linux-kernel - [PATCH for 2.6.18] [2/8] x86_64: On Intel systems when CPU has C3 don't use TSC

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Date:	Sat, 29 Jul 2006 21:42:37 +0200
From:	"Andi Kleen" <ak@...e.de>
To:	torvalds@...l.org
Cc:	discuss@...-64.org, linux-kernel@...r.kernel.org
Subject: [PATCH for 2.6.18] [2/8] x86_64: On Intel systems when CPU has C3 don't use TSC


On Intel systems generally the TSC stops in C3 or deeper, 
so don't use it there. Follows similar logic on i386.

This should fix problems on Meroms.

Signed-off-by: Andi Kleen <ak@...e.de>

---
 arch/x86_64/kernel/time.c |   16 ++++++++++++----
 1 files changed, 12 insertions(+), 4 deletions(-)

Index: linux-2.6.18-rc2-git7/arch/x86_64/kernel/time.c
===================================================================
--- linux-2.6.18-rc2-git7.orig/arch/x86_64/kernel/time.c
+++ linux-2.6.18-rc2-git7/arch/x86_64/kernel/time.c
@@ -28,6 +28,7 @@
 #include <linux/acpi.h>
 #ifdef CONFIG_ACPI
 #include <acpi/achware.h>	/* for PM timer frequency */
+#include <acpi/acpi_bus.h>
 #endif
 #include <asm/8253pit.h>
 #include <asm/pgtable.h>
@@ -953,11 +954,18 @@ __cpuinit int unsynchronized_tsc(void)
 #ifdef CONFIG_SMP
 	if (apic_is_clustered_box())
 		return 1;
- 	/* Intel systems are normally all synchronized. Exceptions
- 	   are handled in the check above. */
- 	if (boot_cpu_data.x86_vendor == X86_VENDOR_INTEL)
- 		return 0;
 #endif
+	/* Most intel systems have synchronized TSCs except for
+	   multi node systems */
+ 	if (boot_cpu_data.x86_vendor == X86_VENDOR_INTEL) {
+#ifdef CONFIG_ACPI
+		/* But TSC doesn't tick in C3 so don't use it there */
+		if (acpi_fadt.length > 0 && acpi_fadt.plvl3_lat < 100)
+			return 1;
+#endif
+ 		return 0;
+	}
+
  	/* Assume multi socket systems are not synchronized */
  	return num_present_cpus() > 1;
 }
-
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