Kernel-with-device-tree

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Background

The idea behind device tree is quite simple. When the kernel starts, it needs to initialize the drivers for the devices on the board. But often on embedded systems, devices can't be discover at running time. In this case, the Linux kernel has a c file that initialize the devices for the board. So the kernel must be modified/compiled for each board.

Another way is to provide a small file that describe the board to the Linux kernel. When the kernel start, it reads this file (called dtb for Device Tree Binary) to know the devices on the board, and initialize usefull drivers.

Some information on device tree may be found here:

Compilation

To use device tree on arm, both bootloader (U-Boot) and kernel should be compiled with the support of device tree.

U-Boot

Note Note: The APF27/U-boot 2012.04.01 patch 3.6, APF28/U-Boot 2012.04.01 patch 1.5 and APF51/U-Boot 2012.04.01 patch 1.5 are already configured to support device tree.
  • If you have an older U-Boot, instructions are kept here.

Linux

The 3.4+ kernels have already the Device Tree support for some ARM SoC.

  • imxl (apf9328) : no
  • imx27 (apf27) : yes
  • imx51 (apf51) : yes
  • imx28 (apf28) : yes

The Linux configuration file should have (should be done by default):

  • CONFIG_MACH_IMX27_DT (on APF27)
  • CONFIG_MACH_IMX51_DT (on APF51)
  • CONFIG_MACH_MXS_DT (on APF28)
  • CONFIG_SERIAL_IMX
  • CONFIG_SERIAL_IMX_CONSOLE

Nothing else needs to be done, because the kernel image is build by the armadeus SDK with the "make" command.

  • Starting with 3.8+ kernels, multi arch support is activated for i.MX(1/2/3/5) processors and so loadaddr should be given at uImage build time:
$ make linux LOADADDR=0xA0008000       (APF27)

$ make linux LOADADDR=0x40008000       (APF28)

$ make linux LOADADDR=0x90008000       (APF51)
  • or configured inside Buildroot (2013.05+), here for APF51:
$ make menuconfig

    Kernel  --->
        (0x90008000) load address (for 3.7+ multi-platform image)

dtb

A dtb (Device Tree Binary) file is created from a dts (Device Tree Source) file.

The 3.8+ kernel already have the support of device tree for i.mx27 (APF27), i.mx28 (APF28) and i.mx51 (APF51).

If your board hasn't a dts file, a <board>.dts must be added in the directory arch/arm/boot/dts/ of the linux kernel.

Only recent Buildroot have direct support for dtb image generation (example for APF51): 

$ make menuconfig

Kernel  --->
    ...
    [*]   Device tree support
          Device tree source (Use a device tree present in the kernel.)  --->
          (imx51-apf51dev) Device Tree Source file names

If your armadeus SDK/BSP doesn't have support for dtb image generation, you will have to do it yourself:

  • go to the directory buildroot/output/build/linux-<version>
  • add the file <soc>-<board>.dts the directory arch/arm/boot/dts/
  • build the dtb with the command:
$ make ARCH=arm CROSS_COMPILE=../../host/usr/bin/arm-linux- <soc>-<board>.dtb

The file <soc>-<board>.dtb should then appear in arch/arm/boot/. Copy it to your TFTP directory: 

$ cp buildroot/output/build/linux-3.5-rc5/arch/arm/boot/imx28-apf28.dtb /tftpboot/apf28.dtb

by naming your dtb file in /tftpboot apfXX.dtb you will be able to use the U-Boot 2013.03 scripts download_dtb, flash_dtb and update_dtb. These scripts are equivalent to usual kernel scripts download_kernel, flash_kernel and update_kernel.

Running

Boot Linux manually

Start the U-Boot previously compiled or use the latest U-Boot 2013.04, and then :

  • load the kernel in memory
  • load the dtb in memory
  • run the command bootm with 3 arguments: <kernel addr> - <dtb addr>

The second argument is "-", it means that there isn't an initrd to load.

For example, on APF27, it should be something like this: 

BIOS> setenv consoledev ttymxc0
BIOS> setenv baudrate 115200
BIOS> setenv dtbaddr 0xa0800000
BIOS> tftp $loadaddr apf27-linux.bin
BIOS> tftp $dtbaddr imx27-apf27.dtb
BIOS> setenv consoledev ttymxc0
BIOS> setenv console console=$consoledev,115200
BIOS> setenv bootargs ${console} earlyprintk
BIOS> run addnfsargs addipargs
BIOS> bootm $loadaddr - $dtbaddr

Boot Linux using U-Boot scripts

On the APF27, 28 and 51 with the latest U-Boot version things are even simpler:

  • Device Tree can be loaded from the Flash memory
  • The variable fdt_addr_r defines the RAM address to load the DTB structure and will be used to notify the kernel of the DT location in RAM
  • APF27: Set the fdt_addr_r to A1000000 - APF28: set the fdt_addr_r to 41000000 - APF51: set the fdt_addr_r to 91000000
  • Some scripts are availaible to update the dtb partition: download_dtb (download the dtb in RAM from your local network) - flash_dtb (tranfert a dtb from RAM to flash) - update_dtb (download and flash a dtb)
  • the boot scripts ubifsboot, nfsboot and mmcboot support booting a kernel with a DT

Here is the example on the APF27: 

BIOS> setenv consoledev ttymxc0
BIOS> setenv fdt_addr_r a1000000 (with the apf27 - 41000000 with an apf28 - 91000000 with an apf51 )
BIOS> saveenv
BIOS> run update_kernel
BIOS> run update_dtb
BIOS> boot

Here is the example on the APF51: 

BIOS> setenv consoledev ttymxc2
BIOS> setenv fdt_addr_r 91000000
BIOS> saveenv
BIOS> run update_kernel
BIOS> run update_dtb
BIOS> boot

Here is the example on the APF28: 

BIOS> setenv consoledev ttyAMA0
BIOS> setenv fdt_addr_r 41000000
BIOS> saveenv
BIOS> run update_kernel
BIOS> run update_dtb
BIOS> boot

Remove DT boot

  • just in case you want to come back to normal boot:
BIOS> setenv fdt_addr_r
BIOS> saveenv
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