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Documentation: i2c: Add doc for I2C sysfs

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This doc helps Linux users navigate through I2C sysfs and learn
the system I2C topology.

Signed-off-by: Alex Qiu <xqiu@google.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wolfram Sang <wsa@kernel.org>
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Alex Qiu 2021-05-17 16:56:05 -07:00 committed by Wolfram Sang
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.. SPDX-License-Identifier: GPL-2.0
===============
Linux I2C Sysfs
===============
Overview
========
I2C topology can be complex because of the existence of I2C MUX
(I2C Multiplexer). The Linux
kernel abstracts the MUX channels into logical I2C bus numbers. However, there
is a gap of knowledge to map from the I2C bus physical number and MUX topology
to logical I2C bus number. This doc is aimed to fill in this gap, so the
audience (hardware engineers and new software developers for example) can learn
the concept of logical I2C buses in the kernel, by knowing the physical I2C
topology and navigating through the I2C sysfs in Linux shell. This knowledge is
useful and essential to use ``i2c-tools`` for the purpose of development and
debugging.
Target audience
---------------
People who need to use Linux shell to interact with I2C subsystem on a system
which the Linux is running on.
Prerequisites
-------------
1. Knowledge of general Linux shell file system commands and operations.
2. General knowledge of I2C, I2C MUX and I2C topology.
Location of I2C Sysfs
=====================
Typically, the Linux Sysfs filesystem is mounted at the ``/sys`` directory,
so you can find the I2C Sysfs under ``/sys/bus/i2c/devices``
where you can directly ``cd`` to it.
There is a list of symbolic links under that directory. The links that
start with ``i2c-`` are I2C buses, which may be either physical or logical. The
other links that begin with numbers and end with numbers are I2C devices, where
the first number is I2C bus number, and the second number is I2C address.
Google Pixel 3 phone for example::
blueline:/sys/bus/i2c/devices $ ls
0-0008 0-0061 1-0028 3-0043 4-0036 4-0041 i2c-1 i2c-3
0-000c 0-0066 2-0049 4-000b 4-0040 i2c-0 i2c-2 i2c-4
``i2c-2`` is an I2C bus whose number is 2, and ``2-0049`` is an I2C device
on bus 2 address 0x49 bound with a kernel driver.
Terminologies
=============
First, let us define a couple of terminologies to avoid confusions in the later
sections.
(Physical) I2C Bus Controller
-----------------------------
The hardware system that the Linux kernel is running on may have multiple
physical I2C bus controllers. The controllers are hardware and physical, and the
system may define multiple registers in the memory space to manipulate the
controllers. Linux kernel has I2C bus drivers under source directory
``drivers/i2c/busses`` to translate kernel I2C API into register
operations for different systems. This terminology is not limited to Linux
kernel only.
I2C Bus Physical Number
-----------------------
For each physical I2C bus controller, the system vendor may assign a physical
number to each controller. For example, the first I2C bus controller which has
the lowest register addresses may be called ``I2C-0``.
Logical I2C Bus
---------------
Every I2C bus number you see in Linux I2C Sysfs is a logical I2C bus with a
number assigned. This is similar to the fact that software code is usually
written upon virtual memory space, instead of physical memory space.
Each logical I2C bus may be an abstraction of a physical I2C bus controller, or
an abstraction of a channel behind an I2C MUX. In case it is an abstraction of a
MUX channel, whenever we access an I2C device via a such logical bus, the kernel
will switch the I2C MUX for you to the proper channel as part of the
abstraction.
Physical I2C Bus
----------------
If the logical I2C bus is a direct abstraction of a physical I2C bus controller,
let us call it a physical I2C bus.
Caveat
------
This may be a confusing part for people who only know about the physical I2C
design of a board. It is actually possible to rename the I2C bus physical number
to a different number in logical I2C bus level in Device Tree Source (DTS) under
section ``aliases``. See
`arch/arm/boot/dts/nuvoton-npcm730-gsj.dts
<../../arch/arm/boot/dts/nuvoton-npcm730-gsj.dts>`_
for an example of DTS file.
