Linux Kernel

Ecosystem

Linux Kernel Archives is the primary site for the Linux kernel source. All Linux kernel versions, including latest snapshot or "release candidate" (-rc) can be easily found here.

The ARM Linux Project lead by Russell King. Russell is in charge to ensure the ARM core support in the Kernel mainline.

The ARM-SOC git tree lead by Arnd Bergmann and Olof Johansson. It is the initiative that managed to unify the work being done on ARM-based System on Chip (SoC) by different vendors. This git tree is the natural way to send AT91-related material upstream.

The Linux-arm-kernel Mailing lists is where the work is actually being done. Discussions, bug reports and patches must be posted there.

Changes introduced by Device Tree support

The Device Tree is a binary file containing the description of your SoC and your board. Before this move to Device Tree, the information was stored in several C files directly in the Linux kernel code. The number of ARM-based SoC growing rapidly it has become clear that the amount of C code could not follow this pace. In addition to consolidating the ARM related kernel code, allowing the common code to be centralized and better maintained, the Device Tree also bring the possibility to run the same kernel binary on several platforms sharing the same ARM instruction set. This single zImage initiative allow the use of the same Linux kernel binary, without having the need for recompiling it, on another SoC, by just providing the proper Device Tree Binary file (.dtb).

Now the boot process has to deal with two Linux files instead of one: the Device Tree Binary file and the kernel image itself. It mainly involves changes for the U-Boot / barebox booting command. If you don't want to care about this new file, there is an Linux kernel option to append the Device Tree Binary at the end of the kernel image.

Useful documentation:

• Getting started information:
  1. http://elinux.org/Device_Trees
  2. Linaro DT presentation
  3. Platform devices and device trees introduction article on LWN
  4. A Tutorial on the Device Tree (part 1, 2, 3, 4 and 5)
  5. Device Tree support on ARM Linux slides

• References
  • ePAPR specification
  • Grant Likely article on LWN
  • Device Tree main website
    http://devicetree.org/Main_Page
    http://devicetree.org/Device_Tree_Usage
  • Linux kernel documentation: Documentation/devicetree/booting-without-of.txt

Pre-built images

Pre-built images are also delivered into uImage U-Boot format. To download them on AT91 boards, see the section How to load Linux kernel with U-Boot in the U-Boot page.

Linux for AT91 source code

The Linux kernel for Atmel ARM-based SoC (aka AT91) is distributed as a GIT tree hosted on GitHub as a fork of the official Linus Torvalds' git tree. The objective of the Atmel Linux team is to integrate all the AT91-related material in the official Linux Kernel.
Before completing this inclusion process, which can take time, this website provides the Linux4SAM Linux Kernel git tree for AT91 devices at:

https://github.com/linux4sam/linux-at91.

linux4sam_4.0 tag - particular tag for SAMA5D3x product family

• ARM Cortex-A5 support
• VFP support
• Timer Counter
• Watchdog (beware, can be deactivated by AT91Bootstrap)
• RTC
• Serial driver with DMA support
• I2C interface with DMA support
• SPI interface with DMA support
• CAN
• ADC (with 12 bits support)
• ISI interface
• MCI interface with DMA support
• NAND flash with hadware ECC and DMA support
• serial flash (SPI - at25)
• Enhanced Ethernet drivers & Enhanced Gigabit Ethernet drivers
• USB: host (OHCI & EHCI)
• USB: device/gadget high speed
• LCD base layer / LCD overlay 1, 2 / LCD High End Overlay
• Crypto engines (AES, DES/TDES, SHA) with DMA support
• Pinctrl (PIO muxing, PIO control and configuration)
• Resistive touchscreen
• Capacitive touchscreen maXTouch - PDA module
• Touch button QT1070
• LEDs
• Audio using SSC/I2S - wm8904 codec
• Power management

Get sources

To get the source code, you have to clone the repository:

$ git clone git://github.com/linux4sam/linux-at91.git
Cloning into 'linux-at91'...
remote: Counting objects: 5239592, done.
remote: Compressing objects: 100% (764014/764014), done.
Receiving objects: 100% (5239592/5239592), 1018.56 MiB | 11.18 MiB/s, done.
remote: Total 5239592 (delta 4448248), reused 5218219 (delta 4427054)
Resolving deltas: 100% (4448248/4448248), done.
Checking out files: 100% (38567/38567), done.
$ cd linux-at91
$ ls
arch     CREDITS        drivers   include  Kbuild   lib          mm      REPORTING-BUGS  security  usr
block    crypto         firmware  init     Kconfig  MAINTAINERS  net     samples         sound     virt
COPYING  Documentation  fs        ipc      kernel   Makefile     README  scripts         tools

The source code has been taken from the master branch which is pointing on the latest branch we use.

