.. _bone-cook-book-kernel:
The Kernel
###########
Introduction
---------------------------------
The kernel is the heart of the Linux operating system. It's the software that takes the
low-level requests, such as reading or writing files, or reading and writing general-purpose
input/output (GPIO) pins, and maps them to the hardware. When you install a new version of the
OS (:ref:`<basics_latest_os>`), you get a certain version of the kernel.
You usually won't need to mess with the kernel, but sometimes you might want to try something new
that requires a different kernel. This chapter shows how to switch kernels. The nice thing is you
can have multiple kernels on your system at the same time and select from among them which to boot up.
// TODO is this still true?
.. note::
We assume here that you are logged on to your Bone as +root+ and superuser privileges.
You also need to be logged in to your Linux host computer as a nonsuperuser.
Updating the Kernel
---------------------
Problem
***********
You have an out-of-date kernel and want to want to make it current.
Solution
***********
Use the following command to determine which kernel you are running:
.. code-block:: bash
bone$ uname -a
Linux beaglebone 3.8.13-bone67 #1 SMP Wed Sep 24 21:30:03 UTC 2014 armv7l
GNU/Linux
The +3.8.13-bone67+ string is the kernel version.
To update to the current kernel, ensure that your Bone is on the Internet
(:ref:`<networking_usb>` or :ref:`<networking_wired>`) and then run the following commands:
.. code-block:: bash
bone$ apt-cache pkgnames | grep linux-image | sort | less
...
linux-image-3.15.8-armv7-x5
linux-image-3.15.8-bone5
linux-image-3.15.8-bone6
...
linux-image-3.16.0-rc7-bone1
...
linux-image-3.8.13-bone60
linux-image-3.8.13-bone61
linux-image-3.8.13-bone62
bone$ sudo apt install linux-image-3.14.23-ti-r35
bone$ sudo reboot
bone$ uname -a
Linux beaglebone 3.14.23-ti-r35 #1 SMP PREEMPT Wed Nov 19 21:11:08 UTC 2014 armv7l
GNU/Linux
The first command lists the versions of the kernel that are available. The second command installs one.
After you have rebooted, the new kernel will be running.
If the current kernel is doing its job adequately, you probably don't need to update, but sometimes a new
software package requires a more up-to-date kernel. Fortunately, precompiled kernels are available and ready to download.
Discussion
***********
.. _kernel_building_modules:
Building and Installing Kernel Modules
---------------------------------
Problem
***********
You need to use a peripheral for which there currently is no driver, or you need to improve the
performance of an interface previously handled in user space.
Solution
***********
The solution is to run in kernel space by building a kernel module. There are entire
`books on writing Linux Device Drivers <http://bit.ly/1Fb0usf>`_. This recipe assumes that
the driver has already been written and shows how to compile and install it. After you've
followed the steps for this simple module, you will be able to apply them to any other module.
For our example module, add the code in :ref:`<kernel_simple_module>` to a file called _hello.c_.
.. _kernel_simple_module:
Simple Kernel Module (hello.c)
.. code-block:: JavaScript
include::code/hello.c
When compiling on the Bone, all you need to do is load the Kernel Headers for the version of the kernel you're running:
.. code-block:: bash
bone$ sudo apt install linux-headers-`uname -r`
.. note::
The quotes around +`uname -r`+ are backtick characters. On a United States keyboard,
the backtick key is to the left of the 1 key.
This took a little more than three minutes on my Bone. The +`uname -r`+ part of the command
looks up what version of the kernel you are running and loads the headers for it.
Next, add the code in :ref:`<kernel_Makefle>` to a file called _Makefile_.
.. _kernel_Makefle:
Simple Kernel Module (_Makefile_)
.. code-block:: JavaScript
include::code/Makefile.display
.. note::
Replace the two instances of +<TAB>+ with a tab character (the key left of the Q key on a United States keyboard).
The tab characters are very important to makefiles and must appear as shown.
Now, compile the kernel module by using the +make+ command:
.. code-block:: bash
bone$ make
make -C /lib/modules/3.8.13-bone67/build \
SUBDIRS=/root/cookbook-atlas/code/hello modules
make[1]: Entering directory `/usr/src/linux-headers-3.8.13-bone67'
CC [M] /root/cookbook-atlas/code/hello/hello.o
Building modules, stage 2.
