Merge branch '2020-05-25-misc-fixes'

[oweals/u-boot.git] / doc / README.fdt-overlays

diff --git a/doc/README.fdt-overlays b/doc/README.fdt-overlays
index ee868eca36f31acd5bff828b006a28cb97b0a3c8..39139cb850cde323f9197fa5023bd94864723570 100644 (file)
--- a/doc/README.fdt-overlays
+++ b/doc/README.fdt-overlays
@@ -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.