Best Practice: **(To kernel software developers)** It is better to keep the I2C
bus physical number the same as their corresponding logical I2C bus number,
instead of renaming or mapping them, so that it may be less confusing to other
users. These physical I2C buses can be served as good starting points for I2C
MUX fanouts. For the following examples, we will assume that the physical I2C
bus has a number same as their I2C bus physical number.
Walk through Logical I2C Bus
============================
For the following content, we will use a more complex I2C topology as an
example. Here is a brief graph for the I2C topology. If you do not understand
this graph at the first glance, do not be afraid to continue reading this doc
and review it when you finish reading.
::
i2c-7 (physical I2C bus controller 7)
`-- 7-0071 (4-channel I2C MUX at 0x71)
|-- i2c-60 (channel-0)
|-- i2c-73 (channel-1)
| |-- 73-0040 (I2C sensor device with hwmon directory)
| |-- 73-0070 (I2C MUX at 0x70, exists in DTS, but failed to probe)
| `-- 73-0072 (8-channel I2C MUX at 0x72)
| |-- i2c-78 (channel-0)
| |-- ... (channel-1...6, i2c-79...i2c-84)
| `-- i2c-85 (channel-7)
|-- i2c-86 (channel-2)
`-- i2c-203 (channel-3)
Distinguish Physical and Logical I2C Bus
----------------------------------------
One simple way to distinguish between a physical I2C bus and a logical I2C bus,
is to read the symbolic link ``device`` under the I2C bus directory by using
command ``ls -l`` or ``readlink``.
An alternative symbolic link to check is ``mux_device``. This link only exists
in logical I2C bus directory which is fanned out from another I2C bus.
Reading this link will also tell you which I2C MUX device created
this logical I2C bus.
If the symbolic link points to a directory ending with ``.i2c``, it should be a
physical I2C bus, directly abstracting a physical I2C bus controller. For
example::
$ readlink /sys/bus/i2c/devices/i2c-7/device
../../f0087000.i2c
$ ls /sys/bus/i2c/devices/i2c-7/mux_device
ls: /sys/bus/i2c/devices/i2c-7/mux_device: No such file or directory
In this case, ``i2c-7`` is a physical I2C bus, so it does not have the symbolic
link ``mux_device`` under its directory. And if the kernel software developer
follows the common practice by not renaming physical I2C buses, this should also
mean the physical I2C bus controller 7 of the system.
On the other hand, if the symbolic link points to another I2C bus, the I2C bus
presented by the current directory has to be a logical bus. The I2C bus pointed
by the link is the parent bus which may be either a physical I2C bus or a
logical one. In this case, the I2C bus presented by the current directory
abstracts an I2C MUX channel under the parent bus.
For example::
$ readlink /sys/bus/i2c/devices/i2c-73/device
../../i2c-7
$ readlink /sys/bus/i2c/devices/i2c-73/mux_device
../7-0071
``i2c-73`` is a logical bus fanout by an I2C MUX under ``i2c-7``
whose I2C address is 0x71.
Whenever we access an I2C device with bus 73, the kernel will always
switch the I2C MUX addressed 0x71 to the proper channel for you as part of the
abstraction.
Finding out Logical I2C Bus Number
----------------------------------
In this section, we will describe how to find out the logical I2C bus number
representing certain I2C MUX channels based on the knowledge of physical
hardware I2C topology.
In this example, we have a system which has a physical I2C bus 7 and not renamed
in DTS. There is a 4-channel MUX at address 0x71 on that bus. There is another
8-channel MUX at address 0x72 behind the channel 1 of the 0x71 MUX. Let us
navigate through Sysfs and find out the logical I2C bus number of the channel 3
of the 0x72 MUX.
First of all, let us go to the directory of ``i2c-7``::
~$ cd /sys/bus/i2c/devices/i2c-7
/sys/bus/i2c/devices/i2c-7$ ls
7-0071 i2c-60 name subsystem
delete_device i2c-73 new_device uevent
device i2c-86 of_node
i2c-203 i2c-dev power
There, we see the 0x71 MUX as ``7-0071``. Go inside it::
/sys/bus/i2c/devices/i2c-7$ cd 7-0071/
/sys/bus/i2c/devices/i2c-7/7-0071$ ls -l
channel-0 channel-3 modalias power
channel-1 driver name subsystem
channel-2 idle_state of_node uevent
Read the link ``channel-1`` using ``readlink`` or ``ls -l``::
/sys/bus/i2c/devices/i2c-7/7-0071$ readlink channel-1
../i2c-73
We find out that the channel 1 of 0x71 MUX on ``i2c-7`` is assigned
with a logical I2C bus number of 73.
