Comment 8 for bug 58386

The purpose of this optional kernel feature, enabled by "CONFIG_ACPI_CUSTOM_DSDT_INITRD=y" in the kernel .config at build-time, is to allow the user to replace the BIOS Advanced Configuration & Power Interface (ACPI) Differentiated System Description Table (DSDT) with one that has been modified to solve problems in the BIOS shipped by manufacturers.

The message you see is correct. As far as the kernel is concerned it is an error because it is told to expect to find a replacement DSDT.aml and it isn't there. However the message could be altered somewhat to make it sound less harsh.

Once a modified DSDT has been built using the Intel iasl tool it is copied to /etc/initramfs-tools/DSDT.aml and the initrd image rebuilt:

$ sudo dpkg-reconfigure linux-image-$(uname -r)

This installs the DSDT to /DSDT.aml in the initrd image.

The functionality (and messages you see) is implemented in drivers/acpi/osl.c::acpi_find_dsdt_initrd(void)

For more information, see: https://help.ubuntu.com/community/ACPIBattery