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