1.. SPDX-License-Identifier: GPL-2.0 2 3=============== 4Linux I2C Sysfs 5=============== 6 7Overview 8======== 9 10I2C topology can be complex because of the existence of I2C MUX 11(I2C Multiplexer). The Linux 12kernel abstracts the MUX channels into logical I2C bus numbers. However, there 13is a gap of knowledge to map from the I2C bus physical number and MUX topology 14to logical I2C bus number. This doc is aimed to fill in this gap, so the 15audience (hardware engineers and new software developers for example) can learn 16the concept of logical I2C buses in the kernel, by knowing the physical I2C 17topology and navigating through the I2C sysfs in Linux shell. This knowledge is 18useful and essential to use ``i2c-tools`` for the purpose of development and 19debugging. 20 21Target audience 22--------------- 23 24People who need to use Linux shell to interact with I2C subsystem on a system 25which the Linux is running on. 26 27Prerequisites 28------------- 29 301. Knowledge of general Linux shell file system commands and operations. 31 322. General knowledge of I2C, I2C MUX and I2C topology. 33 34Location of I2C Sysfs 35===================== 36 37Typically, the Linux Sysfs filesystem is mounted at the ``/sys`` directory, 38so you can find the I2C Sysfs under ``/sys/bus/i2c/devices`` 39where you can directly ``cd`` to it. 40There is a list of symbolic links under that directory. The links that 41start with ``i2c-`` are I2C buses, which may be either physical or logical. The 42other links that begin with numbers and end with numbers are I2C devices, where 43the first number is I2C bus number, and the second number is I2C address. 44 45Google Pixel 3 phone for example:: 46 47 blueline:/sys/bus/i2c/devices $ ls 48 0-0008 0-0061 1-0028 3-0043 4-0036 4-0041 i2c-1 i2c-3 49 0-000c 0-0066 2-0049 4-000b 4-0040 i2c-0 i2c-2 i2c-4 50 51``i2c-2`` is an I2C bus whose number is 2, and ``2-0049`` is an I2C device 52on bus 2 address 0x49 bound with a kernel driver. 53 54Terminology 55=========== 56 57First, let us define some terms to avoid confusion in later sections. 58 59(Physical) I2C Bus Controller 60----------------------------- 61 62The hardware system that the Linux kernel is running on may have multiple 63physical I2C bus controllers. The controllers are hardware and physical, and the 64system may define multiple registers in the memory space to manipulate the 65controllers. Linux kernel has I2C bus drivers under source directory 66``drivers/i2c/busses`` to translate kernel I2C API into register 67operations for different systems. This terminology is not limited to Linux 68kernel only. 69 70I2C Bus Physical Number 71----------------------- 72 73For each physical I2C bus controller, the system vendor may assign a physical 74number to each controller. For example, the first I2C bus controller which has 75the lowest register addresses may be called ``I2C-0``. 76 77Logical I2C Bus 78--------------- 79 80Every I2C bus number you see in Linux I2C Sysfs is a logical I2C bus with a 81number assigned. This is similar to the fact that software code is usually 82written upon virtual memory space, instead of physical memory space. 83 84Each logical I2C bus may be an abstraction of a physical I2C bus controller, or 85an abstraction of a channel behind an I2C MUX. In case it is an abstraction of a 86MUX channel, whenever we access an I2C device via a such logical bus, the kernel 87will switch the I2C MUX for you to the proper channel as part of the 88abstraction. 89 90Physical I2C Bus 91---------------- 92 93If the logical I2C bus is a direct abstraction of a physical I2C bus controller, 94let us call it a physical I2C bus. 95 96Caveat 97------ 98 99This may be a confusing part for people who only know about the physical I2C 100design of a board. It is actually possible to rename the I2C bus physical number 101to a different number in logical I2C bus level in Device Tree Source (DTS) under 102section ``aliases``. See ``arch/arm/boot/dts/nuvoton-npcm730-gsj.dts`` 103for an example of DTS file. 104 105Best Practice: **(To kernel software developers)** It is better to keep the I2C 106bus physical number the same as their corresponding logical I2C bus number, 107instead of renaming or mapping them, so that it may be less confusing to other 108users. These physical I2C buses can be served as good starting points for I2C 109MUX fanouts. For the following examples, we will assume that the physical I2C 110bus has a number same as their I2C bus physical number. 