Difference between revisions of "Kernel module"
m (→Configuration: don't use contractions; add templates; fix some grammar issues and shortening of words; update name of the SCSI mkinitcpio hook to "block") |
m (→Using files in /etc/modprobe.d/: use the term "initramfs" instead of "init ramdisk", etc.) |
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options thinkpad_acpi fan_control=1}} | options thinkpad_acpi fan_control=1}} | ||
− | {{Note|If any of the affected modules is loaded from the | + | {{Note|If any of the affected modules is loaded from the initramfs, then you will need to add the appropriate {{ic|.conf}} file to {{ic|FILES}} in {{ic|/etc/[[mkinitcpio.conf]]}} or use the {{ic|modconf}} [[Mkinitcpio.conf#HOOKS|hook]], so that it will be included in the initramfs.}} |
==== Using kernel command line ==== | ==== Using kernel command line ==== |
Revision as of 18:55, 16 January 2014
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.
Contents
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.
_
) 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
.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
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
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.
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) }