ACPI Advanced Seed

GPIO OperationRegion

How ACPI models GPIO-backed platform state and why GPIO numbering, polarity, wake flags, and controller paths are common firmware bugs.

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Modern platforms use GPIOs for far more than simple high/low outputs.

A single GPIO can represent:

  • lid state
  • tablet mode
  • camera privacy switch
  • touchpad interrupt
  • device reset line
  • power enable line
  • wake source

ACPI can expose GPIOs through _CRS descriptors and through GPIO OperationRegions.

GPIO in _CRS vs GPIO OperationRegion

Item Value Note
GpioInt in _CRS Interrupt resource Tells the OS driver which GPIO interrupt belongs to the device.
GpioIo in _CRS GPIO I/O resource Can describe reset or enable pins consumed by the driver.
GeneralPurposeIo OperationRegion AML-accessible GPIO field Lets AML read or write GPIO-backed state.
Wake flags ExclusiveAndWake / SharedAndWake Important for sleep and wake behavior.

Real World Example: camera privacy switch

A camera privacy switch may be wired to a GPIO. The OS may need to know the state, while firmware may also send notifications when it changes.

OperationRegion (GPRG, GeneralPurposeIo, Zero, One)
Field (GPRG, ByteAcc, NoLock, Preserve)
{
    CPSW, 1
}

Method (GCPS, 0, NotSerialized)
{
    Return (CPSW)
}

If GPIO polarity is wrong, the OS reports privacy on when it is off.

Debug Diary: wake instantly after sleep

Symptom:

System enters S3
Immediately wakes again
Wake source points to GPIO controller

The issue may be:

  • wrong active level
  • wake GPIO not masked before sleep
  • stale event status not cleared
  • _PRW references the wrong GPE
  • _CRS marks a GPIO as wake-capable when it should not

GPIO ACPI debug checklist

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Embedded Controller OperationRegion

How AML talks to EC RAM through EmbeddedControl OperationRegion and how to debug EC timeout, SCI storm, battery, thermal, and hotkey issues.

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Field in ACPI

How Field definitions map bits and bytes inside an OperationRegion to names used by AML methods.

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