xref: /freebsd/sys/contrib/device-tree/Bindings/chosen.txt (revision c66ec88fed842fbaad62c30d510644ceb7bd2d71)

The chosen node
---------------

The chosen node does not represent a real device, but serves as a place
for passing data between firmware and the operating system, like boot
arguments. Data in the chosen node does not represent the hardware.

The following properties are recognized:


kaslr-seed
-----------

This property is used when booting with CONFIG_RANDOMIZE_BASE as the
entropy used to randomize the kernel image base address location. Since
it is used directly, this value is intended only for KASLR, and should
not be used for other purposes (as it may leak information about KASLR
offsets). It is parsed as a u64 value, e.g.

/ {
	chosen {
		kaslr-seed = <0xfeedbeef 0xc0def00d>;
	};
};

Note that if this property is set from UEFI (or a bootloader in EFI
mode) when EFI_RNG_PROTOCOL is supported, it will be overwritten by
the Linux EFI stub (which will populate the property itself, using
EFI_RNG_PROTOCOL).

stdout-path
-----------

Device trees may specify the device to be used for boot console output
with a stdout-path property under /chosen, as described in the Devicetree
Specification, e.g.

/ {
	chosen {
		stdout-path = "/serial@f00:115200";
	};

	serial@f00 {
		compatible = "vendor,some-uart";
		reg = <0xf00 0x10>;
	};
};

If the character ":" is present in the value, this terminates the path.
The meaning of any characters following the ":" is device-specific, and
must be specified in the relevant binding documentation.

For UART devices, the preferred binding is a string in the form:

	<baud>{<parity>{<bits>{<flow>}}}

where

	baud	- baud rate in decimal
	parity	- 'n' (none), 'o', (odd) or 'e' (even)
	bits	- number of data bits
	flow	- 'r' (rts)

For example: 115200n8r

Implementation note: Linux will look for the property "linux,stdout-path" or
on PowerPC "stdout" if "stdout-path" is not found.  However, the
"linux,stdout-path" and "stdout" properties are deprecated. New platforms
should only use the "stdout-path" property.

linux,booted-from-kexec
-----------------------

This property is set (currently only on PowerPC, and only needed on
book3e) by some versions of kexec-tools to tell the new kernel that it
is being booted by kexec, as the booting environment may differ (e.g.
a different secondary CPU release mechanism)

linux,usable-memory-range
-------------------------

This property (arm64 only) holds a base address and size, describing a
limited region in which memory may be considered available for use by
the kernel. Memory outside of this range is not available for use.

This property describes a limitation: memory within this range is only
valid when also described through another mechanism that the kernel
would otherwise use to determine available memory (e.g. memory nodes
or the EFI memory map). Valid memory may be sparse within the range.
e.g.

/ {
	chosen {
		linux,usable-memory-range = <0x9 0xf0000000 0x0 0x10000000>;
	};
};

The main usage is for crash dump kernel to identify its own usable
memory and exclude, at its boot time, any other memory areas that are
part of the panicked kernel's memory.

While this property does not represent a real hardware, the address
and the size are expressed in #address-cells and #size-cells,
respectively, of the root node.

linux,elfcorehdr
----------------

This property (currently used only on arm64) holds the memory range,
the address and the size, of the elf core header which mainly describes
the panicked kernel's memory layout as PT_LOAD segments of elf format.
e.g.

/ {
	chosen {
		linux,elfcorehdr = <0x9 0xfffff000 0x0 0x800>;
	};
};

While this property does not represent a real hardware, the address
and the size are expressed in #address-cells and #size-cells,
respectively, of the root node.

linux,initrd-start and linux,initrd-end
---------------------------------------

These properties hold the physical start and end address of an initrd that's
loaded by the bootloader. Note that linux,initrd-start is inclusive, but
linux,initrd-end is exclusive.
e.g.

/ {
	chosen {
		linux,initrd-start = <0x82000000>;
		linux,initrd-end = <0x82800000>;
	};
};