[PATCH] Use 32-bit FADT values on X86

[Matthew Garrett <mjg59@srcf.ucam.org>]

The ACPI specification says that we should use the 64-bit address offsets
contained within the FADT if they exist. However, Windows uses the legacy
address. Various vendors have left incorrect values in the 64-bit field
which then causes problems later. Since the vast majority of machines have
never been tested with an OS that uses the 64-bit value by default, we should
behave like Windows and ignore the spec by only using the 64-bit address if
it contains something that can't be represented in the legacy field. Since
system io space is only 16 bits on x86, this should be entirely safe.
    
Signed-off-by: Matthew Garrett <mjg@redhat.com>

---

Len, this is a clear case of the spec not matching real-life behaviour. 
I'd be amazed if anyone can find an x86 system that uses system-io space 
for these values and doesn't contain an accurate value in the 32-bit 
field. On the other hand, we've seen machines that assume the 
Windows-style behaviour and we keep finding more. A blacklist isn't the 
correct solution for this problem.

diff --git a/drivers/acpi/tables/tbfadt.c b/drivers/acpi/tables/tbfadt.c
index 2817158..89a3c82 100644
--- a/drivers/acpi/tables/tbfadt.c
+++ b/drivers/acpi/tables/tbfadt.c
@@ -320,9 +320,30 @@ static void acpi_tb_convert_fadt(void)
 		    ACPI_ADD_PTR(struct acpi_generic_address, &acpi_gbl_FADT,
 				 fadt_info_table[i].target);
 
-		/* Expand only if the X target is null */
-
-		if (!target->address) {
+		/*
+		 * The ACPI specification says that we should use the
+		 * 64-bit address offsets if they exists. However,
+		 * Windows uses the legacy address. Various vendors
+		 * have left incorrect values in the 64-bit field,
+		 * which then causes problems later. Since the vast
+		 * majority of machines have never been tested with an
+		 * OS that uses the 64-bit value by default, we should
+		 * behave like Windows and ignore the spec by only
+		 * using the 64-bit address if it contains something
+		 * that can't be represented in the legacy
+		 * field. Since system io space is only 16 bits on
+		 * x86, this should be entirely safe. We also extend
+		 * the 32-bit value into the 64-bit one if no 64-bit
+		 * address is provided.
+		 */
+
+		if (!target->address
+#ifdef CONFIG_X86
+		    || (target->space_id == ACPI_ADR_SPACE_SYSTEM_IO &&
+			*ACPI_ADD_PTR(u32, &acpi_gbl_FADT,
+				      fadt_info_table[i].source))
+#endif
+			) {
 			acpi_tb_init_generic_address(target,
 						     *ACPI_ADD_PTR(u8,
 								   &acpi_gbl_FADT,

-- 
Matthew Garrett | mjg59@srcf.ucam.org