linux-libre/Documentation/devicetree/bindings/arm/fw-cfg.txt

* QEMU Firmware Configuration bindings for ARM

QEMU's arm-softmmu and aarch64-softmmu emulation / virtualization targets
provide the following Firmware Configuration interface on the "virt" machine
type:

- A write-only, 16-bit wide selector (or control) register,
- a read-write, 64-bit wide data register.

QEMU exposes the control and data register to ARM guests as memory mapped
registers; their location is communicated to the guest's UEFI firmware in the
DTB that QEMU places at the bottom of the guest's DRAM.

The authoritative guest-side hardware interface documentation to the fw_cfg
device can be found in "docs/specs/fw_cfg.txt" in the QEMU source tree.


Required properties:

- compatible: "qemu,fw-cfg-mmio".

- reg: the MMIO region used by the device.
  * Bytes 0x0 to 0x7 cover the data register.
  * Bytes 0x8 to 0x9 cover the selector register.
  * Further registers may be appended to the region in case of future interface
    revisions / feature bits.

Example:

/ {
	#size-cells = <0x2>;
	#address-cells = <0x2>;

	fw-cfg@9020000 {
		compatible = "qemu,fw-cfg-mmio";
		reg = <0x0 0x9020000 0x0 0xa>;
	};
};