The implementation of acpi_scan_rsdp() in linux 2.6.3 does not correctly
locate the RSDP on an Acer TravelMate C100 with BIOS versions R01-A4F or
R01-A4K

BIOS version R01-A4K has an RSDP signature at 0xEC2D0, with zero
checksum and RSDT pointer, and another RSDP signature at 0xFE030,
with a valid checksum and RSDT pointer.


Currently acpi_scan_rsdp() in arch/i386/kernel/acpi/boot.c searches
for a RSDP signature, and returns the first one it finds.  If this
does not have a valid checksum, acpi_table_init() will return an error
(and since 2.6.2, this causes ACPI not to be initialised)


ACPI spec 2.0c, section 5.4.2.1 says:

"OSPM finds the Root System Description Pointer (RSDP) structure by
searching physical memory ranges on 16-byte boundaries for a valid Root
System Description Pointer structure signature and checksum match"

It's at least arguably correct to contine the search until a RSDP
signature and valid checksum is found.


The attached patch against 2.6.3 demonstrates a (not-very-good) way of
doing this.
(It also adds a printk which reports when ACPI is disabled because the
RSDP is invalid)


extract from dmesg without patch:

ACPI: RSDP (v000 Acer                                      ) @ 0x000ec2d0
     >>> ERROR: Invalid checksum

extract from dmesg with patch:

ACPI: RSDP (v000 Acer                                      ) @ 0x000fe030
ACPI: RSDT (v001 Acer   TM100    0x00000001 MSFT 0x00000001) @ 0x0ffe0000
ACPI: FADT (v001 Acer   TM100    0x00000001 MSFT 0x00000001) @ 0x0ffe0054
ACPI: BOOT (v001 Acer   TM100    0x00000001 MSFT 0x00000001) @ 0x0ffe002c
ACPI: DSDT (v001   Acer   AN100  0x00001000 MSFT 0x0100000e) @ 0x000000002