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838404054e
The DT spec demands a unit-address in a node name to match the "reg" property in that node. Newer dtc versions will throw warnings if this is not the case. Fix all occurences in the FIT image documentation files where this was not observed, to not give bad examples to the reader. Signed-off-by: Andre Przywara <andre.przywara@arm.com>
225 lines
5.3 KiB
Text
225 lines
5.3 KiB
Text
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 {
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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";
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fdt = "fdt-1";
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};
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foo-revb.dtb {
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kernel = "kernel";
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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";
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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";
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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";
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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";
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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 {
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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";
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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";
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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";
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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";
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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";
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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";
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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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