u-boot/doc/device-tree-bindings/chosen.txt
Philipp Tomsich 2aa4abc1bd spl: document 'chosen/u-boot, spl-boot-device'
To let the full U-Boot know where it was booted from (i.e. which of
the entries in /chosen/u-boot,spl-boot-order' contained a valid
image), we define (and document) /chosen/u-boot,spl-boot-device as the
property that could/should automatically be injected by SPL.

This commit only contains a documentation change, which documents the
new property and the intended usage.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Tested-by: Klaus Goger <klaus.goger@theobroma-systems.com>
2018-07-21 01:55:26 +02:00

85 lines
2.5 KiB
Text

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,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.