111 112Walk through Logical I2C Bus 113============================ 114 115For the following content, we will use a more complex I2C topology as an 116example. Here is a brief graph for the I2C topology. If you do not understand 117this graph at first glance, do not be afraid to continue reading this doc 118and review it when you finish reading. 119 120:: 121 122 i2c-7 (physical I2C bus controller 7) 123 `-- 7-0071 (4-channel I2C MUX at 0x71) 124 |-- i2c-60 (channel-0) 125 |-- i2c-73 (channel-1) 126 | |-- 73-0040 (I2C sensor device with hwmon directory) 127 | |-- 73-0070 (I2C MUX at 0x70, exists in DTS, but failed to probe) 128 | `-- 73-0072 (8-channel I2C MUX at 0x72) 129 | |-- i2c-78 (channel-0) 130 | |-- ... (channel-1...6, i2c-79...i2c-84) 131 | `-- i2c-85 (channel-7) 132 |-- i2c-86 (channel-2) 133 `-- i2c-203 (channel-3) 134 135Distinguish Physical and Logical I2C Bus 136---------------------------------------- 137 138One simple way to distinguish between a physical I2C bus and a logical I2C bus, 139is to read the symbolic link ``device`` under the I2C bus directory by using 140command ``ls -l`` or ``readlink``. 141 142An alternative symbolic link to check is ``mux_device``. This link only exists 143in logical I2C bus directory which is fanned out from another I2C bus. 144Reading this link will also tell you which I2C MUX device created 145this logical I2C bus. 146 147If the symbolic link points to a directory ending with ``.i2c``, it should be a 148physical I2C bus, directly abstracting a physical I2C bus controller. For 149example:: 150 151 $ readlink /sys/bus/i2c/devices/i2c-7/device 152 ../../f0087000.i2c 153 $ ls /sys/bus/i2c/devices/i2c-7/mux_device 154 ls: /sys/bus/i2c/devices/i2c-7/mux_device: No such file or directory 155 156In this case, ``i2c-7`` is a physical I2C bus, so it does not have the symbolic 157link ``mux_device`` under its directory. And if the kernel software developer 158follows the common practice by not renaming physical I2C buses, this should also 159mean the physical I2C bus controller 7 of the system. 160 161On the other hand, if the symbolic link points to another I2C bus, the I2C bus 162presented by the current directory has to be a logical bus. The I2C bus pointed 163by the link is the parent bus which may be either a physical I2C bus or a 164logical one. In this case, the I2C bus presented by the current directory 165abstracts an I2C MUX channel under the parent bus. 166 167For example:: 168 169 $ readlink /sys/bus/i2c/devices/i2c-73/device 170 ../../i2c-7 171 $ readlink /sys/bus/i2c/devices/i2c-73/mux_device 172 ../7-0071 173 174``i2c-73`` is a logical bus fanout by an I2C MUX under ``i2c-7`` 175whose I2C address is 0x71. 176Whenever we access an I2C device with bus 73, the kernel will always 177switch the I2C MUX addressed 0x71 to the proper channel for you as part of the 178abstraction. 179 180Finding out Logical I2C Bus Number 181---------------------------------- 182 183In this section, we will describe how to find out the logical I2C bus number 184representing certain I2C MUX channels based on the knowledge of physical 185hardware I2C topology. 