ACPI / sleep: EC-based wakeup from suspend-to-idle on recent systems

Some recent Dell laptops, including the XPS13 model numbers 9360 and
9365, cannot be woken up from suspend-to-idle by pressing the power
button which is unexpected and makes that feature less usable on
those systems.  Moreover, on the 9365 ACPI S3 (suspend-to-RAM) is
not expected to be used at all (the OS these systems ship with never
exercises the ACPI S3 path in the firmware) and suspend-to-idle is
the only viable system suspend mechanism there.

The reason why the power button wakeup from suspend-to-idle doesn't
work on those systems is because their power button events are
signaled by the EC (Embedded Controller), whose GPE (General Purpose
Event) line is disabled during suspend-to-idle transitions in Linux.
That is done on purpose, because in general the EC tends to be noisy
for various reasons (battery and thermal updates and similar, for
example) and all events signaled by it would kick the CPUs out of
deep idle states while in suspend-to-idle, which effectively might
defeat its purpose.

Of course, on the Dell systems in question the EC GPE must be enabled
during suspend-to-idle transitions for the button press events to
be signaled while suspended at all, but fortunately there is a way
out of this puzzle.

First of all, those systems have the ACPI_FADT_LOW_POWER_S0 flag set
in their ACPI tables, which means that the OS is expected to prefer
the "low power S0 idle" system state over ACPI S3 on them.  That
causes the most recent versions of other OSes to simply ignore ACPI
S3 on those systems, so it is reasonable to expect that it should not
be necessary to block GPEs during suspend-to-idle on them.

Second, in addition to that, the systems in question provide a special
firmware interface that can be used to indicate to the platform that
the OS is transitioning into a system-wide low-power state in which
certain types of activity are not desirable or that it is leaving
such a state and that (in principle) should allow the platform to
adjust its operation mode accordingly.

That interface is a special _DSM object under a System Power
Management Controller device (PNP0D80).  The expected way to use it
is to invoke function 0 from it on system initialization, functions
3 and 5 during suspend transitions and functions 4 and 6 during
resume transitions (to reverse the actions carried out by the
former).  In particular, function 5 from the "Low-Power S0" device
_DSM is expected to cause the platform to put itself into a low-power
operation mode which should include making the EC less verbose (so to
speak).  Next, on resume, function 6 switches the platform back to
the "working-state" operation mode.

In accordance with the above, modify the ACPI suspend-to-idle code
to look for the "Low-Power S0" _DSM interface on platforms with the
ACPI_FADT_LOW_POWER_S0 flag set in the ACPI tables.  If it's there,
use it during suspend-to-idle transitions as prescribed and avoid
changing the GPE configuration in that case.  [That should reflect
what the most recent versions of other OSes do.]

Also modify the ACPI EC driver to make it handle events during
suspend-to-idle in the usual way if the "Low-Power S0" _DSM interface
is going to be used to make the power button events work while
suspended on the Dell machines mentioned above

Link: http://www.uefi.org/sites/default/files/resources/Intel_ACPI_Low_Power_S0_Idle.pdf
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/ec.c

@@ -1835,7 +1835,7 @@ static int acpi_ec_suspend(struct device *dev)
 	struct acpi_ec *ec =
 		acpi_driver_data(to_acpi_device(dev));
 
-	if (ec_freeze_events)
+	if (acpi_sleep_no_ec_events() && ec_freeze_events)
 		acpi_ec_disable_event(ec);
 	return 0;
 }

drivers/acpi/internal.h

@@ -199,9 +199,11 @@ void acpi_ec_remove_query_handler(struct acpi_ec *ec, u8 query_bit);
   -------------------------------------------------------------------------- */
 #ifdef CONFIG_ACPI_SYSTEM_POWER_STATES_SUPPORT
 extern bool acpi_s2idle_wakeup(void);
+extern bool acpi_sleep_no_ec_events(void);
 extern int acpi_sleep_init(void);
 #else
 static inline bool acpi_s2idle_wakeup(void) { return false; }
+static inline bool acpi_sleep_no_ec_events(void) { return true; }
 static inline int acpi_sleep_init(void) { return -ENXIO; }
 #endif
 

drivers/acpi/sleep.c

@@ -650,18 +650,108 @@ static const struct platform_suspend_ops acpi_suspend_ops_old = {
 	.recover = acpi_pm_finish,
 };
 
+static bool s2idle_in_progress;
 static bool s2idle_wakeup;
 
