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- U-Boot FDT Overlay usage
- =============================================
- 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
- overlapping anything.
- => setenv fdtaddr 0x87f00000
- => setenv fdtovaddr 0x87fc0000
- 2. Load the base blob and overlay blobs
- => load ${devtype} ${bootpart} ${fdtaddr} ${bootdir}/base.dtb
- => load ${devtype} ${bootpart} ${fdtovaddr} ${bootdir}/overlay.dtb
- 3. Set it as the working fdt tree.
- => fdtaddr $fdtaddr
- 4. Grow it enough so it can 'fit' all the applied overlays
- => fdt resize 8192
- 5. You are now ready to apply the overlay.
- => 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.
- Pantelis Antoniou
- pantelis.antoniou@konsulko.com
- 11/7/2017
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