186 187In this example, we have a system which has a physical I2C bus 7 and not renamed 188in DTS. There is a 4-channel MUX at address 0x71 on that bus. There is another 1898-channel MUX at address 0x72 behind the channel 1 of the 0x71 MUX. Let us 190navigate through Sysfs and find out the logical I2C bus number of the channel 3 191of the 0x72 MUX. 192 193First of all, let us go to the directory of ``i2c-7``:: 194 195 ~$ cd /sys/bus/i2c/devices/i2c-7 196 /sys/bus/i2c/devices/i2c-7$ ls 197 7-0071 i2c-60 name subsystem 198 delete_device i2c-73 new_device uevent 199 device i2c-86 of_node 200 i2c-203 i2c-dev power 201 202There, we see the 0x71 MUX as ``7-0071``. Go inside it:: 203 204 /sys/bus/i2c/devices/i2c-7$ cd 7-0071/ 205 /sys/bus/i2c/devices/i2c-7/7-0071$ ls -l 206 channel-0 channel-3 modalias power 207 channel-1 driver name subsystem 208 channel-2 idle_state of_node uevent 209 210Read the link ``channel-1`` using ``readlink`` or ``ls -l``:: 211 212 /sys/bus/i2c/devices/i2c-7/7-0071$ readlink channel-1 213 ../i2c-73 214 215We find out that the channel 1 of 0x71 MUX on ``i2c-7`` is assigned 216with a logical I2C bus number of 73. 217Let us continue the journey to directory ``i2c-73`` in either ways:: 218 219 # cd to i2c-73 under I2C Sysfs root 220 /sys/bus/i2c/devices/i2c-7/7-0071$ cd /sys/bus/i2c/devices/i2c-73 221 /sys/bus/i2c/devices/i2c-73$ 222 223 # cd the channel symbolic link 224 /sys/bus/i2c/devices/i2c-7/7-0071$ cd channel-1 225 /sys/bus/i2c/devices/i2c-7/7-0071/channel-1$ 226 227 # cd the link content 228 /sys/bus/i2c/devices/i2c-7/7-0071$ cd ../i2c-73 229 /sys/bus/i2c/devices/i2c-7/i2c-73$ 230 231Either ways, you will end up in the directory of ``i2c-73``. Similar to above, 232we can now find the 0x72 MUX and what logical I2C bus numbers 233that its channels are assigned:: 234 235 /sys/bus/i2c/devices/i2c-73$ ls 236 73-0040 device i2c-83 new_device 237 73-004e i2c-78 i2c-84 of_node 238 73-0050 i2c-79 i2c-85 power 239 73-0070 i2c-80 i2c-dev subsystem 240 73-0072 i2c-81 mux_device uevent 241 delete_device i2c-82 name 242 /sys/bus/i2c/devices/i2c-73$ cd 73-0072 243 /sys/bus/i2c/devices/i2c-73/73-0072$ ls 244 channel-0 channel-4 driver of_node 245 channel-1 channel-5 idle_state power 246 channel-2 channel-6 modalias subsystem 247 channel-3 channel-7 name uevent 248 /sys/bus/i2c/devices/i2c-73/73-0072$ readlink channel-3 249 ../i2c-81 250 251There, we find out the logical I2C bus number of the channel 3 of the 0x72 MUX 252is 81. We can later use this number to switch to its own I2C Sysfs directory or 253issue ``i2c-tools`` commands. 254 255Tip: Once you understand the I2C topology with MUX, command 256`i2cdetect -l 257<https://manpages.debian.org/unstable/i2c-tools/i2cdetect.8.en.html>`_ 258in 259`I2C Tools 260<https://i2c.wiki.kernel.org/index.php/I2C_Tools>`_ 261can give you 262an overview of the I2C topology easily, if it is available on your system. For 263example:: 264 265 $ i2cdetect -l | grep -e '\-73' -e _7 | sort -V 266 i2c-7 i2c npcm_i2c_7 I2C adapter 267 i2c-73 i2c i2c-7-mux (chan_id 1) I2C adapter 268 i2c-78 i2c i2c-73-mux (chan_id 0) I2C adapter 269 i2c-79 i2c i2c-73-mux (chan_id 1) I2C adapter 270 i2c-80 i2c i2c-73-mux (chan_id 2) I2C adapter 271 i2c-81 i2c i2c-73-mux (chan_id 3) I2C adapter 272 i2c-82 i2c i2c-73-mux (chan_id 4) I2C adapter 273 i2c-83 i2c i2c-73-mux (chan_id 5) I2C adapter 274 i2c-84 i2c i2c-73-mux (chan_id 6) I2C adapter 275 i2c-85 i2c i2c-73-mux (chan_id 7) I2C adapter 276 277Pinned Logical I2C Bus Number 278----------------------------- 279 280If not specified in DTS, when an I2C MUX driver is applied and the MUX device is 281successfully probed, the kernel will assign the MUX channels with a logical bus 282number based on the current biggest logical bus number incrementally. For 283example, if the system has ``i2c-15`` as the highest logical bus number, and a 2844-channel MUX is applied successfully, we will have ``i2c-16`` for the 285MUX channel 0, and all the way to ``i2c-19`` for the MUX channel 3. 286 287The kernel software developer is able to pin the fanout MUX channels to a static 288logical I2C bus number in the DTS. This doc will not go through the details on 289how to implement this in DTS, but we can see an example in: 290``arch/arm/boot/dts/aspeed-bmc-facebook-wedge400.dts`` 291 292In the above example, there is an 8-channel I2C MUX at address 0x70 on physical 293I2C bus 2. The channel 2 of the MUX is defined as ``imux18`` in DTS, 294and pinned to logical I2C bus number 18 with the line of ``i2c18 = &imux18;`` 295in section ``aliases``. 