Note that you can also add this Linux4SAM repository as a remote GIT repository to your usual Linux git tree. It will save you a lot of bandwidth and download time:

$ git remote add linux4sam git://github.com/linux4sam/linux-at91.git
$ git remote update linux4sam
Fetching linux4sam
From git://github.com/linux4sam/linux-at91
 * [new branch]      at91-3.4-trunk_merge -> linux4sam/at91-3.4-trunk_merge
 * [new branch]      linus2639_5series_1.x -> linux4sam/linus2639_5series_1.x
 * [new branch]      linux-3.4.9-at91 -> linux4sam/linux-3.4.9-at91
 * [new branch]      linux-3.6.9-at91 -> linux4sam/linux-3.6.9-at91
 * [new branch]      master     -> linux4sam/master

If you want to use an other branch, you can list them and use one of them by doing this:

$ git branch -r
  origin/HEAD -> origin/master
  origin/at91-3.4-trunk_merge
  origin/linus2639_5series_1.x
  origin/linux-3.4.9-at91
  origin/linux-3.6.9-at91
  origin/master
$ git checkout origin/linux-3.6.9-at91 -b linux-3.6.9-at91
Branch linux-3.6.9-at91 set up to track remote branch linux-3.6.9-at91 from origin.
Switched to a new branch 'linux-3.6.9-at91'

Configure and Build the Linux kernel

Now you have to configure the Linux kernel according to your hardware. The default configuration for a particular SoC is a good starting point that you will be able to adapt to your needs afterwards.

$ cd linux-at91
$ ls arch/arm/configs/+(at91|sam)*
arch/arm/configs/at91_dt_defconfig      arch/arm/configs/at91sam9g45_defconfig
arch/arm/configs/at91rm9200_defconfig   arch/arm/configs/at91sam9rl_defconfig
arch/arm/configs/at91sam9260_defconfig  arch/arm/configs/at91x40_defconfig
arch/arm/configs/at91sam9261_defconfig  arch/arm/configs/sam9_l9260_defconfig
arch/arm/configs/at91sam9263_defconfig  arch/arm/configs/sama5d3_defconfig
arch/arm/configs/at91sam9g20_defconfig
$
$ make ARCH=arm <name of your SoC_defconfig>  # sama5d3_defconfig for instance
  HOSTCC  scripts/basic/fixdep
  HOSTCC  scripts/kconfig/conf.o
  SHIPPED scripts/kconfig/zconf.tab.c
  SHIPPED scripts/kconfig/zconf.lex.c
  SHIPPED scripts/kconfig/zconf.hash.c
  HOSTCC  scripts/kconfig/zconf.tab.o
  HOSTLD  scripts/kconfig/conf
#
# configuration written to .config
#

At this step, you can modify default configuration using the menuconfig

$ make ARCH=arm menuconfig 

And build the Linux kernel image

$ make ARCH=arm  CROSS_COMPILE=<path_to_cross-compiler/cross-compiler-prefix-> zImage

[..]

  Kernel: arch/arm/boot/Image is ready
  Kernel: arch/arm/boot/zImage is ready

make ARCH=arm  CROSS_COMPILE=<path_to_cross-compiler/cross-compiler-prefix-> uImage

[..]

  Kernel: arch/arm/boot/Image is ready
  Kernel: arch/arm/boot/zImage is ready
  UIMAGE  arch/arm/boot/uImage
Image Name:   Linux-3.6.9+
Created:      Mon Feb  4 12:26:46 2013
Image Type:   ARM Linux Kernel Image (uncompressed)
Data Size:    2837352 Bytes = 2770.85 kB = 2.71 MB
Load Address: 20008000
Entry Point:  20008000
  Image arch/arm/boot/uImage is ready

make ARCH=arm  CROSS_COMPILE=<path_to_cross-compiler/cross-compiler-prefix-> dtbs

[..]