MODPOST 1 modules
CC /root/cookbook-atlas/code/hello/hello.mod.o
LD [M] /root/cookbook-atlas/code/hello/hello.ko
make[1]: Leaving directory `/usr/src/linux-headers-3.8.13-bone67'
bone$ ls
Makefile hello.c hello.mod.c hello.o
Module.symvers hello.ko hello.mod.o modules.order
Notice that several files have been created. _hello.ko_ is the one you want. Try a couple of commands with it:
.. code-block:: bash
bone$ modinfo hello.ko
filename: /root/hello/hello.ko
srcversion: 87C6AEED7791B4B90C3B50C
depends:
vermagic: 3.8.13-bone67 SMP mod_unload modversions ARMv7 thumb2 p2v8
bone$ sudo insmod hello.ko
bone$ dmesg | tail -4
[419313.320052] bone-iio-helper helper.15: ready
[419313.322776] bone-capemgr bone_capemgr.9: slot #8: Applied #1 overlays.
[491540.999431] Loading hello module...
[491540.999476] Hello world
The first command displays information about the module. The +insmod+ command inserts the module into the running kernel.
If all goes well, nothing is displayed, but the module does print something in the kernel log. The +dmesg+ command displays
the messages in the log, and the +tail -4+ command shows the last four messages. The last two messages are from the module. It worked!
Discussion
***********
.. _kernel_LEDs:
Controlling LEDs by Using SYSFS Entries
---------------------------------
Problem
***********
You want to control the onboard LEDs from the command line.
Solution
***********
On Linux, http://bit.ly/1AjhWUW[everything is a file]; that is, you can access all the inputs and outputs, the LEDs,
and so on by opening the right _file_ and reading or writing to it. For example, try the following:
.. code-block:: bash
bone$ cd /sys/class/leds/
bone$ ls
beaglebone:green:usr0 beaglebone:green:usr2
beaglebone:green:usr1 beaglebone:green:usr3
What you are seeing are four directories, one for each onboard LED. Now try this:
.. code-block:: bash
bone$ cd beaglebone\:green\:usr0
bone$ ls
brightness device max_brightness power subsystem trigger uevent
bone$ cat trigger
none nand-disk mmc0 mmc1 timer oneshot [heartbeat]
backlight gpio cpu0 default-on transient
The first command changes into the directory for LED +usr0+, which is the LED closest to the edge of the board.
The +[heartbeat]+ indicates that the default trigger (behavior) for the LED is to blink in the heartbeat pattern.
Look at your LED. Is it blinking in a heartbeat pattern?
Then try the following:
.. code-block:: bash
bone$ echo none > trigger
bone$ cat trigger
[none] nand-disk mmc0 mmc1 timer oneshot heartbeat
backlight gpio cpu0 default-on transient
This instructs the LED to use +none+ for a trigger. Look again. It should be no longer blinking.
Now, try turning it on and off:
.. code-block:: bash
bone$ echo 1 > brightness
bone$ echo 0 > brightness
The LED should be turning on and off with the commands.
Discussion
***********
.. _kernel_gpio_sysfs:
Controlling GPIOs by Using SYSFS Entries
-----------------------------------------
Problem
***********
You want to control a GPIO pin from the command line.
Solution
***********
:ref:`<kernel_LEDs>` introduces the +sysfs+. This recipe shows how to read and write a GPIO pin.
Reading a GPIO Pin via sysfs
******************************
Suppose that you want to read the state of the +P9_42+ GPIO pin. (:ref:`<sensors_pushbutton>` shows how to wire a switch to +P9_42+.)
First, you need to map the +P9+ header location to GPIO number using :ref:`<kernel_gpio_map_fig>`, which shows that +P9_42+ maps to GPIO 7.
.. _kernel_gpio_map_fig:
.Mapping P9_42 header position to GPIO 7
.. figure:: figures/cape-headers-digitalGPIO7.png
:align: center
:alt: Mapping Header Position to GPIO Numbers
Next, change to the GPIO +sysfs+ directory:
.. code-block:: bash
bone$ cd /sys/class/gpio/
bone$ ls
export gpiochip0 gpiochip32 gpiochip64 gpiochip96 unexport
The +ls+ command shows all the GPIO pins that have be exported. In this case, none have,
so you see only the four GPIO controllers. Export using the +export+ command:
.. code-block:: bash
bone$ echo 7 > export
bone$ ls
export gpio7 gpiochip0 gpiochip32 gpiochip64 gpiochip96 unexport
Now you can see the _gpio7_ directory. Change into the _gpio7_ directory and look around:
.. code-block:: bash
bone$ cd gpio7
bone$ ls
active_low direction edge power subsystem uevent value
bone$ cat direction
in
bone$ cat value
0
Notice that the pin is already configured to be an input pin. (If it wasn't already configured that way,
use +echo in > direction+ to configure it.) You can also see that its current value is +0+—that is, it
isn't pressed. Try pressing and holding it and running again:
.. code-block:: bash
bone$ cat value
1
The +1+ informs you that the switch is pressed. When you are done with GPIO 7, you can always +unexport+ it:
.. code-block:: bash
bone$ cd ..
