2 files changed, 20 insertions, 1 deletions

diff --git a/drivers/acpi/acpica/tbfadt.c b/drivers/acpi/acpica/tbfadt.c
index 428d44e2d162..6f5588e62c0a 100644
--- a/drivers/acpi/acpica/tbfadt.c
+++ b/drivers/acpi/acpica/tbfadt.c
@@ -384,8 +384,11 @@ static void acpi_tb_convert_fadt(void)
          *
          * The ACPI 1.0 reserved fields that will be zeroed are the bytes located at
          * offset 45, 55, 95, and the word located at offset 109, 110.
+         *
+         * Note: The FADT revision value is unreliable. Only the length can be
+         * trusted.
          */
-        if (acpi_gbl_FADT.header.revision < FADT2_REVISION_ID) {
+        if (acpi_gbl_FADT.header.length <= ACPI_FADT_V2_SIZE) {
                 acpi_gbl_FADT.preferred_profile = 0;
                 acpi_gbl_FADT.pstate_control = 0;
                 acpi_gbl_FADT.cst_control = 0;
diff --git a/include/acpi/actbl.h b/include/acpi/actbl.h
index 7e42bfee0e29..d41c94885211 100644
--- a/include/acpi/actbl.h
+++ b/include/acpi/actbl.h
@@ -343,4 +343,20 @@ struct acpi_table_desc {
 #include <acpi/actbl1.h>
 #include <acpi/actbl2.h>
 
+/*
+ * Sizes of the various flavors of FADT. We need to look closely
+ * at the FADT length because the version number essentially tells
+ * us nothing because of many BIOS bugs where the version does not
+ * match the expected length. In other words, the length of the
+ * FADT is the bottom line as to what the version really is.
+ *
+ * For reference, the values below are as follows:
+ *     FADT V1  size: 0x74
+ *     FADT V2  size: 0x84
+ *     FADT V3+ size: 0xF4
+ */
+#define ACPI_FADT_V1_SIZE       (u32) (ACPI_FADT_OFFSET (flags) + 4)
+#define ACPI_FADT_V2_SIZE       (u32) (ACPI_FADT_OFFSET (reserved4[0]) + 3)
+#define ACPI_FADT_V3_SIZE       (u32) (sizeof (struct acpi_table_fadt))
+
 #endif                          /* __ACTBL_H__ */