Let us continue the journey to directory ``i2c-73`` in either ways::
# cd to i2c-73 under I2C Sysfs root
/sys/bus/i2c/devices/i2c-7/7-0071$ cd /sys/bus/i2c/devices/i2c-73
/sys/bus/i2c/devices/i2c-73$
# cd the channel symbolic link
/sys/bus/i2c/devices/i2c-7/7-0071$ cd channel-1
/sys/bus/i2c/devices/i2c-7/7-0071/channel-1$
# cd the link content
/sys/bus/i2c/devices/i2c-7/7-0071$ cd ../i2c-73
/sys/bus/i2c/devices/i2c-7/i2c-73$
Either ways, you will end up in the directory of ``i2c-73``. Similar to above,
we can now find the 0x72 MUX and what logical I2C bus numbers
that its channels are assigned::
/sys/bus/i2c/devices/i2c-73$ ls
73-0040 device i2c-83 new_device
73-004e i2c-78 i2c-84 of_node
73-0050 i2c-79 i2c-85 power
73-0070 i2c-80 i2c-dev subsystem
73-0072 i2c-81 mux_device uevent
delete_device i2c-82 name
/sys/bus/i2c/devices/i2c-73$ cd 73-0072
/sys/bus/i2c/devices/i2c-73/73-0072$ ls
channel-0 channel-4 driver of_node
channel-1 channel-5 idle_state power
channel-2 channel-6 modalias subsystem
channel-3 channel-7 name uevent
/sys/bus/i2c/devices/i2c-73/73-0072$ readlink channel-3
../i2c-81
There, we find out the logical I2C bus number of the channel 3 of the 0x72 MUX
is 81. We can later use this number to switch to its own I2C Sysfs directory or
issue ``i2c-tools`` commands.
Tip: Once you understand the I2C topology with MUX, command
`i2cdetect -l
<https://manpages.debian.org/unstable/i2c-tools/i2cdetect.8.en.html>`_
in
`I2C Tools
<https://i2c.wiki.kernel.org/index.php/I2C_Tools>`_
can give you
an overview of the I2C topology easily, if it is available on your system. For
example::
$ i2cdetect -l | grep -e '\-73' -e _7 | sort -V
i2c-7 i2c npcm_i2c_7 I2C adapter
i2c-73 i2c i2c-7-mux (chan_id 1) I2C adapter
i2c-78 i2c i2c-73-mux (chan_id 0) I2C adapter
i2c-79 i2c i2c-73-mux (chan_id 1) I2C adapter
i2c-80 i2c i2c-73-mux (chan_id 2) I2C adapter
i2c-81 i2c i2c-73-mux (chan_id 3) I2C adapter
i2c-82 i2c i2c-73-mux (chan_id 4) I2C adapter
i2c-83 i2c i2c-73-mux (chan_id 5) I2C adapter
i2c-84 i2c i2c-73-mux (chan_id 6) I2C adapter
i2c-85 i2c i2c-73-mux (chan_id 7) I2C adapter
Pinned Logical I2C Bus Number
-----------------------------
If not specified in DTS, when an I2C MUX driver is applied and the MUX device is
successfully probed, the kernel will assign the MUX channels with a logical bus
number based on the current biggest logical bus number incrementally. For
example, if the system has ``i2c-15`` as the highest logical bus number, and a
4-channel MUX is applied successfully, we will have ``i2c-16`` for the
MUX channel 0, and all the way to ``i2c-19`` for the MUX channel 3.
The kernel software developer is able to pin the fanout MUX channels to a static
logical I2C bus number in the DTS. This doc will not go through the details on
how to implement this in DTS, but we can see an example in:
`arch/arm/boot/dts/aspeed-bmc-facebook-wedge400.dts
<../../arch/arm/boot/dts/aspeed-bmc-facebook-wedge400.dts>`_
In the above example, there is an 8-channel I2C MUX at address 0x70 on physical
I2C bus 2. The channel 2 of the MUX is defined as ``imux18`` in DTS,
and pinned to logical I2C bus number 18 with the line of ``i2c18 = &imux18;``
in section ``aliases``.
