doc/device-tree-bindings/chosen.txt

The chosen node
---------------
The chosen node does not represent a real device, but serves as a place
for passing data like which serial device to used to print the logs etc


stdout-path property
--------------------
Device trees may specify the device to be used for boot console output
with a stdout-path property under /chosen.

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

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

tick-timer property
-------------------
In a system there are multiple timers, specify which timer to be used
as the tick-timer. Earlier it was hardcoded in the timer driver now
since device tree has all the timer nodes. Specify which timer to be
used as tick timer.

Example
-------
/ {
	chosen {
		tick-timer = "/timer2@f00";
	};

	timer2@f00 {
		compatible = "vendor,some-timer";
		reg = <0xf00 0x10>;
	};
};

u-boot,bootcount-device property
--------------------------------

In a DM-based system, the bootcount may be stored in a device known to
the DM framework (e.g. in a battery-backed SRAM area within a RTC
device) managed by a device conforming to UCLASS_BOOTCOUNT.  If
multiple such devices are present in a system concurrently, then the
u-boot,bootcount-device property can select the preferred target.

Example
-------
/ {
	chosen {
	        u-boot,bootcount-device = &bootcount-rv3029;
	};

	bootcount-rv3029: bootcount@0 {
		compatible = "u-boot,bootcount-rtc";
		rtc = &rv3029;
		offset = <0x38>;
	};

	i2c2 {
	        rv3029: rtc@56 {
		                compatible = "mc,rv3029";
		                reg = <0x56>;
		};
	};
};

u-boot,spl-boot-order property
------------------------------

In a system using an SPL stage and having multiple boot sources
(e.g. SPI NOR flash, on-board eMMC and a removable SD-card), the boot
device may be probed by reading the image and verifying an image
signature.

If the SPL is configured through the device-tree, the boot-order can
be configured with the spl-boot-order property under the /chosen node.
Each list element of the property should specify a device to be probed
in the order they are listed: references (i.e. implicit paths), a full
path or an alias is expected for each entry.

A special specifier "same-as-spl" can be used at any position in the
boot-order to direct U-Boot to insert the device the SPL was booted
from there.  Whether this is indeed inserted or silently ignored (if
it is not supported on any given SoC/board or if the boot-device is
not available to continue booting from) is implementation-defined.
Note that if "same-as-spl" expands to an actual node for a given
board, the corresponding node may appear multiple times in the
boot-order (as there currently exists no mechanism to suppress
duplicates from the list).

Example
-------
/ {
	chosen {
		u-boot,spl-boot-order = "same-as-spl", &sdmmc, "/sdhci@fe330000";
	};
};

u-boot,spl-boot-device property
-------------------------------

This property is a companion-property to the u-boot,spl-boot-order and
will be injected automatically by the SPL stage to notify a later stage
of where said later stage was booted from.

You should not define this property yourself in the device-tree, as it
may be overwritten without warning.

firmware-loader property
------------------------
Multiple file system firmware loader nodes could be defined in device trees for
multiple storage type and their default partition, then a property
"firmware-loader" can be used to pass default firmware loader
node(default storage type) to the firmware loader driver.

Example
-------
/ {
	chosen {
		firmware-loader = &fs_loader0;
	};

	fs_loader0: fs-loader@0 {
		u-boot,dm-pre-reloc;
		compatible = "u-boot,fs-loader";
		phandlepart = <&mmc 1>;
	};
};