bone$ echo 7 > unexport
bone$ ls
export gpiochip0 gpiochip32 gpiochip64 gpiochip96 unexport
Writing a GPIO Pin via sysfs
******************************
Now, suppose that you want to control an external LED. :ref:`<displays_externalLED>` shows
how to wire an LED to +P9_14+. :ref:`<kernel_gpio_map_fig>` shows +P9_14+ is GPIO 50. Following
the approach in :ref:`<kernel_gpio_sysfs>`, enable GPIO 50 and make it an output:
.. code-block:: bash
bone$ cd /sys/class/gpio/
bone$ echo 50 > export
bone$ ls
gpio50 gpiochip0 gpiochip32 gpiochip64 gpiochip96
bone$ cd gpio50
bone$ ls
active_low direction edge power subsystem uevent value
bone$ cat direction
in
By default, +P9_14+ is set as an input. Switch it to an output and turn it on:
.. code-block:: bash
bone$ echo out > direction
bone$ echo 1 > value
bone$ echo 0 > value
The LED turns on when a +1+ is written to +value+ and turns off when a +0+ is written.
Discussion
***********
.. _kernel_compiling:
Compiling the Kernel
----------------------
Problem
***********
You need to download, patch, and compile the kernel from its source code.
Solution
***********
This is easier than it sounds, thanks to some very powerful scripts.
.. warning::
Be sure to run this recipe on your host computer. The Bone has enough computational
power to compile a module or two, but compiling the entire kernel takes lots of time and resourses.
Downloading and Compiling the Kernel
**************************************
To download and compile the kernel, follow these steps:
.. code-block:: bash
host$ git clone https://github.com/RobertCNelson/bb-kernel.git # <1>
host$ cd bb-kernel
host$ git tag # <2>
host$ git checkout 3.8.13-bone60 -b v3.8.13-bone60 # <3>
host$ ./build_kernel.sh # <4>
1. The first command clones a repository with the tools to build the kernel for the Bone.
2. This command lists all the different versions of the kernel that you can build. You'll need to pick one of these. How do you know which one to pick? A good first step is to choose the one you are currently running. +uname -a+ will reveal which one that is. When you are able to reproduce the current kernel, go to http://kernelnewbies.org/[Linux Kernel Newbies] to see what features are available in other kernels. http://bit.ly/1AjiL00[LinuxChanges] shows the features in the newest kernel and http://bit.ly/1MrIHx3[LinuxVersions] links to features of pervious kernels.
3. When you know which kernel to try, use +git checkout+ to check it out. This command checks out at tag +3.8.13-bone60+ and creates a new branch, +v3.8.13-bone60+.
4. +build_kernel+ is the master builder. If needed, it will download the cross compilers needed to compile the kernel (linaro [http://www.linaro.org/] is the current cross compiler). If there is a kernel at _~/linux-dev_, it will use it; otherwise, it will download a copy to _bb-kernel/ignore/linux-src_. It will then patch the kernel so that it will run on the Bone.
After the kernel is patched, you'll see a screen similar to :ref:`<kernel_config_fig>`, on which you can configure the kernel.
.. _kernel_config_fig:
Kernel configuration menu
.. figure:: figures/KernelConfig3.16.png
:align: center
:alt: Kernel configuration menu
You can use the arrow keys to navigate. No changes need to be made, so you can just press the right
arrow and Enter to start the kernel compiling. The entire process took about 25 minutes on my 8-core host.
The _bb-kernel/KERNEL_ directory contains the source code for the kernel. The _bb-kernel/deploy_
directory contains the compiled kernel and the files needed to run it.
.. _kernel_install:
Installing the Kernel on the Bone
***********************************
To copy the new kernel and all its files to the microSD card, you need to halt the Bone,
and then pull the microSD card out and put it in an microSD card reader on your host computer.
