ACPI: create "processor.bm_check_disable" boot param

processor.bm_check_disable=1" prevents Linux from checking BM_STS
before entering C3-type cpu power states.

This may be useful for a system running acpi_idle
where the BIOS exports FADT C-states, _CST IO C-states,
or _CST FFH C-states with the BM_STS bit set;
while configuring the chipset to set BM_STS
more frequently than perhaps is optimal.

Note that such systems may have been developed
using a tickful OS that would quickly clear BM_STS,
rather than a tickless OS that may go for some time
between checking and clearing BM_STS.

Note also that an alternative for newer systems
is to use the intel_idle driver, which always
ignores BM_STS, relying Linux device drivers
to register constraints explicitly via PM_QOS.

https://bugzilla.kernel.org/show_bug.cgi?id=15886Signed-off-by: Len Brown <len.brown@intel.com>

drivers/acpi/processor_idle.c

@@ -76,6 +76,8 @@ static unsigned int max_cstate __read_mostly = ACPI_PROCESSOR_MAX_POWER;
 module_param(max_cstate, uint, 0000);
 static unsigned int nocst __read_mostly;
 module_param(nocst, uint, 0000);
+static int bm_check_disable __read_mostly;
+module_param(bm_check_disable, uint, 0000);
 
 static unsigned int latency_factor __read_mostly = 2;
 module_param(latency_factor, uint, 0644);
@@ -763,6 +765,9 @@ static int acpi_idle_bm_check(void)
 {
 	u32 bm_status = 0;
 
+	if (bm_check_disable)
+		return 0;
+
 	acpi_read_bit_register(ACPI_BITREG_BUS_MASTER_STATUS, &bm_status);
 	if (bm_status)
 		acpi_write_bit_register(ACPI_BITREG_BUS_MASTER_STATUS, 1);