Re: Why ACPI is in the kernel, notes from 2001

[Jeff Garzik]

Moore, Robert wrote:
> Here's some notes from 2001:
>
> Why ACPI is in the kernel
>
> ACPI and the AML interpreter are required very early during kernel
> initialization, before the device drivers are loaded.  Control methods
> are executed by the interpreter at this time (such as all device _INI
> methods).
>
> ACPI owns the ACPI hardware and ACPI interrupt (SCI), and therefore this
> part of the ACPI subsystem is similar to a device driver.
>
> Control methods that are executed via the AML interpreter are allowed
> direct access to all of physical memory, all I/O space, and all PCI
> configuration space (via Operation Regions.)
>
> Device drivers such as the Embedded Controller, Battery, and Thermal use
> ACPI services and execute AML control methods during their operation.
>
> Device driver callback routines are invoked directly from the AML
> interpreter when the ASL Notify operation is executed.
>
> ACPI and the AML interpreter cannot be paged out.  How do you wake up
> the disk used for paging?  Not a good idea for other device drivers to
> depend on code that may be paged out.
>
> Also,
>
> As of May 2000, the AML interpreter was in acpid! Shortly after that, we
> made a conscious decision to move it into the driver for the reasons
> above.  The code may be large (for Linux), but it is necessary and must
> remain resident (it is non-pageable).


None of this implies that the interpreter cannot be in initramfs-like 
userspace, which neither requires a device driver nor will ever be paged 
out.

	Jeff


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