How to Install and Use lm_sensors

Overview

This article describes why you might want to add sensor data to your appliance, and how to install and use lm_sensors.

Introduction to LM Sensors

Most newer motherboards and disk drives have on-board sensors to measure system voltages, temperatures, and fan speeds. The first common part used to make these measurements was the Nation Semiconductor LM74, hence the "lm" in the name. Why bother with sensors? Because you will be able to catch a computer failure before it occurs. Many computer components slip out of tolerance before failure. This is especially true of moving components. For example, the CPU fan is usually the critical component most likely to fail. The lubrication in the bearings drys out and the increased friction in the fan causes it to heat up and to eventually fail. The failure is easy to predict since the fan speed will drop off as the bearing friction goes up.

You get a failure warning when the fan speed drops by more than 10 percent. While this example focuses on fan speed, the other sensors can be used to predict other types of failures. Out of tolerance voltages may predict a power supply failure or a cooling problem in the power supply. A high case temperature may indicate dirty or blocked air filters.

There are a few requirements for using sensors. First, you need to care. After all, you don't want to spend the cost and effort of including sensors if the cost of system failure is low. For example, a consumer set-top box is expected to only last a couple of years and consumers are relatively unbothered by appliance failures. A second, more critical, requirement is that the information be "actionable". That is, if you can predict a system failure, there needs to be a way for someone to prevent it and there needs to be a reliable way to tell someone of the impending failure. This where logs and alarms enter the picture. If your system does not include logs and alarms, you probably don't need to support sensors.

Installing LM Sensors

Sensors are really just devices so an "installation" should leave us with the proper device drivers loaded. The next two sections show how to identify and install the proper device drivers for your motherboard.

Installation Requirements

Make sure your motherboard has sensors. You can usually determine this from one of the BIOS menus at system power-on. The motherboard manual should also tell you if sensors are on the board or not. If you use one of the standard Linux distributions, be sure to have your installation disks handy. You can also get the packages you'll need with app-get or from www.rpmfind.org. Two packages are required, liblm_sensors, and lm_sensors itself, and you will need to be root to install lm_sensors. The sensor detection process tries to install several modules, so you will need to have modules enabled on your system.

Installation Procedure

We need to determine the types of sensors used on our motherboard and the program to do that is sensors-detect from the lm_sensors package. We install liblm_sensors first. The lm_sensors library installs only a single file and a symbolic link to that file.

# rpm -qlp liblm_sensors3-2.9.1-4mdk.i586.rpm 
/usr/lib64/libsensors.so.3
/usr/lib64/libsensors.so.3.0.8
# rpm -i liblm_sensors-2.9.1-4mdk.i586.rpm 

The lm_sensors package nominally requires the "Round Robin Database". In fact, it does not, and it's unfortunate that the maintainer of lm_sensors lists it as a requirement. Install lm_sensors with --nodeps to keep it from complaining about the lack of the rrd package.

# rpm -i --nodeps lm_sensors-2.9.1-4mdk.i586.rpm

With lm_sensors installed, you can run sensors-detect to locate and identify the sensors on your system. The sensors-detect program asks lots of questions about what it should probe for. Take the default answers the first time you try the sensor detection. We show below just the start of the sensors-detect run.

# sensors-detect 

This program will help you to determine which I2C/SMBus modules you need to load to use lm_sensors most effectively. If you are installing from a tarball or have a custom kernel, you may need to do a `make install', issue a `depmod -a' and made sure that `/etc/conf.modules' (or `/etc/modules.conf') contains the appropriate module path before you can use sensors-detect.

If you are using a standard distribution, you almost certainly do not need to "make install" in order to use sensors-detect. Sensors-detect will try to detect what is on your system and which modules to load. If all goes well, sensors-detect will output a list of which modules you need to load. For example, on my system the final output was:

To make the sensors modules behave correctly, add these lines to 
either /etc/modules.conf or /etc/conf.modules:

#----cut here----
# I2C module options
alias char-major-89 i2c-dev
#----cut here----

To load everything that is needed, add this to some /etc/rc* file:

#----cut here----
# I2C adapter drivers
modprobe i2c-viapro
modprobe i2c-isa
# I2C chip drivers
modprobe w83781d
# sleep 2 # optional
/usr/local/bin/sensors -s # recommended
#----cut here----

If sensors-detect does not find any sensors, you may need to run it again answering the questions in such a way as to ask it to do more extensive probing. Since some of the extensive probes can interfere with some VGA hardware, you should run extensive probes from a system console and not from X-Windows.

