ACPI Clock Input Resources

This page is a work in progress!

ACPI v6.5 spec. introduced ClockInput resource descriptor to describe clocks required for devices.

ClockInput (FrequencyNumerator, FrequencyDivisor, Scale, FixedMode, ResourceSource, ˓ResourceSourceIndex)

Examples from ACPI spec.

Case 1: Simple clock input descriptor, with a fixed frequency of 33 1/3 MHz and no Device shown as a clock source.

ClockInput (100, 3, MHz, Fixed,,)

Case 2: Clock input descriptor referencing a variable clock input of 19.2 kHz, with a source device and index.

ClockInput (19200, 1, Hz, Variable, “\\_SB.PCI0.CLK4”, 3)

SCMI Clock Driver with ClockInput resources

Device (SCMI) {
  Name (_HID, "PNP####")
  ...
}

Device(I2C) {
  ...
  Name(_CLK, 13) // SCMI clock ID for I2C root clock
  Method (_CRS, 0, NotSerialized)
  {
    Name (RBUF, ResourceTemplate () {
      MEMORY32FIXED(ReadWrite, 0x30A40000, 0x14, )
      Interrupt(ResourceConsumer, Level, ActiveHigh, Exclusive) { 69 }
      ClockInput (24, 1, MHz, Variable, “\\_SB.SCMI”, _CLK)
    })
    Return(RBUF)
  }
  ...
}

ACPI ClockInput resources with variable clock rate would require support from the operating system. Clock drivers and clock framework support will be required from OS.

ClockInput resources with _PRS, SRS, and _CRS

Another approach is to support ClockInput resources from AML without requiring operating system infrastructures.

CLK:
  Method(GCLK, ...) // Get Clock Rate
  Method(SCLK, ...) // Set Clock Rate
 
DEV:

  _PRS:
    ClockInput(FREQ_1, 1, MHz, Variable, CLK,)
    ClockInput(FREQ_2, 1, MHz, Variable, CLK, )
    ClockInput(FREQ_3, 1, MHz, Variable, CLK, )
       
  _CRS:
    freq_num, freq_div = CLK.GCLK(...)
    ClockInput(freq_num, freq_div, ... , Variable, "//SB.CLK", )

  _SRS:
    ...
    local0 = decode(Arg0)
    CLK.SCLK(local0)

Challenges

ClockInput resource doesn't have resource names so we cannot index them for modification.
Linux kernel bindings for drivers to select and set _PRS and _SRS seem to be missing.