CODEWITHSUNDEEP
linux-kernelbootloaderdevice-treearm-linux
BakaOsaka
BakaOsaka

Reputation: 53

How does the linux kernel know about the initrd when booting with a device tree?

I am writing a bootloader for my Arm board (32-bit i.MX6) and want to boot the Linux kernel using a device tree and an initrd file located at a static location in memory.

I looked to U-Boot as a reference and I see I can use the bootm command to provide a kernel, device tree and ramdisk:

Boot application image from memory:
        bootm [addr [arg ...]] - boot application image stored in memory passing arguments 'arg
        ...'; when booting a Linux kernel,‘arg' can be the address of an initrd image

I know there are two ways to pass information from a bootloader to the kernel, the legacy ATAGS method and the modern device tree method.

With ATAGS this can be achieved with the ATAG_INITRD2 tag which describes the address that a ramdisk is stored in memory. However, with a flattened device tree no ATAGS are passed to the kernel (which is shown in the boot log with the "No ATAGs?" message). I do not see any method to specify a ramdisk when using a device tree.

If I look at the documentation for booting the Linux kernel on Arm I see the following interface is specified:

- CPU register settings
  r0 = 0,
  r1 = machine type number discovered in (3) above.
  r2 = physical address of tagged list in system RAM, or
       physical address of device tree block (dtb) in system RAM

In fact, the same document states that an initramfs must be configured prior to booting, yet includes no details on how to do so when booting with a device tree.

Is there some alternative way to achieve this? Is the required information appended to the device tree automatically by U-Boot, or is there an alternative way to notify the kernel of the ramdisk location?

Upvotes: 0

Views: 1856

Answers (1)

ensc
ensc

Reputation: 6984

It will be patched in the dtree by the bootloader; e.g.

/ {
    chosen {
        linux,initrd-start = <0x....>;
        linux,initrd-end = <0x....>;
    };

Upvotes: 1

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