ACPI: skip checking BM_STS if the BIOS doesn't ask for it

It turns out that there is a bit in the _CST for Intel FFH C3
that tells the OS if we should be checking BM_STS or not.

Linux has been unconditionally checking BM_STS.
If the chip-set is configured to enable BM_STS,
it can retard or completely prevent entry into
deep C-states -- as illustrated by turbostat:

http://userweb.kernel.org/~lenb/acpi/utils/pmtools/turbostat/

ref: Intel Processor Vendor-Specific ACPI Interface Specification
table 4 "_CST FFH GAS Field Encoding"
Bit 1: Set to 1 if OSPM should use Bus Master avoidance for this C-state

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

Signed-off-by: Len Brown <len.brown@intel.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/arch/x86/kernel/acpi/cstate.c b/arch/x86/kernel/acpi/cstate.c
index 2e837f5080fe..fb7a5f052e2b 100644
--- a/arch/x86/kernel/acpi/cstate.c
+++ b/arch/x86/kernel/acpi/cstate.c
@@ -145,6 +145,15 @@ int acpi_processor_ffh_cstate_probe(unsigned int cpu,
                 percpu_entry->states[cx->index].eax = cx->address;
                 percpu_entry->states[cx->index].ecx = MWAIT_ECX_INTERRUPT_BREAK;
         }
+
+        /*
+         * For _CST FFH on Intel, if GAS.access_size bit 1 is cleared,
+         * then we should skip checking BM_STS for this C-state.
+         * ref: "Intel Processor Vendor-Specific ACPI Interface Specification"
+         */
+        if ((c->x86_vendor == X86_VENDOR_INTEL) && !(reg->access_size & 0x2))
+                cx->bm_sts_skip = 1;
+
         return retval;
 }
 EXPORT_SYMBOL_GPL(acpi_processor_ffh_cstate_probe);