Take it further, it is possible to design a logical I2C bus number schema that
can be easily remembered by humans or calculated arithmetically. For example, we
can pin the fanout channels of a MUX on bus 3 to start at 30. So 30 will be the
logical bus number of the channel 0 of the MUX on bus 3, and 37 will be the
logical bus number of the channel 7 of the MUX on bus 3.
I2C Devices
===========
In previous sections, we mostly covered the I2C bus. In this section, let us see
what we can learn from the I2C device directory whose link name is in the format
of ``${bus}-${addr}``. The ``${bus}`` part in the name is a logical I2C bus
decimal number, while the ``${addr}`` part is a hex number of the I2C address
of each device.
I2C Device Directory Content
----------------------------
Inside each I2C device directory, there is a file named ``name``.
This file tells what device name it was used for the kernel driver to
probe this device. Use command ``cat`` to read its content. For example::
/sys/bus/i2c/devices/i2c-73$ cat 73-0040/name
ina230
/sys/bus/i2c/devices/i2c-73$ cat 73-0070/name
pca9546
/sys/bus/i2c/devices/i2c-73$ cat 73-0072/name
pca9547
There is a symbolic link named ``driver`` to tell what Linux kernel driver was
used to probe this device::
/sys/bus/i2c/devices/i2c-73$ readlink -f 73-0040/driver
/sys/bus/i2c/drivers/ina2xx
/sys/bus/i2c/devices/i2c-73$ readlink -f 73-0072/driver
/sys/bus/i2c/drivers/pca954x
But if the link ``driver`` does not exist at the first place,
it may mean that the kernel driver failed to probe this device due to
some errors. The error may be found in ``dmesg``::
/sys/bus/i2c/devices/i2c-73$ ls 73-0070/driver
ls: 73-0070/driver: No such file or directory
/sys/bus/i2c/devices/i2c-73$ dmesg | grep 73-0070
pca954x 73-0070: probe failed
pca954x 73-0070: probe failed
Depending on what the I2C device is and what kernel driver was used to probe the
device, we may have different content in the device directory.
I2C MUX Device
--------------
While you may be already aware of this in previous sections, an I2C MUX device
will have symbolic link ``channel-*`` inside its device directory.
These symbolic links point to their logical I2C bus directories::
/sys/bus/i2c/devices/i2c-73$ ls -l 73-0072/channel-*
lrwxrwxrwx ... 73-0072/channel-0 -> ../i2c-78
lrwxrwxrwx ... 73-0072/channel-1 -> ../i2c-79
lrwxrwxrwx ... 73-0072/channel-2 -> ../i2c-80
lrwxrwxrwx ... 73-0072/channel-3 -> ../i2c-81
lrwxrwxrwx ... 73-0072/channel-4 -> ../i2c-82
lrwxrwxrwx ... 73-0072/channel-5 -> ../i2c-83
lrwxrwxrwx ... 73-0072/channel-6 -> ../i2c-84
lrwxrwxrwx ... 73-0072/channel-7 -> ../i2c-85
I2C Sensor Device / Hwmon
-------------------------
I2C sensor device is also common to see. If they are bound by a kernel hwmon
(Hardware Monitoring) driver successfully, you will see a ``hwmon`` directory
inside the I2C device directory. Keep digging into it, you will find the Hwmon
Sysfs for the I2C sensor device::
/sys/bus/i2c/devices/i2c-73/73-0040/hwmon/hwmon17$ ls
curr1_input in0_lcrit_alarm name subsystem
device in1_crit power uevent
in0_crit in1_crit_alarm power1_crit update_interval
in0_crit_alarm in1_input power1_crit_alarm
in0_input in1_lcrit power1_input
in0_lcrit in1_lcrit_alarm shunt_resistor
For more info on the Hwmon Sysfs, refer to the doc:
`Naming and data format standards for sysfs files
<../hwmon/sysfs-interface.rst>`_
Instantiate I2C Devices in I2C Sysfs
------------------------------------
Refer to the doc:
`How to instantiate I2C devices, Method 4: Instantiate from user-space
<instantiating-devices.rst#method-4-instantiate-from-user-space>`_