[2.6.21.stable PATCH] ACPI: Fix 2.6.21 boot regression on P4/HT

[Len Brown]

Up through 2.6.20 we cleared the FADT.CSTATE_CONTROL field
for FADT versions before r3, because it made no sense
for that reserved field to be set for pre-ACPI 2.0 systems.

It turns out that not clearing this field exposes
Linux to SMM BIOS failures, so do the same in 2.6.21.

http://bugzilla.kernel.org/show_bug.cgi?id=8346

Signed-off-by: Len Brown <len.brown@xxxxxxxxx>
---
diff --git a/drivers/acpi/tables/tbfadt.c b/drivers/acpi/tables/tbfadt.c
index 807c711..d341491 100644
--- a/drivers/acpi/tables/tbfadt.c
+++ b/drivers/acpi/tables/tbfadt.c
@@ -347,6 +347,20 @@ static void acpi_tb_convert_fadt(void)
 		acpi_gbl_xpm1b_enable.space_id = acpi_gbl_FADT.xpm1a_event_block.space_id;
 
 	}
+	/*
+	 * _CST object and C States change notification start with
+	 * ACPI 2.0 (FADT r3).  Although the field should be Reserved
+	 * and 0 before then, some pre-r3 FADT set this field and
+	 * it results in SMM-related boot failures.  For them, clear it.
+	 */
+	if ((acpi_gbl_FADT.header.revision < 3) &&
+		(acpi_gbl_FADT.cst_control != 0)) {
+			ACPI_WARNING((AE_INFO,
+				"Ignoring BIOS FADT r%u C-state control",
+				acpi_gbl_FADT.header.revision));
+		 	acpi_gbl_FADT.cst_control = 0;
+	}
+
 }
 
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