If you have a very recent motherboard, your distribution's kernel might not have the drivers for the sensors on your motherboard. It is a lot of work, but you can get around this by downloading and building the latest kernel and lm_sensors. A web search on the make and model of your motherboard and lm_sensors may give you a clue how others have solved the same problem.

Using Sensors

After running sensors-detect, do a modprobe on each of the modules it recommends.

#modprobe i2c-viapro
#modprobe i2c-isa
#modprobe w83627hf

Getting Sensor Data

With the modules successfully installed, you can finally see the values of your motherboard's sensors with the sensors command.

# sensors
w83697hf-isa-0290
Adapter: ISA adapter
VCore:     +1.50 V  (min =  +0.59 V, max =  +0.06 V)
+3.3V:     +3.25 V  (min =  +0.56 V, max =  +2.05 V)
+5V:       +4.95 V  (min =  +0.91 V, max =  +0.05 V)
+12V:     +11.86 V  (min =  +1.46 V, max =  +0.06 V)
-12V:     -11.70 V  (min = -14.58 V, max =  -7.01 V)
-5V:       -5.20 V  (min =  -2.49 V, max =  -6.50 V)
V5SB:      +5.46 V  (min =  +0.00 V, max =  +0.86 V)
VBat:      +3.55 V  (min =  +0.26 V, max =  +1.09 V)
fan1:      3850 RPM  (min = 112500 RPM, div = 2)
fan2:      0 RPM  (min = 3461 RPM, div = 2)
temp1:     +33°C  (high =   +32°C, hyst =    +8°C)   sensor = thermistor
temp2:     +48.0°C  (high =   +80°C, hyst =   +75°C)   sensor = diode
alarms:
beep_enable:
          Sound alarm disabled

The sensors command is fine if a user wants to see the data, but a more convenient way for programs and scripts to get the data is from the /proc filesystem. Depending on the version of your kernel, the sensor data will appear in either /proc/sys/dev/sensors/xxxx, where xxxx a string based on the type and location of the sensors detected, or in a similar location in /sys/devices/platform/i2c-2/. On my system the sensor data is in /sys/devices/platform/i2c-2/2-0290. A listing the contents of the directory show a file for each of the sensors on the system.

# ls /sys/devices/platform/i2c-2/2-0290
alarms       fan2_min   in4_input  in7_max      temp1_max
beep_enable  in0_input  in4_max    in7_min      temp1_max_hyst
beep_mask    in0_max    in4_min    in8_input    temp1_type
bus@         in0_min    in5_input  in8_max      temp2_input
driver@      in2_input  in5_max    in8_min      temp2_max
fan1_div     in2_max    in5_min    name         temp2_max_hyst
fan1_input   in2_min    in6_input  power/       temp2_type
fan1_min     in3_input  in6_max    pwm1
fan2_div     in3_max    in6_min    pwm2
fan2_input   in3_min    in7_input  temp1_input

You can just cat the file corresponding to the sensor that you want to read.

# cat /sys/devices/platform/i2c-2/2-0290/temp1_input
33000

There are two warning to heed if you use /sys or /proc to read the sensor values. The first is that the values in the files may need to be calibrated. On my system, temp1, the CPU temperature, clearly does not read the real value. Be sure to calibrate your reading against the values that you see in the BIOS screen with sensor data. The other problem is that taking a reading can sometimes take a long time. The chip that does the actual measurement sometime needs hundreds of milliseconds to take a reading. Do not use blocking IO if your program needs to respond to other, faster events.

Undoing the Installation

Once you've run sensors-detect and found which modules to load, you can remove the lm_sensors packages if you wish, You'll lose the sensors program but if you're working on an appliance you don't really need it.

# rpm -e lm_sensors-2.9.1-4mdk
# rpm -e liblm_sensors3-2.9.1-4mdk 

We usually remove the sensors packages from the shipping appliances as a way to save flash disk space. We also use the "tbl2file" utility to map the sensors values in /sys into values that we can read as a DB table. Treating the sensor data as values in a DB makes some aspects of user interface design much simpler.