|
@@ -1,9 +1,76 @@
|
|
|
-U-Boot FDT Overlay usage (without FIT images)
|
|
|
+U-Boot FDT Overlay usage
|
|
|
=============================================
|
|
|
|
|
|
-FDT overlays are now available for use even without FIT images.
|
|
|
-It is not as automagic as with FIT images though and require a few
|
|
|
-manual steps.
|
|
|
+Overlays Syntax
|
|
|
+---------------
|
|
|
+
|
|
|
+Overlays require slightly different syntax compared to traditional overlays.
|
|
|
+Please refer to dt-object-internal.txt in the dtc sources for information
|
|
|
+regarding the internal format of overlays:
|
|
|
+https://git.kernel.org/pub/scm/utils/dtc/dtc.git/tree/Documentation/dt-object-internal.txt
|
|
|
+
|
|
|
+Building Overlays
|
|
|
+-----------------
|
|
|
+
|
|
|
+In a nutshell overlays provides a means to manipulate a symbol a previous dtb
|
|
|
+or overlay has defined. It requires both the base and all the overlays
|
|
|
+to be compiled with the -@ command line switch so that symbol information is
|
|
|
+included.
|
|
|
+
|
|
|
+Note support for -@ option can only be found in dtc version 1.4.4 or newer.
|
|
|
+Only version 4.14 or higher of the Linux kernel includes a built in version
|
|
|
+of dtc that meets this requirement.
|
|
|
+
|
|
|
+Building an overlay follows the same process as building a traditional dtb.
|
|
|
+
|
|
|
+For example:
|
|
|
+
|
|
|
+base.dts
|
|
|
+--------
|
|
|
+
|
|
|
+ /dts-v1/;
|
|
|
+ / {
|
|
|
+ foo: foonode {
|
|
|
+ foo-property;
|
|
|
+ };
|
|
|
+ };
|
|
|
+
|
|
|
+ $ dtc -@ -I dts -O dtb -o base.dtb base.dts
|
|
|
+
|
|
|
+bar.dts
|
|
|
+-------
|
|
|
+
|
|
|
+ /dts-v1/;
|
|
|
+ /plugin/;
|
|
|
+ / {
|
|
|
+ fragment@1 {
|
|
|
+ target = <&foo>;
|
|
|
+ __overlay__ {
|
|
|
+ overlay-1-property;
|
|
|
+ bar: barnode {
|
|
|
+ bar-property;
|
|
|
+ };
|
|
|
+ };
|
|
|
+ };
|
|
|
+ };
|
|
|
+
|
|
|
+ $ dtc -@ -I dts -O dtb -o bar.dtb bar.dts
|
|
|
+
|
|
|
+Ways to Utilize Overlays in U-boot
|
|
|
+----------------------------------
|
|
|
+
|
|
|
+There are two ways to apply overlays in U-boot.
|
|
|
+1. Include and define overlays within a FIT image and have overlays
|
|
|
+ automatically applied.
|
|
|
+
|
|
|
+2. Manually load and apply overlays
|
|
|
+
|
|
|
+The remainder of this document will discuss using overlays via the manual
|
|
|
+approach. For information on using overlays as part of a FIT image please see:
|
|
|
+doc/uImage.FIT/overlay-fdt-boot.txt
|
|
|
+
|
|
|
+Manually Loading and Applying Overlays
|
|
|
+--------------------------------------
|
|
|
|
|
|
1. Figure out where to place both the base device tree blob and the
|
|
|
overlay. Make sure you have enough space to grow the base tree without
|
|
@@ -29,6 +96,16 @@ overlapping anything.
|
|
|
|
|
|
=> fdt apply $fdtovaddr
|
|
|
|
|
|
+6. Boot system like you would do with a traditional dtb.
|
|
|
+
|
|
|
+For bootm:
|
|
|
+
|
|
|
+=> bootm ${kerneladdr} - ${fdtaddr}
|
|
|
+
|
|
|
+For bootz:
|
|
|
+
|
|
|
+=> bootz ${kerneladdr} - ${fdtaddr}
|
|
|
+
|
|
|
Please note that in case of an error, both the base and overlays are going
|
|
|
to be invalidated, so keep copies to avoid reloading.
|
|
|
|