Changes between Version 1 and Version 2 of linux/devicetree


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Timestamp:
10/21/2017 11:16:56 PM (12 months ago)
Author:
Chris Lang
Comment:

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  • linux/devicetree

    v1 v2  
    44The modern linux kernel uses a system called 'devicetree' to describe hardware in a consistent fashion to avoid needing custom 'board support' files for boards.
    55
    6 Typically the bootloader will pass a 'flattened device tree' (a compiled binary representation of a device-tree) to the kernel so that the kernel can configure all the components on the board. 
     6Typically the bootloader will pass a 'flattened device tree' (a compiled binary representation of a device-tree) to the kernel so that the kernel can configure all the components on the board.
    77
    88The Gateworks Ventana product family based off the Freescale i.MX6 CPU, uses devicetree (with the exception of the early Yocto 1.3 and Android jellybean BSP's which were based on a 3.0.35 non-device-tree kernel)
     
    1717
    1818== Example Device Tree File ==
    19 You can find the device tree files in arch/arm/boot/dts
     19You can find the device tree files in {{{arch/arm/boot/dts}}}
    2020
    2121For an example, see here: [https://github.com/Gateworks/linux-imx6/blob/gateworks_3.10.53_1.1.0_ga/arch/arm/boot/dts/imx6qdl-gw54xx.dtsi]
    2222
    2323
    24 == Accessing devicetree from the Bootloader ==
     24== Accessing Device Tree from the Bootloader ==
    2525The Ventana u-boot has fdt support enabled and uses this through default bootscripts to load an fdt (based on the board model) from the filesystem of the boot media.  The bootloader then modifies the devicetree to disable components that are possible on the board, but perhaps not loaded on the bill-of-materials (for example, a Gateworks Special build).
    2626
    27 If desired you can use the ftd u-boot command to access/modify the devicetree before the kernel is booted.
     27If desired you can use the {{{ftd}}} u-boot command to access/modify the devicetree before the kernel is booted.
    2828
    2929Examples:
    3030 * display the fdt of a Ventana board (loaded from NAND flash ubifs):
    31 {{{
     31{{{#!bash
    3232setenv fsload 'ubifsload'
    3333ubi part rootfs && ubifsmount ubi0:rootfs
     
    3636}}}
    3737 * disable PCI via device-tree:
    38 {{{
     38{{{#!bash
    3939setenv fsload 'ubifsload'
    4040ubi part rootfs && ubifsmount ubi0:rootfs
     
    4444}}}
    4545
    46 Note the Ventana bootloader has a '''fixfdt''' script that can make integrating fixups like this into the boot process easy. See [http://trac.gateworks.com/wiki/ventana/bootloader#Specifyinganalternatedevice-treeblobDTB here] for details.
     46Note the Ventana bootloader has a {{{fixfdt}}} script that can make integrating fixups like this into the boot process easy. See [http://trac.gateworks.com/wiki/ventana/bootloader#Specifyinganalternatedevice-treeblobDTB here] for details.
    4747
    4848
    49 == Accessing devicetree from Linux ==
    50 If the kernel is configured with CONFIG_PROC_DEVICETREE (which the Ventana OpenWrt BSP does configure) and procfs is enabled, you can access the devicetree via /proc/device-tree.  This can be useful to obtain information about the board that the bootloader configured, such as board model and serial number.
     49== Accessing Device Tree from Linux ==
     50If the kernel is configured with CONFIG_PROC_DEVICETREE (which the Ventana OpenWrt BSP does configure) and procfs is enabled, you can access the devicetree via {{{/proc/device-tree}}}.  This can be useful to obtain information about the board that the bootloader configured, such as board model and serial number.
    5151
    5252Examples:
    5353 * show board model:
    54 {{{
     54{{{#!bash
    5555echo $(cat /proc/device-tree/board)
    5656}}}
    5757 * show board serialnumber
    58 {{{
     58{{{#!bash
    5959echo $(cat /proc/device-tree/system-serial)
    6060}}}
    6161 * show devicetree compatible node (this describes which device-tree was used as there is one per base-board design):
    62 {{{
     62{{{#!bash
    6363echo $(cat /proc/device-tree/compatible)
    6464}}}
    65  * show chosen bootargs (the bootargs passed in by the bootloader, same as /proc/cmdline):
    66 {{{
     65 * show chosen bootargs (the bootargs passed in by the bootloader, same as {{{/proc/cmdline}}}):
     66{{{#!bash
    6767echo $(cat /proc/device-tree/chosen/bootargs)
     68}}}
     69 * Print the whole device tree:
     70{{{#!bash
     71find /proc/device-tree/
    6872}}}
    6973
    7074
    71 == Specifying the Device-tree that is used ==
    72 The [wiki:/linux/bootloader bootloader] is responsible for loading the device-tree blob (DTB) and executing the kernel 
     75== Specifying the Device Tree Used ==
     76The [wiki:/linux/bootloader bootloader] is responsible for loading the device-tree blob (DTB) and executing the kernel
    7377The Gateworks ventana bootloader in a way that it knows where the DTB is loaded.
    7478
    7579Therefore, its the bootloader that decides which DTB to load and from where.
    7680
    77 See [wiki:/ventana/bootloader] for details on how the DTB filename is chosen and loaded.
     81See [wiki:/ventana/bootloader] for details on how the DTB filename is chosen and loaded.
     82
     83== Adding New Devices to the Device Tree ==
     84For customers interested in adding a new device to an existing controller, see the [wiki:SPI] wiki page for an example of the process.
     85
     86== Compiling the Device Tree ==
     87If you need to change the device-tree you can easily compile it on a Linux system using the {{{dtc}}} app from the device-tree-compiler package:
     88{{{#!bash
     89apt-get install device-tree-compiler
     90dtc -O dtb -o imx6dl-gw51xx.dtb imqx6dl-gw51xx.dts
     91}}}
     92
     93You can also de-compile a dtb back to a dts:
     94{{{#!bash
     95$ dtc -I dtb -O dts imx6dl-gw51xx.dtb
     96}}}