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Commit 718be4aa authored by Len Brown's avatar Len Brown
Browse files

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: default avatarLen Brown <len.brown@intel.com>
parent f4b23cc2

arch/x86/kernel/acpi/cstate.c

+9 −0
Original line number Diff line number Diff line
@@ -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);

drivers/acpi/processor_idle.c

+1 −1
Original line number Diff line number Diff line
@@ -947,7 +947,7 @@ static int acpi_idle_enter_bm(struct cpuidle_device *dev, 
	if (acpi_idle_suspend)

		return(acpi_idle_enter_c1(dev, state));



	if (acpi_idle_bm_check()) {

	if (!cx->bm_sts_skip && acpi_idle_bm_check()) {

		if (dev->safe_state) {

			dev->last_state = dev->safe_state;

			return dev->safe_state->enter(dev, dev->safe_state);

include/acpi/processor.h

+2 −1
Original line number Diff line number Diff line
@@ -48,7 +48,7 @@ struct acpi_power_register { 
	u8 space_id;

	u8 bit_width;

	u8 bit_offset;

	u8 reserved;

	u8 access_size;

	u64 address;

} __attribute__ ((packed));
@@ -63,6 +63,7 @@ struct acpi_processor_cx { 
	u32 power;

	u32 usage;

	u64 time;

	u8 bm_sts_skip;

	char desc[ACPI_CX_DESC_LEN];

};