296 297Take it further, it is possible to design a logical I2C bus number schema that 298can be easily remembered by humans or calculated arithmetically. For example, we 299can pin the fanout channels of a MUX on bus 3 to start at 30. So 30 will be the 300logical bus number of the channel 0 of the MUX on bus 3, and 37 will be the 301logical bus number of the channel 7 of the MUX on bus 3. 302 303I2C Devices 304=========== 305 306In previous sections, we mostly covered the I2C bus. In this section, let us see 307what we can learn from the I2C device directory whose link name is in the format 308of ``${bus}-${addr}``. The ``${bus}`` part in the name is a logical I2C bus 309decimal number, while the ``${addr}`` part is a hex number of the I2C address 310of each device. 311 312I2C Device Directory Content 313---------------------------- 314 315Inside each I2C device directory, there is a file named ``name``. 316This file tells what device name it was used for the kernel driver to 317probe this device. Use command ``cat`` to read its content. For example:: 318 319 /sys/bus/i2c/devices/i2c-73$ cat 73-0040/name 320 ina230 321 /sys/bus/i2c/devices/i2c-73$ cat 73-0070/name 322 pca9546 323 /sys/bus/i2c/devices/i2c-73$ cat 73-0072/name 324 pca9547 325 326There is a symbolic link named ``driver`` to tell what Linux kernel driver was 327used to probe this device:: 328 329 /sys/bus/i2c/devices/i2c-73$ readlink -f 73-0040/driver 330 /sys/bus/i2c/drivers/ina2xx 331 /sys/bus/i2c/devices/i2c-73$ readlink -f 73-0072/driver 332 /sys/bus/i2c/drivers/pca954x 333 334But if the link ``driver`` does not exist at the first place, 335it may mean that the kernel driver failed to probe this device due to 336some errors. The error may be found in ``dmesg``:: 337 338 /sys/bus/i2c/devices/i2c-73$ ls 73-0070/driver 339 ls: 73-0070/driver: No such file or directory 340 /sys/bus/i2c/devices/i2c-73$ dmesg | grep 73-0070 341 pca954x 73-0070: probe failed 342 pca954x 73-0070: probe failed 343 344Depending on what the I2C device is and what kernel driver was used to probe the 345device, we may have different content in the device directory. 346 347I2C MUX Device 348-------------- 349 350While you may be already aware of this in previous sections, an I2C MUX device 351will have symbolic link ``channel-*`` inside its device directory. 352These symbolic links point to their logical I2C bus directories:: 353 354 /sys/bus/i2c/devices/i2c-73$ ls -l 73-0072/channel-* 355 lrwxrwxrwx ... 73-0072/channel-0 -> ../i2c-78 356 lrwxrwxrwx ... 73-0072/channel-1 -> ../i2c-79 357 lrwxrwxrwx ... 73-0072/channel-2 -> ../i2c-80 358 lrwxrwxrwx ... 73-0072/channel-3 -> ../i2c-81 359 lrwxrwxrwx ... 73-0072/channel-4 -> ../i2c-82 360 lrwxrwxrwx ... 73-0072/channel-5 -> ../i2c-83 361 lrwxrwxrwx ... 73-0072/channel-6 -> ../i2c-84 362 lrwxrwxrwx ... 73-0072/channel-7 -> ../i2c-85 363 364I2C Sensor Device / Hwmon 365------------------------- 366 367I2C sensor device is also common to see. If they are bound by a kernel hwmon 368(Hardware Monitoring) driver successfully, you will see a ``hwmon`` directory 369inside the I2C device directory. Keep digging into it, you will find the Hwmon 370Sysfs for the I2C sensor device:: 371 372 /sys/bus/i2c/devices/i2c-73/73-0040/hwmon/hwmon17$ ls 373 curr1_input in0_lcrit_alarm name subsystem 374 device in1_crit power uevent 375 in0_crit in1_crit_alarm power1_crit update_interval 376 in0_crit_alarm in1_input power1_crit_alarm 377 in0_input in1_lcrit power1_input 378 in0_lcrit in1_lcrit_alarm shunt_resistor 379 380For more info on the Hwmon Sysfs, refer to the doc: 381 382../hwmon/sysfs-interface.rst 383 384Instantiate I2C Devices in I2C Sysfs 385------------------------------------ 386 387Refer to section "Method 4: Instantiate from user-space" of instantiating-devices.rst 388