Run +Disk+ (see :ref:`<basics_latest_os>`) to learn where the microSD card appears on your host
(mine appears in _/dev/sdb_). Then open the _bb-kernel/system.sh_ file and find this line near the end:
.. code-block:: bash
MMC=/dev/sde
Change that line to look like this (where +/dev/sdb+ is the path to your device):
.. code-block:: bash
MMC=/dev/sdb
Now, while in the _bb-kernel_ directory, run the following command:
.. code-block:: bash
host$ tools/install_kernel.sh
[sudo] password for yoder:
I see...
fdisk -l:
Disk /dev/sda: 160.0 GB, 160041885696 bytes
Disk /dev/sdb: 3951 MB, 3951034368 bytes
Disk /dev/sdc: 100 MB, 100663296 bytes
lsblk:
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sda 8:0 0 149.1G 0 disk
├─sda1 8:1 0 141.1G 0 part /
├─sda2 8:2 0 1K 0 part
└─sda5 8:5 0 8G 0 part [SWAP]
sdb 8:16 1 3.7G 0 disk
├─sdb1 8:17 1 16M 0 part
└─sdb2 8:18 1 3.7G 0 part
sdc 8:32 1 96M 0 disk
-----------------------------
Are you 100% sure, on selecting [/dev/sdb] (y/n)? y
The script lists the partitions it sees and asks if you have the correct one.
If you are sure, press Y, and the script will uncompress and copy the files to
the correct locations on your card. When this is finished, eject your card, plug
it into the Bone, and boot it up. Run +uname -a+, and you
will see that you are running your compiled kernel.
Discussion
***********
.. _kernel_using_cross_compiler:
Using the Installed Cross Compiler
------------------------------------
Problem
***********
You have followed the instructions in :ref:`<kernel_compiling>`
and want to use the cross compiler it has downloaded.
[TIP]
You can cross-compile without installing the entire kernel source by running the following:
.. code-block:: bash
host$ sudo apt install gcc-arm-linux-gnueabihf
Then skip down to :ref:`<kernel_skip_to_here>`.
Solution
***********
:ref:`<kernel_compiling>` installs a cross compiler, but you need to set up a
couple of things so that it can be found. :ref:`<kernel_compiling>` installed the
kernel and other tools in a directory called _bb-kernel_. Run the
following commands to find the path to the cross compiler:
.. code-block:: bash
host$ cd bb-kernel/dl
host$ ls
gcc-linaro-arm-linux-gnueabihf-4.7-2013.04-20130415_linux
gcc-linaro-arm-linux-gnueabihf-4.7-2013.04-20130415_linux.tar.xz
Here, the path to the cross compiler contains the version number
of the compiler. Yours might be different from mine. +cd+ into it:
.. code-block:: bash
host$ cd gcc-linaro-arm-linux-gnueabihf-4.7-2013.04-20130415_linux
host$ ls
20130415-gcc-linaro-arm-linux-gnueabihf bin libexec
arm-linux-gnueabihf lib share
At this point, we are interested in what's in _bin_:
.. code-block:: bash
host$ cd bin
host$ ls
arm-linux-gnueabihf-addr2line arm-linux-gnueabihf-gfortran
arm-linux-gnueabihf-ar arm-linux-gnueabihf-gprof
arm-linux-gnueabihf-as arm-linux-gnueabihf-ld
arm-linux-gnueabihf-c++ arm-linux-gnueabihf-ld.bfd
arm-linux-gnueabihf-c++filt arm-linux-gnueabihf-ldd
arm-linux-gnueabihf-cpp arm-linux-gnueabihf-ld.gold
arm-linux-gnueabihf-ct-ng.config arm-linux-gnueabihf-nm
arm-linux-gnueabihf-elfedit arm-linux-gnueabihf-objcopy
arm-linux-gnueabihf-g++ arm-linux-gnueabihf-objdump
arm-linux-gnueabihf-gcc arm-linux-gnueabihf-pkg-config
arm-linux-gnueabihf-gcc-4.7.3 arm-linux-gnueabihf-pkg-config-real
arm-linux-gnueabihf-gcc-ar arm-linux-gnueabihf-ranlib
arm-linux-gnueabihf-gcc-nm arm-linux-gnueabihf-readelf
arm-linux-gnueabihf-gcc-ranlib arm-linux-gnueabihf-size
arm-linux-gnueabihf-gcov arm-linux-gnueabihf-strings
arm-linux-gnueabihf-gdb arm-linux-gnueabihf-strip
What you see are all the cross-development tools. You need to add this directory
to the +$PATH+ the shell uses to find the commands it runs:
.. code-block:: bash
host$ pwd
/home/yoder/BeagleBoard/bb-kernel/dl/\
gcc-linaro-arm-linux-gnueabihf-4.7-2013.04-20130415_linux/bin
host$ echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:\
/usr/games:/usr/local/games
The first command displays the path to the directory where the cross-development
tools are located. The second shows which directories are searched to find commands
to be run. Currently, the cross-development tools are not in the +$PATH+. Let's add it:
.. code-block:: bash
host$ export PATH=`pwd`:$PATH
host$ echo $PATH
/home/yoder/BeagleBoard/bb-kernel/dl/\
gcc-linaro-arm-linux-gnueabihf-4.7-2013.04-20130415_linux/bin:\
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:\
/usr/games:/usr/local/games
.. note::
Those are backtick characters (left of the "1" key on your keyboard) around +pwd+.
