On 5/4/21 6:12 PM, Alex Qiu wrote:
This doc helps Linux users navigate through I2C sysfs and learn the system I2C topology. Signed-off-by: Alex Qiu <xqiu@xxxxxxxxxx> Cc: Guenter Roeck <linux@xxxxxxxxxxxx>
lgtm. I was struggling with the idea of renaming instances of MUX to "multiplexer" or "multiplexers", but ultimately that doesn't seem to be worth the effort, and at the end I wasn't sure if it would be an improvement at all. The only improvement I can think of would be to introduce the term "MUX" as shortcut for multiplexer(s) in the "Overview" section. Reviewed-by: Guenter Roeck <linux@xxxxxxxxxxxx>
--- Documentation/i2c/i2c-sysfs.rst | 394 ++++++++++++++++++++++++++++++++ 1 file changed, 394 insertions(+) create mode 100644 Documentation/i2c/i2c-sysfs.rst diff --git a/Documentation/i2c/i2c-sysfs.rst b/Documentation/i2c/i2c-sysfs.rst new file mode 100644 index 000000000000..58a90af8d966 --- /dev/null +++ b/Documentation/i2c/i2c-sysfs.rst @@ -0,0 +1,394 @@ +.. SPDX-License-Identifier: GPL-2.0 + +=============== +Linux I2C Sysfs +=============== + +Overview +======== + +I2C topology can be complex because of the existence of I2C MUX. 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>`_
