Documentation: i2c: Add doc for I2C sysfs

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>

1 files changed, 395 insertions, 0 deletions

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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>`_