Add ACPI support for _CLS

[andrew]

Summary

Add support to read the _CLS entry if it's present. It is used by
memory-mapped devices that are normally PCIe drives. Devices can then
use the existing pci_get_class, etc. accessors to query this data.

The ivar values are different enough from the existing ACPI and ISA
values to not conflict.

[jhb]

My only thought is if you want to read _CLS once during device enumeration and cache it's value in 'struct acpi_device' rather than invoking it for each ivar access. Aside from that this looks fine.

[andrew]

Read _CLS once

[jhb]

sys/dev/acpica/acpi.c

930 ↗

(On Diff #22851)

While this would seem useful, it's probably not in practice. Accessors like 'pci_get_class()' have no way to return an error. They always return whatever is in '*result' so the caller will just get garbage. I think it would be better to always return a value, but use specific invalid values for devices without _CLS (namely 0xff). You can do this by just setting acpi_cls_class to '0xffffff' when enumerating a device if _CLS isn't present or has an error and then having these bits of code return the acpi_cls_class value unconditionally.

sys/dev/acpica/acpivar.h

89 ↗

(On Diff #22851)

If you do the above suggestion you won't need this variable.

[andrew]

Return 0xff if _CLS is missing

[jhb]

I should have noticed earlier, but it would be nice to use the same 'ad_' prefix as the rest of the structure members, and you probably don't need a blank line between ad_flags and the new field.