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* ARM architected timer

ARM cores may have a per-core architected timer, which provides per-cpu timers,
or a memory mapped architected timer, which provides up to 8 frames with a
physical and optional virtual timer per frame.

The per-core architected timer is attached to a GIC to deliver its
per-processor interrupts via PPIs. The memory mapped timer is attached to a GIC
to deliver its interrupts via SPIs.

** CP15 Timer node properties:

- compatible : Should at least contain one of
	"arm,armv7-timer"
	"arm,armv8-timer"

- interrupts : Interrupt list for secure, non-secure, virtual and
  hypervisor timers, in that order.

- clock-frequency : The frequency of the main counter, in Hz. Should be present
  only where necessary to work around broken firmware which does not configure
  CNTFRQ on all CPUs to a uniform correct value. Use of this property is
  strongly discouraged; fix your firmware unless absolutely impossible.

- always-on : a boolean property. If present, the timer is powered through an
  always-on power domain, therefore it never loses context.

** Optional properties:

- arm,cpu-registers-not-fw-configured : Firmware does not initialize
  any of the generic timer CPU registers, which contain their
  architecturally-defined reset values. Only supported for 32-bit
  systems which follow the ARMv7 architected reset values.


Example:

	timer {
		compatible = "arm,cortex-a15-timer",
			     "arm,armv7-timer";
		interrupts = <1 13 0xf08>,
			     <1 14 0xf08>,
			     <1 11 0xf08>,
			     <1 10 0xf08>;
		clock-frequency = <100000000>;
	};

** Memory mapped timer node properties:

- compatible : Should at least contain "arm,armv7-timer-mem".

- clock-frequency : The frequency of the main counter, in Hz. Should be present
  only when firmware has not configured the MMIO CNTFRQ registers.

- reg : The control frame base address.

Note that #address-cells, #size-cells, and ranges shall be present to ensure
the CPU can address a frame's registers.

A timer node has up to 8 frame sub-nodes, each with the following properties:

- frame-number: 0 to 7.

- interrupts : Interrupt list for physical and virtual timers in that order.
  The virtual timer interrupt is optional.

- reg : The first and second view base addresses in that order. The second view
  base address is optional.

- status : "disabled" indicates the frame is not available for use. Optional.

Example:

	timer@f0000000 {
		compatible = "arm,armv7-timer-mem";
		#address-cells = <1>;
		#size-cells = <1>;
		ranges;
		reg = <0xf0000000 0x1000>;
		clock-frequency = <50000000>;

		frame@f0001000 {
			frame-number = <0>
			interrupts = <0 13 0x8>,
				     <0 14 0x8>;
			reg = <0xf0001000 0x1000>,
			      <0xf0002000 0x1000>;
		};

		frame@f0003000 {
			frame-number = <1>
			interrupts = <0 15 0x8>;
			reg = <0xf0003000 0x1000>;
			status = "disabled";
		};
	};