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=15886
Signed-off-by: Len Brown <len.brown@intel.com>
diff --git a/drivers/acpi/processor_idle.c b/drivers/acpi/processor_idle.c
index b351342..1d41048 100644
--- a/drivers/acpi/processor_idle.c
+++ b/drivers/acpi/processor_idle.c
@@ -76,6 +76,8 @@
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 @@
{
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);