The second line shows the +$PATH+ now contains the directory with the cross-development tools.
.. _kernel_skip_to_here:
Setting Up Variables
*********************
Now, set up a couple of variables to know which compiler you are using:
.. code-block:: bash
host$ export ARCH=arm
host$ export CROSS_COMPILE=arm-linux-gnueabihf-
These lines set up the standard environmental variables so that you can determine which cross-development
tools to use. Test the cross compiler by adding :ref:`<kernel_helloWorld>` to a file named _helloWorld.c_.
.. _kernel_helloWorld:
Simple helloWorld.c to test cross compiling (helloWorld.c)
.. code-block:: C
include::code/helloWorld.c
You can then cross-compile by using the following commands:
.. code-block:: bash
host$ ${CROSS_COMPILE}gcc helloWorld.c
host$ file a.out
a.out: ELF 32-bit LSB executable, ARM, version 1 (SYSV),
dynamically linked (uses shared libs), for GNU/Linux 2.6.31,
BuildID[sha1]=0x10182364352b9f3cb15d1aa61395aeede11a52ad, not stripped
The +file+ command shows that +a.out+ was compiled for an ARM processor.
Discussion
***********
.. _kernel_patches:
Applying Patches
--------------------
Problem
***********
You have a patch file that you need to apply to the kernel.
Solution
***********
:ref:`<kernel_hello_patch>` shows a patch file that you can use on the kernel.
.. _kernel_hello_patch:
Simple kernel patch file (hello.patch)
.. code-block:: C
include::code/hello.patch[]
Here's how to use it:
- Install the kernel sources (:ref:`<kernel_compiling>`).
- Change to the kernel directory (+cd bb-kernel/KERNEL+).
- Add :ref:`<kernel_hello_patch>` to a file named _hello.patch_ in the _bb-kernel/KERNEL_ directory.
- Run the following commands:
.. code-block:: bash
host$ cd bb-kernel/KERNEL
host$ patch -p1 < hello.patch
patching file hello/Makefile
patching file hello/hello.c
The output of the +patch+ command apprises you of what it's doing.
Look in the _hello_ directory to see what was created:
.. code-block:: bash
host$ cd hello
host$ ls
hello.c Makefile
Discussion
***********
:ref:`<kernel_building_modules>` shows how to build and install a module, and :ref:`<kernel_create_patch>`
shows how to create your own patch file.
.. _kernel_create_patch:
Creating Your Own Patch File
---------------------------------
Problem
***********
You made a few changes to the kernel, and you want to share them with your friends.
Solution
***********
Create a patch file that contains just the changes you have made. Before making your changes, check out a new branch:
.. code-block:: bash
host$ cd bb-kernel/KERNEL
host$ git status
# On branch master
nothing to commit (working directory clean)
Good, so far no changes have been made. Now, create a new branch:
.. code-block:: bash
host$ git checkout -b hello1
host$ git status
# On branch hello1
nothing to commit (working directory clean)
You've created a new branch called _hello1_ and checked it out. Now, make whatever changes
to the kernel you want. I did some work with a simple character driver that we can use as an example:
.. code-block:: bash
host$ cd bb-kernel/KERNEL/drivers/char/
host$ git status
# On branch hello1
# Changes not staged for commit:
# (use "git add file..." to update what will be committed)
# (use "git checkout -- file..." to discard changes in working directory)
#
# modified: Kconfig
# modified: Makefile
#
# Untracked files:
# (use "git add file..." to include in what will be committed)
#
# examples/
no changes added to commit (use "git add" and/or "git commit -a")
Add the files that were created and commit them:
.. code-block:: bash
host$ git add Kconfig Makefile examples
host$ git status
# On branch hello1
# Changes to be committed:
# (use "git reset HEAD file..." to unstage)
#
# modified: Kconfig
# modified: Makefile
# new file: examples/Makefile
# new file: examples/hello1.c
#
host$ git commit -m "Files for hello1 kernel module"
[hello1 99346d5] Files for hello1 kernel module
4 files changed, 33 insertions(+)
create mode 100644 drivers/char/examples/Makefile
create mode 100644 drivers/char/examples/hello1.c
Finally, create the patch file:
.. code-block:: bash
host$ git format-patch master --stdout > hello1.patch
Discussion
***********