This exception appears to be originating somewhere in the EC driver:

ACPI Exception (evregion-0420): AE_NOT_FOUND, Returned by Handler for
[EmbeddedControl] [20070126]



quoted text
> -----Original Message-----
> From: linux-acpi-owner@vger.kernel.org [mailto:linux-acpi-
> owner@vger.kernel.org] On Behalf Of Meelis Roos
> Sent: Tuesday, February 27, 2007 12:50 PM
> To: Linux Kernel list; linux-acpi@vger.kernel.org
> Subject: 2.6.21-rc1: more ACPI errors (EC__)
> 
> I tested 2.6.21-rc1 on my laptop (IBM X20 with 440BX) and found two
> problems:
> 
> First, a seemingly harmless one - ACPI error messages during bootup:
> 
> ACPI Exception (evregion-0420): AE_NOT_FOUND, Returned by Handler for
> [EmbeddedControl] [20070126]
> ACPI Exception (dswexec-0462): AE_NOT_FOUND, While resolving operands
for
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> [OpcodeName unavailable] [20070126]
> ACPI Error (psparse-0537): Method parse/execution failed
> [\_SB_.PCI0.ISA_.EC__.BAT0._STA] (Node d3c435e0), AE_NOT_FOUND
> ACPI Exception (evregion-0420): AE_NOT_FOUND, Returned by Handler for
> [EmbeddedControl] [20070126]
> ACPI Error (psparse-0537): Method parse/execution failed
> [\_SB_.PCI0.ISA_.EC__.AC__._PSR] (Node d3c43540), AE_NOT_FOUND
> ACPI Exception (ac-0095): AE_NOT_FOUND, Error reading AC Adapter state
> [20070126]
> 
> And second, there were ACPI error messages about EC__ during shutdown.
> These messages appeared, then the "acpi_power_off called" message
> appeared, then it waited for about 5 seconds (this does not happen
> normally) and then it succeeded shutting down. Captured a screenshot
of
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> these messages, temporarily available at
> http://www.cs.ut.ee/~mroos/x20-shutdown-acpi-errors.jpg
> 
> --
> Meelis Roos (mroos@linux.ee)
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