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Expand Up @@ -95,7 +95,7 @@ During boot, the user can specify a specific ARM device tree to use via the `dev

In addition to loading an ARM dtb, `start.elf` supports loading additional Device Tree 'overlays' via the `dtoverlay` parameter in `config.txt`, for example adding as many `dtoverlay=myoverlay` lines as required as overlays to `config.txt`, noting that overlays live in `/overlays` and are suffixed `-overlay.dtb` e.g. `/overlays/myoverlay-overlay.dtb`. Overlays are merged with the base dtb file before the data is passed to the Linux kernel when it starts.

Overlays are used to add data to the base dtb that (nominally) describes non board-specific hardware. This includes GPIO pins used and their function, as well as the device(s) attached, so that the correct drivers can be loaded. The convention is that on a Raspberry Pi, all hardware attached to the Bank0 GPIOs (the GPIO header) should be described using an overlay. On a Compute Module all hardware attached to the Bank0 and Bank1 GPIOs should be described in an overlay file. You don't have to follow these conventions: you can roll all the information into one single dtb file, as previously described, replacing `bcm2708-rpi-cm.dtb`. However, following the conventions means that you can use a 'standard' Raspberry Pi OS release, with its standard base dtb and all the product-specific information contained in a separate overlay. Occasionally the base dtb might change - usually in a way that will not break overlays - which is why using an overlay is suggested.
Overlays are used to add data to the base dtb that (nominally) describes non-board-specific hardware. This includes GPIO pins used and their function, as well as the device(s) attached, so that the correct drivers can be loaded. The convention is that on a Raspberry Pi, all hardware attached to the Bank0 GPIOs (the GPIO header) should be described using an overlay. On a Compute Module all hardware attached to the Bank0 and Bank1 GPIOs should be described in an overlay file. You don't have to follow these conventions: you can roll all the information into one single dtb file, as previously described, replacing `bcm2708-rpi-cm.dtb`. However, following the conventions means that you can use a 'standard' Raspberry Pi OS release, with its standard base dtb and all the product-specific information contained in a separate overlay. Occasionally the base dtb might change - usually in a way that will not break overlays - which is why using an overlay is suggested.

=== dt-blob.bin

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