  DTC     arch/arm/boot/at91sam9g20ek.dtb
DTC: dts->dtb  on file "arch/arm/boot/dts/at91sam9g20ek.dts"
  DTC     arch/arm/boot/at91sam9g20ek_2mmc.dtb
DTC: dts->dtb  on file "arch/arm/boot/dts/at91sam9g20ek_2mmc.dts"

[..]

DTC: dts->dtb  on file "arch/arm/boot/dts/at91sam9m10g45ek.dts"
  DTC     arch/arm/boot/at91sam9n12ek.dtb
DTC: dts->dtb  on file "arch/arm/boot/dts/at91sam9n12ek.dts"
  DTC     arch/arm/boot/at91sam9g15ek.dtb
DTC: dts->dtb  on file "arch/arm/boot/dts/at91sam9g15ek.dts"
  DTC     arch/arm/boot/at91sam9g25ek.dtb
DTC: dts->dtb  on file "arch/arm/boot/dts/at91sam9g25ek.dts"
  DTC     arch/arm/boot/at91sam9g35ek.dtb
DTC: dts->dtb  on file "arch/arm/boot/dts/at91sam9g35ek.dts"
  DTC     arch/arm/boot/at91sam9x25ek.dtb
DTC: dts->dtb  on file "arch/arm/boot/dts/at91sam9x25ek.dts"
  DTC     arch/arm/boot/at91sam9x35ek.dtb
DTC: dts->dtb  on file "arch/arm/boot/dts/at91sam9x35ek.dts"
  DTC     arch/arm/boot/sama5d31ek.dtb
DTC: dts->dtb  on file "arch/arm/boot/dts/sama5d31ek.dts"
  DTC     arch/arm/boot/sama5d31ek_pda.dtb
DTC: dts->dtb  on file "arch/arm/boot/dts/sama5d31ek_pda.dts"
  DTC     arch/arm/boot/sama5d33ek.dtb
DTC: dts->dtb  on file "arch/arm/boot/dts/sama5d33ek.dts"
  DTC     arch/arm/boot/sama5d33ek_pda.dtb
DTC: dts->dtb  on file "arch/arm/boot/dts/sama5d33ek_pda.dts"
  DTC     arch/arm/boot/sama5d34ek.dtb
DTC: dts->dtb  on file "arch/arm/boot/dts/sama5d34ek.dts"
  DTC     arch/arm/boot/sama5d34ek_pda.dtb
DTC: dts->dtb  on file "arch/arm/boot/dts/sama5d34ek_pda.dts"
  DTC     arch/arm/boot/sama5d35ek.dtb
DTC: dts->dtb  on file "arch/arm/boot/dts/sama5d35ek.dts"

path_to_cross-compiler is only needed if it is not in your PATH.
Usually cross-compiler-prefix- looks like arm-none-linux-gnueabi- or arm-linux-gnueabi-

If the building process is successful, the final images can be found under arch/arm/boot/ directory.

$ ls arch/arm/boot/
aks-cdu.dtb             at91sam9m10g45ek.dtb  ethernut5.dtb       sama5d33ek.dtb      tny_a9g20.dtb
at91sam9263ek.dtb       at91sam9n12ek.dtb     evk-pro3.dtb        sama5d33ek_pda.dtb  usb_a9260.dtb
at91sam9g15ek.dtb       at91sam9x25ek.dtb     install.sh          sama5d34ek.dtb      usb_a9263.dtb
at91sam9g20ek_2mmc.dtb  at91sam9x35ek.dtb     kizbox.dtb          sama5d34ek_pda.dtb  usb_a9g20.dtb
at91sam9g20ek.dtb       bootp                 Makefile            sama5d35ek.dtb
at91sam9g25ek.dtb       compressed            sama5d31ek.dtb      tny_a9260.dtb
at91sam9g35ek.dtb       dts                   sama5d31ek_pda.dtb  tny_a9263.dtb