Re: [Devel] Why are empty packages invalid for predefined ACPI objects?

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On 11/08/2010 05:23, Moore, Robert wrote:
> I'd have to go through the spec, but thinking about it, I'm not sure that
> any predefined name allows an empty package. All the definitions are
> either 1) Fixed length package, or 2) Variable length package that is
> defined as "one or more..."

The practical example that triggered this question is the method _DOD
(ACPI 4.0a, Appendix B, p. 698). For this method, the return value is:

     "A package containing a variable-length list of Integers, each of
      which contains the 32-bit device attribute of a child device".

As far as I can see, the specification does not require the list to be
non-empty.


On some laptop (see report [1]), the BIOS declares a device as follows:

              Device (VID2)
              {
                  Name (_ADR, 0x00020001)
                  Method (_DOS, 1, NotSerialized)
                  {
                  }


                  Method (_DOD, 0, NotSerialized)
                  {
                      Return (Package (0x00) {})
                  }
              }

The device may appear useless since _DOS does nothing and _DOD returns an
empty package. But VID2 is notified in the rest of the BIOS ASL. Maybe
the methods _DOS and _DOD under VID2 are there to identify VID2 as a display
adapter device?

On this laptop, the _DOD method triggers an ACPICA warning

ACPI Warning for \_SB_.PCI0.VID2._DOD: Return Package has no elements
(empty) (20100528/nspredef-572)

but, since VID2 conforms to the spec (afaics), is this warning really
deserved?

Best regards,

Grégoire

[1] http://gnats.netbsd.org/cgi-bin/query-pr-single.pl?number=44042