Kernel-with-device-tree

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 "machine" file that initializes/describes the devices for the board. So the kernel must be modified/compiled for each board.

Another way is to provide a small file that describes 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 initializes useful drivers.

Some informations on device tree may be found here:

• http://elinux.org/Device_Trees
• http://devicetree.org/Main_Page
• https://wiki.linaro.org/Boards/Origen/DeviceTree
• http://elinux.org/images/4/48/Experiences_With_Device_Tree_Support_Development_For_ARM-Based_SOC%27s.pdf

Compilation

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

U-Boot

Note: The APF27/U-boot 2012.04.01 patch 3.6, APF28/U-Boot 2012.04.01 patch 1.5, APF51/U-Boot 2012.04.01 patch 1.5 and APF6/All U-Boots 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.

• i.mxl (apf9328) : no
• i.mx27 (apf27) : yes
• i.mx51 (apf51) : yes
• i.mx28 (APF28) : yes
• i.mx6 (APF6) : yes
• i.mx6ul (OPOS6UL) : yes

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 all Armadeus platforms except APF9328.

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 (deprecated)

• Infos are kept here.

Boot Linux using U-Boot scripts

On the APF27/28/51, APF6 and OPOS6UL with the latest U-Boot version things are even simpler:

• Device Tree can be loaded from the Flash/eMMC 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
  • APF6: fdt_addr is set to 0x18000000 by default
  • OPOS6UL: fdt_addr is set to 0x88000000 by default
• Some scripts are available to update the dtb partition:
  • download_dtb (download the dtb into the RAM from your local network)
  • flash_dtb (transfer 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
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

Runtime changes

U-Boot

U-boot has support for live device tree content modifying through fdt command set. Following instructions apply for OPOS6UL and show how to change USB functionality of USB OTG connector (from "otg" to "host") of OPOS6ULDev.

BIOS> load mmc ${mmcdev}:${mmcpart} ${fdt_addr} /boot/imx6ul-${fdt_name}.dtb
BIOS> fdt addr ${fdt_addr}

BIOS> fdt get value toto /soc/aips-bus@02100000/usb@02184000/ dr_mode
BIOS> printenv toto
BIOS> fdt resize

BIOS> fdt set /soc/aips-bus@02100000/usb@02184000/ dr_mode host

# Remove loading of device tree from emmcboot command, as we already loaded it:
BIOS> setenv emmcboot 'run initargs; run addmmcargs; load mmc ${mmcdev}:${mmcpart} ${loadaddr} /boot/${kernelimg} && bootz ${loadaddr} - ${fdt_addr};'

BIOS> boot