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.Fn iicbus_request_bus .
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This request is communicated to the bus's parent, which is the
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.Nm
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framework
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mux driver.
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Once exclusive bus ownership is obtained, the mux driver
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connects the upstream I2C bus to the downstream bus which hosts the
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slave device that requested bus ownership.
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The mux hardware maintains that upstream-to-downstream connection until
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the slave device calls
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.Fn iicbus_release_bus .
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Before releasing ownership, the mux driver returns the mux hardware to
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the idle state.
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.Sh FDT CONFIGURATION
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On an
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.Xr fdt 4
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based system, an I2C mux device node is defined as a child node of its
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upstream I2C bus when the mux device is an I2C slave itself.
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It may be defined as a child node of any other bus or device in the
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system when it is not an I2C slave, in which case the
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.Va i2c-parent
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property indicates which upstream bus the mux is attached to.
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In either case, the children of the mux node are additional I2C buses, which
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will have one or more I2C slave devices described in their child nodes.
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.Pp
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Drivers using the
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.Nm
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framework conform to the standard
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.Bk -words
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.Li i2c/i2c-mux.txt
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.Ek
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bindings document.
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.Sh HINTS CONFIGURATION
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On a
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.Xr device.hints 5
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based system, these values are configurable for
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.Nm
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framework drivers :
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.Bl -tag -width indent
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.It Va hint.<driver>.<unit>.at
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The upstream
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.Xr iicbus 4
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the
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.Nm
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instance is attached to.
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.El
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.Pp
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When configured via hints, the driver automatically adds an iicbus
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instance for every downstream bus supported by the chip.
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There is currently no way to indicate used versus unused downstream buses.
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.Sh SEE ALSO
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.Xr iicbus 4 ,
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.Sh HISTORY
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The
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.Nm
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framework first appeared in
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.Fx 13.0 .
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