Kernel module

Related articles

• Boot Debugging
• Kernels
• Kernel parameters

Kernel modules are pieces of code that can be loaded and unloaded into the kernel upon demand. They extend the functionality of the kernel without the need to reboot the system.

Overview

To create a kernel module, you can read this guide. A module can be configured to be build-in or loadable. To dynamically load or remove a module, it has to be configured as a loadable module in the kernel configuration (the line related to the module will therefore display the letter M).

Modules are stored in /usr/lib/modules/kernel_release. You can use the command uname -r to get your current kernel release version.

Note: Module names often use underscores (_) or dashes (-), however those symbols are interchangeable both when using the modprobe command and in configuration files in /etc/modprobe.d/.

Obtaining information

To show what kernel modules are currently loaded:

$ lsmod

To show information about a module:

$ modinfo module_name

To list the options that are set for a loaded module:

$ systool -v -m module_name

To display the comprehensive configuration of all the modules:

$ modprobe -c | less

To display the configuration of a particular module:

$ modprobe -c | grep module_name

List the dependencies of a module (or alias), including the module itself:

$ modprobe --show-depends module_name

Configuration

Today, all necessary module loading is handled automatically by udev, so if you do not want or need to use any out-of-tree kernel modules, there is no need to put modules that should be loaded at boot in any configuration file. However, there are cases where you might want to load an extra module during the boot process, or blacklist another one for your computer to function properly.

Loading

Extra kernel modules to be loaded during boot are configured as a static list in files under /etc/modules-load.d/. Each configuration file is named in the style of /etc/modules-load.d/<program>.conf. Configuration files simply contain a list of kernel module names to load, separated by newlines. Empty lines and lines whose first non-whitespace character is # or ; are ignored.

/etc/modules-load.d/virtio-net.conf

# Load virtio-net.ko at boot
virtio-net

See modules-load.d(5) for more details.

Setting module options

To pass a parameter to a kernel module, you can use a modprobe configuration file or use the kernel command line.

Using files in /etc/modprobe.d/

Files in /etc/modprobe.d/ directory can be used to pass module settings to udev, which will use modprobe to manage the loading of the modules during system boot. Configuration files in this directory can have any name, given that they end with the .conf extension. The syntax is:

/etc/modprobe.d/myfilename.conf

options modname parametername=parametervalue

For example:

/etc/modprobe.d/thinkfan.conf

# On ThinkPads, this lets the 'thinkfan' daemon control fan speed
options thinkpad_acpi fan_control=1

Note: If any of the affected modules is loaded from the initramfs, then you will need to add the appropriate .conf file to FILES in /etc/mkinitcpio.conf or use the modconf hook, so that it will be included in the initramfs.

Using kernel command line

If the module is built into the kernel, you can also pass options to the module using the kernel command line. For all common bootloaders, the following syntax is correct:

modname.parametername=parametercontents

For example:

thinkpad_acpi.fan_control=1

Simply add this to your bootloader's kernel-line, as described in Kernel Parameters.

Aliasing

Aliases are alternate names for a module. For example: alias my-mod really_long_modulename means you can use modprobe my-mod instead of modprobe really_long_modulename. You can also use shell-style wildcards, so alias my-mod* really_long_modulename means that modprobe my-mod-something has the same effect. Create an alias:

/etc/modprobe.d/myalias.conf

alias mymod really_long_module_name

Some modules have aliases which are used to automatically load them when they are needed by an application. Disabling these aliases can prevent automatic loading but will still allow the modules to be manually loaded.

/etc/modprobe.d/modprobe.conf

# Prevent Bluetooth autoload
alias net-pf-31 off

Blacklisting

Blacklisting, in the context of kernel modules, is a mechanism to prevent the kernel module from loading. This could be useful if, for example, the associated hardware is not needed, or if loading that module causes problems: for instance there may be two kernel modules that try to control the same piece of hardware, and loading them together would result in a conflict.

Some modules are loaded as part of the initramfs. mkinitcpio -M will print out all automatically detected modules: to prevent the initramfs from loading some of those modules, blacklist them in /etc/modprobe.d/modprobe.conf. Running mkinitcpio -v will list all modules pulled in by the various hooks (e.g. filesystems hook, block hook, etc.). Remember to add that .conf file to the FILES section in /etc/mkinitcpio.conf, if you have not done so already, and rebuild the initramfs once you have blacklisted the modules, and reboot afterwards.

Using files in /etc/modprobe.d/

Create a .conf file inside /etc/modprobe.d/ and append a line for each module you want to blacklist, using the blacklist keyword. If for example you want to prevent the pcspkr module from loading:

/etc/modprobe.d/nobeep.conf

# Do not load the 'pcspkr' module on boot.
blacklist pcspkr

Note: The blacklist command will blacklist a module so that it will not be loaded automatically, but the module may be loaded if another non-blacklisted module depends on it or if it is loaded manually.

However, there is a workaround for this behaviour; the install command instructs modprobe to run a custom command instead of inserting the module in the kernel as normal, so you can force the module to always fail loading with:

/etc/modprobe.d/blacklist.conf

...
install module_name /bin/false
...

This will effectively blacklist that module and any other that depends on it.

