ec/lenovo/h8: Guard against EC bugs in the battery status logic.

On my Thinkpad with an H8-compatible ENE KB9012 EC (GDHT92WW 1.52), when
the battery is nearly full and we switch from battery to AC by plugging
in the cable, the current rate will not drop to 0 immediately, but the
discharging state is cleared immediately.

This leads to the code trying to process an invalid rate value >0x8000,
leading to a displayed rate of >1000W.

This patch changes the logic to deal with these corner cases.

Change-Id: Ideb588d00757f259792e5ae97729e371b63a096c
Signed-off-by: Tobias Diedrich <ranma+coreboot@tdiedrich.de>
Reviewed-on: https://review.coreboot.org/18349
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Martin Roth <martinroth@google.com>

src/ec/lenovo/h8/acpi/battery.asl

@@ -97,21 +97,36 @@ Method(BSTA, 4, NotSerialized)
 	Store(BAMA, Local1)
 	Store(Arg0, PAGE) /* Battery dynamic information */
 
+	/*
+	 * Present rate is a 16bit signed int, positive while charging
+	 * and negative while discharging.
+	 */
 	Store(BAPR, Local2)
 
-	if (Arg2) // charging
+	If (Arg2) // Charging
 	{
 		Or(2, Local0, Local0)
-
+	}
-		If (LGreaterEqual (Local2, 0x8000)) {
+	Else
+	{
+		If (Arg3) // Discharging
+		{
+			Or(1, Local0, Local0)
+			// Negate present rate
+			Subtract(0x10000, Local2, Local2)
+		}
+		Else // Full battery, force to 0
+		{
 			Store(0, Local2)
 		}
 	}
 
-	if (Arg3) // discharging
+	/*
-	{
+	 * The present rate value must be positive now, if it is not we have an
-		Or(1, Local0, Local0)
+	 * EC bug or inconsistency and force the value to 0.
-		Subtract(0x10000, Local2, Local2)
+	 */
+	If (LGreaterEqual (Local2, 0x8000)) {
+		Store(0, Local2)
 	}
 
 	Store(Local0, Index(Arg1, 0x00))