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Pull some information regarding overlays from commit messages and put them directly within the documentation. Also add some information regarding required dtc version to properly use overlays. Signed-off-by: Franklin S Cooper Jr <fcooper@ti.com> Acked-by: Simon Glass <sjg@chromium.org>
226 lines
5.3 KiB
Plaintext
226 lines
5.3 KiB
Plaintext
U-Boot FDT Overlay FIT usage
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============================
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Introduction
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------------
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In many cases it is desirable to have a single FIT image support a multitude
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of similar boards and their expansion options. The same kernel on DT enabled
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platforms can support this easily enough by providing a DT blob upon boot
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that matches the desired configuration.
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This document focuses on specifically using overlays as part of a FIT image.
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General information regarding overlays including its syntax and building it
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can be found in doc/README.fdt-overlays
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Configuration without overlays
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------------------------------
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Take a hypothetical board named 'foo' where there are different supported
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revisions, reva and revb. Assume that both board revisions can use add a bar
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add-on board, while only the revb board can use a baz add-on board.
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Without using overlays the configuration would be as follows for every case.
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/dts-v1/;
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/ {
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images {
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kernel@1 {
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data = /incbin/("./zImage");
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type = "kernel";
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arch = "arm";
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os = "linux";
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load = <0x82000000>;
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entry = <0x82000000>;
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};
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fdt@1 {
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data = /incbin/("./foo-reva.dtb");
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type = "flat_dt";
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arch = "arm";
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};
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fdt@2 {
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data = /incbin/("./foo-revb.dtb");
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type = "flat_dt";
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arch = "arm";
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};
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fdt@3 {
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data = /incbin/("./foo-reva-bar.dtb");
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type = "flat_dt";
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arch = "arm";
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};
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fdt@4 {
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data = /incbin/("./foo-revb-bar.dtb");
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type = "flat_dt";
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arch = "arm";
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};
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fdt@5 {
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data = /incbin/("./foo-revb-baz.dtb");
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type = "flat_dt";
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arch = "arm";
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};
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fdt@6 {
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data = /incbin/("./foo-revb-bar-baz.dtb");
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type = "flat_dt";
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arch = "arm";
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};
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};
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configurations {
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default = "foo-reva.dtb;
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foo-reva.dtb {
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kernel = "kernel@1";
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fdt = "fdt@1";
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};
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foo-revb.dtb {
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kernel = "kernel@1";
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fdt = "fdt@2";
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};
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foo-reva-bar.dtb {
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kernel = "kernel@1";
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fdt = "fdt@3";
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};
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foo-revb-bar.dtb {
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kernel = "kernel@1";
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fdt = "fdt@4";
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};
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foo-revb-baz.dtb {
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kernel = "kernel@1";
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fdt = "fdt@5";
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};
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foo-revb-bar-baz.dtb {
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kernel = "kernel@1";
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fdt = "fdt@6";
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};
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};
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};
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Note the blob needs to be compiled for each case and the combinatorial explosion of
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configurations. A typical device tree blob is in the low hunderds of kbytes so a
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multitude of configuration grows the image quite a bit.
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Booting this image is done by using
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# bootm <addr>#<config>
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Where config is one of:
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foo-reva.dtb, foo-revb.dtb, foo-reva-bar.dtb, foo-revb-bar.dtb,
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foo-revb-baz.dtb, foo-revb-bar-baz.dtb
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This selects the DTB to use when booting.
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Configuration using overlays
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----------------------------
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Device tree overlays can be applied to a base DT and result in the same blob
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being passed to the booting kernel. This saves on space and avoid the combinatorial
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explosion problem.
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/dts-v1/;
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/ {
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images {
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kernel@1 {
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data = /incbin/("./zImage");
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type = "kernel";
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arch = "arm";
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os = "linux";
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load = <0x82000000>;
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entry = <0x82000000>;
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};
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fdt@1 {
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data = /incbin/("./foo.dtb");
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type = "flat_dt";
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arch = "arm";
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load = <0x87f00000>;
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};
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fdt@2 {
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data = /incbin/("./reva.dtbo");
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type = "flat_dt";
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arch = "arm";
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load = <0x87fc0000>;
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};
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fdt@3 {
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data = /incbin/("./revb.dtbo");
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type = "flat_dt";
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arch = "arm";
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load = <0x87fc0000>;
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};
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fdt@4 {
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data = /incbin/("./bar.dtbo");
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type = "flat_dt";
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arch = "arm";
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load = <0x87fc0000>;
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};
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fdt@5 {
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data = /incbin/("./baz.dtbo");
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type = "flat_dt";
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arch = "arm";
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load = <0x87fc0000>;
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};
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};
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configurations {
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default = "foo-reva.dtb;
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foo-reva.dtb {
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kernel = "kernel@1";
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fdt = "fdt@1", "fdt@2";
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};
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foo-revb.dtb {
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kernel = "kernel@1";
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fdt = "fdt@1", "fdt@3";
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};
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foo-reva-bar.dtb {
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kernel = "kernel@1";
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fdt = "fdt@1", "fdt@2", "fdt@4";
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};
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foo-revb-bar.dtb {
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kernel = "kernel@1";
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fdt = "fdt@1", "fdt@3", "fdt@4";
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};
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foo-revb-baz.dtb {
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kernel = "kernel@1";
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fdt = "fdt@1", "fdt@3", "fdt@5";
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};
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foo-revb-bar-baz.dtb {
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kernel = "kernel@1";
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fdt = "fdt@1", "fdt@3", "fdt@4", "fdt@5";
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};
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bar {
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fdt = "fdt@4";
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};
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baz {
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fdt = "fdt@5";
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};
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};
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};
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Booting this image is exactly the same as the non-overlay example.
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u-boot will retrieve the base blob and apply the overlays in sequence as
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they are declared in the configuration.
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Note the minimum amount of different DT blobs, as well as the requirement for
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the DT blobs to have a load address; the overlay application requires the blobs
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to be writeable.
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Configuration using overlays and feature selection
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--------------------------------------------------
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Although the configuration in the previous section works is a bit inflexible
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since it requires all possible configuration options to be laid out before
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hand in the FIT image. For the add-on boards the extra config selection method
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might make sense.
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Note the two bar & baz configuration nodes. To boot a reva board with
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the bar add-on board enabled simply use:
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# bootm <addr>#foo-reva.dtb#bar
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While booting a revb with bar and baz is as follows:
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# bootm <addr>#foo-revb.dtb#bar#baz
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The limitation for a feature selection configuration node is that a single
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fdt option is currently supported.
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Pantelis Antoniou
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pantelis.antoniou@konsulko.com
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12/6/2017
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