Driving Me Nuts - Device Classes

With the i2c-dev driver and two i2c adapter drivers (the i2c-piix4 and i2c-isa drivers) loaded, the /sys/class/i2c-dev directory might look like the following:

$ tree /sys/class/i2c-dev/
/sys/class/i2c-dev/
|-- i2c-0
|   |-- dev
|   |-- device -> ../../../devices/pci0/00:07.3
|   `-- driver -> ../../../bus/pci/drivers/piix4-smbus
`-- i2c-2
    |-- dev
    |-- device -> ../../../devices/legacy/i2c-2
    `-- driver -> ../../../bus/i2c/drivers/i2c_adapter

The dev file in the /sys/class/i2c-dev/i2c-2/ directory would contain the following string:

$ cat /sys/class/i2c-dev/i2c-2/dev
5902

which corresponds to major number 86 and minor number 2, the character major and minor numbers for this specific device.

Also, the /sys/bus/i2c/ directory with a few i2c client drivers loaded looks like:

$ tree /sys/bus/i2c/
/sys/bus/i2c/
|-- devices
|   |-- 0-0050 -> ../../../devices/pci0/00:07.3/i2c-0/0-0050
|   |-- 0-0051 -> ../../../devices/pci0/00:07.3/i2c-0/0-0051
|   |-- 0-0052 -> ../../../devices/pci0/00:07.3/i2c-0/0-0052
|   |-- 0-0053 -> ../../../devices/pci0/00:07.3/i2c-0/0-0053
|   `-- 2-0290 -> ../../../devices/legacy/i2c-2/2-0290
`-- drivers
    |-- dev driver
    |-- eeprom
    |   |-- 0-0050 -> ../../../../devices/pci0/00:07.3/i2c-0/0-0050
    |   |-- 0-0051 -> ../../../../devices/pci0/00:07.3/i2c-0/0-0051
    |   |-- 0-0052 -> ../../../../devices/pci0/00:07.3/i2c-0/0-0052
    |   `-- 0-0053 -> ../../../../devices/pci0/00:07.3/i2c-0/0-0053
    |-- i2c_adapter
    `-- w83781d
        `-- 2-0290 -> ../../../../devices/legacy/i2c-2/2-0290

And, the actual /sys/devices/ directories for the i2c adapters look like:

$ tree /sys/devices/pci0/00:07.3
/sys/devices/pci0/00:07.3
|-- class
|-- device
|-- i2c-0
|   |-- 0-0050
|   |   |-- eeprom_00
|   |   |-- name
|   |   `-- power
|   |-- 0-0051
|   |   |-- eeprom_00
|   |   |-- name
|   |   `-- power
|   |-- 0-0052
|   |   |-- eeprom_00
|   |   |-- name
|   |   `-- power
|   |-- 0-0053
|   |   |-- eeprom_00
|   |   |-- name
|   |   `-- power
|   |-- name
|   `-- power
|-- irq
|-- name
|-- power
|-- resource
|-- subsystem_device
|-- subsystem_vendor
`-- vendor

and:

$ tree /sys/devices/legacy/i2c-2/
/sys/devices/legacy/i2c-2/
|-- 2-0290
|   |-- alarms
|   |-- beep_enable
|   |-- beep_mask
|   |-- fan_div1
|   |-- fan_div2
|   |-- fan_div3
|   |-- fan_input1
|   |-- fan_input2
|   |-- fan_input3
|   |-- fan_min1
|   |-- fan_min2
|   |-- fan_min3
|   |-- in_input0
|   |-- in_input1
|   |-- in_input2
|   |-- in_input3
|   |-- in_input4
|   |-- in_input5
|   |-- in_input6
|   |-- in_input7
|   |-- in_input8
|   |-- in_max0
|   |-- in_max1
|   |-- in_max2
|   |-- in_max3
|   |-- in_max4
|   |-- in_max5
|   |-- in_max6
|   |-- in_max7
|   |-- in_max8
|   |-- in_min0
|   |-- in_min1
|   |-- in_min2
|   |-- in_min3
|   |-- in_min4
|   |-- in_min5
|   |-- in_min6
|   |-- in_min7
|   |-- in_min8
|   |-- name
|   |-- power
|   |-- pwm1
|   |-- pwm2
|   |-- pwm_enable2
|   |-- sensor1
|   |-- sensor2
|   |-- sensor3
|   |-- temp_input1
|   |-- temp_input2
|   |-- temp_input3
|   |-- temp_max1
|   |-- temp_max2
|   |-- temp_max3
|   |-- temp_min1
|   |-- temp_min2
|   |-- temp_min3
|   |-- vid
|   `-- vrm
|-- name
`-- power

I think the best description of the kernel driver model's use of interconnected structure pointers and representation to the user was issued by Jonathan Corbet: “web woven by a spider on drugs” (lwn.net/Articles/31185/). Hopefully, these two articles have helped you unravel the loony web, showing the true interconnectedness of all devices within the kernel.

I would like to thank Pat Mochel for creating such a powerful and complete framework in which all kernel drivers and devices easily can be shown to the user. Also, a big thanks to all of the kernel driver subsystem maintainers who have gladly converted their subsystems over to this model; without their help, the driver core code would have been little more than a nice academic exercise.