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U-Boot FDT Overlay usage
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=============================================
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Overlays Syntax
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---------------
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Overlays require slightly different syntax compared to traditional overlays.
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Please refer to dt-object-internal.txt in the dtc sources for information
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regarding the internal format of overlays:
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https://git.kernel.org/pub/scm/utils/dtc/dtc.git/tree/Documentation/dt-object-internal.txt
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Building Overlays
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-----------------
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In a nutshell overlays provides a means to manipulate a symbol a previous dtb
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or overlay has defined. It requires both the base and all the overlays
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to be compiled with the -@ command line switch so that symbol information is
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included.
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Note support for -@ option can only be found in dtc version 1.4.4 or newer.
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Only version 4.14 or higher of the Linux kernel includes a built in version
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of dtc that meets this requirement.
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Building an overlay follows the same process as building a traditional dtb.
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For example:
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base.dts
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--------
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/dts-v1/;
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/ {
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foo: foonode {
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foo-property;
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};
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};
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$ dtc -@ -I dts -O dtb -o base.dtb base.dts
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bar.dts
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-------
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/dts-v1/;
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/plugin/;
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/ {
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fragment@1 {
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target = <&foo>;
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__overlay__ {
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overlay-1-property;
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bar: barnode {
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bar-property;
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};
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};
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};
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};
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$ dtc -@ -I dts -O dtb -o bar.dtb bar.dts
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Ways to Utilize Overlays in U-boot
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----------------------------------
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There are two ways to apply overlays in U-boot.
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1. Include and define overlays within a FIT image and have overlays
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automatically applied.
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2. Manually load and apply overlays
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The remainder of this document will discuss using overlays via the manual
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approach. For information on using overlays as part of a FIT image please see:
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doc/uImage.FIT/overlay-fdt-boot.txt
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Manually Loading and Applying Overlays
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--------------------------------------
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1. Figure out where to place both the base device tree blob and the
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overlay. Make sure you have enough space to grow the base tree without
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overlapping anything.
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=> setenv fdtaddr 0x87f00000
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=> setenv fdtovaddr 0x87fc0000
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2. Load the base blob and overlay blobs
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=> load ${devtype} ${bootpart} ${fdtaddr} ${bootdir}/base.dtb
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=> load ${devtype} ${bootpart} ${fdtovaddr} ${bootdir}/overlay.dtb
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3. Set it as the working fdt tree.
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=> fdtaddr $fdtaddr
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4. Grow it enough so it can 'fit' all the applied overlays
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=> fdt resize 8192
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5. You are now ready to apply the overlay.
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=> fdt apply $fdtovaddr
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6. Boot system like you would do with a traditional dtb.
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For bootm:
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=> bootm ${kerneladdr} - ${fdtaddr}
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For bootz:
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=> bootz ${kerneladdr} - ${fdtaddr}
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Please note that in case of an error, both the base and overlays are going
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to be invalidated, so keep copies to avoid reloading.
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Pantelis Antoniou
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pantelis.antoniou@konsulko.com
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11/7/2017
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