Using kernel command line

Tip: This can be very useful if a broken module makes it impossible to boot your system.

You can also blacklist modules from the bootloader.

Simply add modprobe.blacklist=modname1,modname2,modname3 to your bootloader's kernel line, as described in Kernel parameters.

Note: When you are blacklisting more than one module, note that they are separated by commas only. Spaces or anything else might presumably break the syntax.

Manual module handling

Kernel modules are handled by tools provided by kmod package. You can use these tools manually.

To load a module:

# modprobe module_name

To unload a module:

# modprobe -r module_name

Or, alternatively:

# rmmod module_name

Tips and tricks

Bash function to list module parameters

Here is a nice bash function to be run as root that will show a list of all the currently loaded modules and all of their parameters, including the current value of the parameter. It uses /proc/modules to retrieve the current list of loaded modules, then access the module file directly with modinfo to grab a description of the module and descriptions for each param (if available), finally it accesses the sysfs filesystem to grab the actual parameter names and currently loaded values.

function aa_mod_parameters () 
{ 
    N=/dev/null;
    C=`tput op` O=$(echo -en "\n`tput setaf 2`>>> `tput op`");
    for mod in $(cat /proc/modules|cut -d" " -f1);
    do
        md=/sys/module/$mod/parameters;
        [[ ! -d $md ]] && continue;
        m=$mod;
        d=`modinfo -d $m 2>$N | tr "\n" "\t"`;
        echo -en "$O$m$C";
        [[ ${#d} -gt 0 ]] && echo -n " - $d";
        echo;
        for mc in $(cd $md; echo *);
        do
            de=`modinfo -p $mod 2>$N | grep ^$mc 2>$N|sed "s/^$mc=//" 2>$N`;
            echo -en "\t$mc=`cat $md/$mc 2>$N`";
            [[ ${#de} -gt 1 ]] && echo -en " - $de";
            echo;
        done;
    done
}

Here is some sample output:

# aa_mod_parameters

>>> ehci_hcd - USB 2.0 'Enhanced' Host Controller (EHCI) Driver
        hird=0 - hird:host initiated resume duration, +1 for each 75us (int)
        ignore_oc=N - ignore_oc:ignore bogus hardware overcurrent indications (bool)
        log2_irq_thresh=0 - log2_irq_thresh:log2 IRQ latency, 1-64 microframes (int)
        park=0 - park:park setting; 1-3 back-to-back async packets (uint)

>>> processor - ACPI Processor Driver
        ignore_ppc=-1 - ignore_ppc:If the frequency of your machine gets wronglylimited by BIOS, this should help (int)
        ignore_tpc=0 - ignore_tpc:Disable broken BIOS _TPC throttling support (int)
        latency_factor=2 - latency_factor: (uint)

>>> usb_storage - USB Mass Storage driver for Linux
        delay_use=1 - delay_use:seconds to delay before using a new device (uint)
        option_zero_cd=1 - option_zero_cd:ZeroCD mode (1=Force Modem (default), 2=Allow CD-Rom (uint)
        quirks= - quirks:supplemental list of device IDs and their quirks (string)
        swi_tru_install=1 - swi_tru_install:TRU-Install mode (1=Full Logic (def), 2=Force CD-Rom, 3=Force Modem) (uint)

>>> video - ACPI Video Driver
        allow_duplicates=N - allow_duplicates: (bool)
        brightness_switch_enabled=Y - brightness_switch_enabled: (bool)
        use_bios_initial_backlight=Y - use_bios_initial_backlight: (bool)

The following is a variation of the previous function that includes a full description for each parameter. The output is formatted a little differently and uses more colors.

function show_mod_parameter_info ()
{
  if tty -s <&1
  then
    green="\e[1;32m"
    yellow="\e[1;33m"
    cyan="\e[1;36m"
    reset="\e[0m"
  else
    green=
    yellow=
    cyan=
    reset=
  fi
  newline="
"

  while read mod
  do
    md=/sys/module/$mod/parameters
    [[ ! -d $md ]] && continue
    d="$(modinfo -d $mod 2>/dev/null | tr "\n" "\t")"
    echo -en "$green$mod$reset"
    [[ ${#d} -gt 0 ]] && echo -n " - $d"
    echo
    pnames=()
    pdescs=()
    pvals=()
    pdesc=
    add_desc=false
    while IFS="$newline" read p
    do
      if [[ $p =~ ^[[:space:]] ]]
      then
        pdesc+="$newline    $p"
      else
        $add_desc && pdescs+=("$pdesc")
        pname="${p%%:*}"
        pnames+=("$pname")
        pdesc=("    ${p#*:}")
        pvals+=("$(cat $md/$pname 2>/dev/null)")
      fi
      add_desc=true
    done < <(modinfo -p $mod 2>/dev/null)
    $add_desc && pdescs+=("$pdesc")
    for ((i=0; i<${#pnames[@]}; i++))
    do
      printf "  $cyan%s$reset = $yellow%s$reset\n%s\n" \
        ${pnames[i]} \
        "${pvals[i]}" \
        "${pdescs[i]}"
    done
    echo

  done < <(cut -d' ' -f1 /proc/modules | sort)
}

See also

• modprobe man page
• Disable PC Speaker Beep