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From: Len Brown on 21 Jul 2010 17:40
From: Len Brown <len.brown(a)intel.com>

The BIOS exports deep C-states on modern Intel processors
as "C3-type" to satisfy various legacy Operating Systems.

However, the hardware actually supports C2-type, and does
not require the extra costs of C3-type.

One of the costs is to check the BM_STS (Bus Master Status)
bit before entering C3, and instead choose a shallower C-state
if there was "recent bus master activity".

We have found a number of systems in the field that erroneously
set BM_STS and prevent entry into deep C-states.
Re-define BIOS presented C3-type states as C2-type states
on modern processors to avoid this issue.

If a device in the system really does want to prevent use
of a deep C-state, its Linux driver should register its
constraints via pm_qos_add_request().

https://bugzilla.kernel.org/show_bug.cgi?id=15886

Signed-off-by: Len Brown <len.brown(a)intel.com>
---
drivers/acpi/processor_idle.c | 38 ++++++++++++++++++++++++++++++++++++++
1 files changed, 38 insertions(+), 0 deletions(-)

diff --git a/drivers/acpi/processor_idle.c b/drivers/acpi/processor_idle.c
index b1b3856..14d1a0c 100644
--- a/drivers/acpi/processor_idle.c
+++ b/drivers/acpi/processor_idle.c
@@ -607,6 +607,38 @@ static void acpi_processor_power_verify_c3(struct acpi_processor *pr,
return;
}

+/*
+ * Modern Intel processors support only ACPI C2-type C-states.
+ * But the BIOS tends to report its deepest C-state as C3-type
+ * to satisfy various old operating systems. We can skip
+ * C3 OS overhead by treating the deep-states as C2-type.
+ * Also, we can avoid checking BM_STS, which on some systems
+ * erroneously prevents entry into C3-type states.
+ */
+static int acpi_c3type_is_really_c2type(void) {
+
+ if (boot_cpu_data.x86_vendor != X86_VENDOR_INTEL)
+ return 0;
+
+ if (boot_cpu_data.x86 != 6)
+ return 0;
+
+ switch(boot_cpu_data.x86_model) {
+ case 0x1A: /* Core i7, Xeon 5500 series */
+ case 0x1E: /* Core i7 and i5 Processor */
+ case 0x1F: /* Core i7 and i5 Processor */
+ case 0x2E: /* NHM-EX Xeon */
+ case 0x2F: /* WSM-EX Xeon */
+ case 0x25: /* WSM */
+ case 0x2C: /* WSM */
+ case 0x2A: /* SNB */
+ case 0x2D: /* SNB Xeon */
+ return 1;
+ default:
+ return 0;
+ }
+}
+
static int acpi_processor_power_verify(struct acpi_processor *pr)
{
unsigned int i;
@@ -617,6 +649,12 @@ static int acpi_processor_power_verify(struct acpi_processor *pr)
for (i = 1; i < ACPI_PROCESSOR_MAX_POWER && i <= max_cstate; i++) {
struct acpi_processor_cx *cx = &pr->power.states[i];

+ if ((cx->type == ACPI_STATE_C3)
+ && acpi_c3type_is_really_c2type()) {
+ ACPI_DEBUG_PRINT((ACPI_DB_INFO, "Redefining C3-type to C2\n"));
+ cx->type = ACPI_STATE_C2;
+ }
+
switch (cx->type) {
case ACPI_STATE_C1:
cx->valid = 1;
--
1.7.2.rc3.43.g24e7a


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