commit 7ba0dea4158155a68b833982199691dbc2d4e6dc
Author: Matthew Garrett <mjg@redhat.com>
Date:   Mon Apr 19 16:51:39 2010 -0400

    acpi: Fall back to manually changing SCI_EN
    
    The ACPI spec tells us that the ACPI SCI_EN bit is under hardware control
    and shouldn't be touched by the OS. It seems that the Leading Other OS
    ignores this and some machines expect this behaviour. We have a blacklist
    for these, but given that we're able to detect the failure case and the
    alternative to breaking the spec is letting the machine crash and burn,
    let's try falling back when we know the alternative is a mostly-dead
    machine.
    
    Signed-off-by: Matthew Garrett <mjg@redhat.com>

diff --git a/drivers/acpi/sleep.c b/drivers/acpi/sleep.c
index f74834a..79df8d4 100644
--- a/drivers/acpi/sleep.c
+++ b/drivers/acpi/sleep.c
@@ -227,6 +227,7 @@ static int acpi_suspend_begin(suspend_state_t pm_state)
 static int acpi_suspend_enter(suspend_state_t pm_state)
 {
 	acpi_status status = AE_OK;
+	acpi_status enable_status = AE_OK;
 	unsigned long flags = 0;
 	u32 acpi_state = acpi_target_sleep_state;
 
@@ -254,10 +255,19 @@ static int acpi_suspend_enter(suspend_state_t pm_state)
 	}
 
 	/* If ACPI is not enabled by the BIOS, we need to enable it here. */
-	if (set_sci_en_on_resume)
+	if (!set_sci_en_on_resume)
+		enable_status = acpi_enable();
+
+	if (set_sci_en_on_resume || enable_status == AE_NO_HARDWARE_RESPONSE)
+		/* If we're still in legacy mode then we have a problem. The
+		 * spec tells us that this bit is under hardware control, but
+		 * there's no plausible way that the OS can transition back to
+		 * legacy mode so our choices here are to either ignore the
+		 * spec or crash and burn horribly. The latter doesn't seem
+		 * like it's ever going to be the preferable choice, so let's
+		 * live dangerously.
+		 */
 		acpi_write_bit_register(ACPI_BITREG_SCI_ENABLE, 1);
-	else
-		acpi_enable();
 
 	/* Reprogram control registers and execute _BFS */
 	acpi_leave_sleep_state_prep(acpi_state);