ACPI idle: permit sparse C-state sub-state numbers

Linux uses CPUID.MWAIT.EDX to validate the C-states
reported by ACPI, silently discarding states which
are not supported by the HW.

This test is too restrictive, as some HW now uses
sparse sub-state numbering, so the sub-state number
may be higher than the number of sub-states...

Also, rather than silently ignoring an invalid state,
we should complain about a firmware bug.

In practice...

Bay Trail systems originally supported C6-no-shrink as
MWAIT sub-state 0x58, and in CPUID.MWAIT.EDX 0x03000000
indicated that there were 3 MWAIT-C6 sub-states.
So acpi_idle would discard that C-state because 8 >= 3.

Upon discovering this issue, the ucode was updated so that
C6-no-shrink was also exported as 0x51, and the BIOS was
updated to match.  However, systems shipped with 0x58,
will never get a BIOS update, and this patch allows
Linux to see C6-no-shrink on early Bay Trail.

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

1 files changed, 3 insertions, 1 deletions

diff --git a/arch/x86/kernel/acpi/cstate.c b/arch/x86/kernel/acpi/cstate.c
index e69182f..4b28159 100644
--- a/arch/x86/kernel/acpi/cstate.c
+++ b/arch/x86/kernel/acpi/cstate.c
@@ -87,7 +87,9 @@ static long acpi_processor_ffh_cstate_probe_cpu(void *_cx)
 	num_cstate_subtype = edx_part & MWAIT_SUBSTATE_MASK;
 
 	retval = 0;
-	if (num_cstate_subtype < (cx->address & MWAIT_SUBSTATE_MASK)) {
+	/* If the HW does not support any sub-states in this C-state */
+	if (num_cstate_subtype == 0) {
+		pr_warn(FW_BUG "ACPI MWAIT C-state 0x%x not supported by HW (0x%x)\n", cx->address, edx_part);
 		retval = -1;
 		goto out;
 	}