+/*
+ * On platforms supporting the Low Power S0 Idle interface there is an ACPI
+ * device object with the PNP0D80 compatible device ID (System Power Management
+ * Controller) and a specific _DSM method under it.  That method, if present,
+ * can be used to indicate to the platform that the OS is transitioning into a
+ * low-power state in which certain types of activity are not desirable or that
+ * it is leaving such a state, which allows the platform to adjust its operation
+ * mode accordingly.
+ */
+static const struct acpi_device_id lps0_device_ids[] = {
+	{"PNP0D80", },
+	{"", },
+};
+
+#define ACPI_LPS0_DSM_UUID	"c4eb40a0-6cd2-11e2-bcfd-0800200c9a66"
+
+#define ACPI_LPS0_SCREEN_OFF	3
+#define ACPI_LPS0_SCREEN_ON	4
+#define ACPI_LPS0_ENTRY		5
+#define ACPI_LPS0_EXIT		6
+
+#define ACPI_S2IDLE_FUNC_MASK	((1 << ACPI_LPS0_ENTRY) | (1 << ACPI_LPS0_EXIT))
+
+static acpi_handle lps0_device_handle;
+static guid_t lps0_dsm_guid;
+static char lps0_dsm_func_mask;
+
+static void acpi_sleep_run_lps0_dsm(unsigned int func)
+{
+	union acpi_object *out_obj;
+
+	if (!(lps0_dsm_func_mask & (1 << func)))
+		return;
+
+	out_obj = acpi_evaluate_dsm(lps0_device_handle, &lps0_dsm_guid, 1, func, NULL);
+	ACPI_FREE(out_obj);
+
+	acpi_handle_debug(lps0_device_handle, "_DSM function %u evaluation %s\n",
+			  func, out_obj ? "successful" : "failed");
+}
+
+static int lps0_device_attach(struct acpi_device *adev,
+			      const struct acpi_device_id *not_used)
+{
+	union acpi_object *out_obj;
+
+	if (lps0_device_handle)
+		return 0;
+
+	if (!(acpi_gbl_FADT.flags & ACPI_FADT_LOW_POWER_S0))
+		return 0;
+
+	guid_parse(ACPI_LPS0_DSM_UUID, &lps0_dsm_guid);
+	/* Check if the _DSM is present and as expected. */
+	out_obj = acpi_evaluate_dsm(adev->handle, &lps0_dsm_guid, 1, 0, NULL);
+	if (out_obj && out_obj->type == ACPI_TYPE_BUFFER) {
+		char bitmask = *(char *)out_obj->buffer.pointer;
+
+		if ((bitmask & ACPI_S2IDLE_FUNC_MASK) == ACPI_S2IDLE_FUNC_MASK) {
+			lps0_dsm_func_mask = bitmask;
+			lps0_device_handle = adev->handle;
+		}
+
+		acpi_handle_debug(adev->handle, "_DSM function mask: 0x%x\n",
+				  bitmask);
+	} else {
+		acpi_handle_debug(adev->handle,
+				  "_DSM function 0 evaluation failed\n");
+	}
+	ACPI_FREE(out_obj);
+	return 0;
+}
+
+static struct acpi_scan_handler lps0_handler = {
+	.ids = lps0_device_ids,
+	.attach = lps0_device_attach,
+};
+
 static int acpi_freeze_begin(void)
 {
 	acpi_scan_lock_acquire();
+	s2idle_in_progress = true;
 	return 0;
 }
 
 static int acpi_freeze_prepare(void)
 {
-	acpi_enable_all_wakeup_gpes();
-	acpi_os_wait_events_complete();
+	if (lps0_device_handle) {
+		acpi_sleep_run_lps0_dsm(ACPI_LPS0_SCREEN_OFF);
+		acpi_sleep_run_lps0_dsm(ACPI_LPS0_ENTRY);
+	} else {
+		/*
+		 * The configuration of GPEs is changed here to avoid spurious
+		 * wakeups, but that should not be necessary if this is a
+		 * "low-power S0" platform and the low-power S0 _DSM is present.
+		 */
+		acpi_enable_all_wakeup_gpes();
+		acpi_os_wait_events_complete();
+	}
 	if (acpi_sci_irq_valid())
 		enable_irq_wake(acpi_sci_irq);
 
@@ -700,11 +790,17 @@ static void acpi_freeze_restore(void)
 	if (acpi_sci_irq_valid())
 		disable_irq_wake(acpi_sci_irq);
 
-	acpi_enable_all_runtime_gpes();
+	if (lps0_device_handle) {
+		acpi_sleep_run_lps0_dsm(ACPI_LPS0_EXIT);
+		acpi_sleep_run_lps0_dsm(ACPI_LPS0_SCREEN_ON);
+	} else {
+		acpi_enable_all_runtime_gpes();
+	}
 }
 
 static void acpi_freeze_end(void)
 {
+	s2idle_in_progress = false;
 	acpi_scan_lock_release();
 }
 
@@ -727,11 +823,15 @@ static void acpi_sleep_suspend_setup(void)
 
 	suspend_set_ops(old_suspend_ordering ?
 		&acpi_suspend_ops_old : &acpi_suspend_ops);
+
+	acpi_scan_add_handler(&lps0_handler);
 	freeze_set_ops(&acpi_freeze_ops);
 }
 
 #else /* !CONFIG_SUSPEND */
-#define s2idle_wakeup	(false)
+#define s2idle_in_progress	(false)
+#define s2idle_wakeup		(false)
+#define lps0_device_handle	(NULL)
 static inline void acpi_sleep_suspend_setup(void) {}
 #endif /* !CONFIG_SUSPEND */
 
@@ -740,6 +840,11 @@ bool acpi_s2idle_wakeup(void)
 	return s2idle_wakeup;
 }
 
+bool acpi_sleep_no_ec_events(void)
+{
+	return !s2idle_in_progress || !lps0_device_handle;
+}
+
 #ifdef CONFIG_PM_SLEEP
 static u32 saved_bm_rld;