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aux/acpi: attempt to call _PTS and _TTS before shutdown

The ACPI specification requires us to call _PTS (prepare to sleep)
and _TTS (transition to state) before we write to PM1a_CNT_BLK
and PM1b_CNT_BLK.

--- a/sys/src/cmd/aux/acpi.c

+++ b/sys/src/cmd/aux/acpi.c

@@ -256,6 +256,7 @@

 poweroff(void)

 {

 	int n;

+	void *tts, *pts;

 	if(facp == 0){

 		werrstr("no FACP");

@@ -263,6 +264,16 @@

 	}

 	wirecpu0();

+

+	/* The ACPI spec requires we call _TTS and _PTS to prepare

+	 * the system to go to _S5 state. If they fail, too bad,

+	 * try to go to _S5 state anyway. */

+	pts = amlval(amlwalk(amlroot, "_PTS"));

+	tts = amlval(amlwalk(amlroot, "_TTS"));

+	if(pts)

+		amleval(pts, "i", 5, nil);

+	if(tts)

+		amleval(tts, "i", 5, nil);

 	/* disable GPEs */

 	for(n = 0; GPE0_BLK > 0 && n < GPE0_BLK_LEN/2; n += 2){