Index: sys/dev/acpica/acpi_cmbat.c
===================================================================
--- sys/dev/acpica/acpi_cmbat.c
+++ sys/dev/acpica/acpi_cmbat.c
@@ -62,6 +62,7 @@
 #define	ACPI_BATTERY_BST_CHANGE	0x80
 #define	ACPI_BATTERY_BIF_CHANGE	0x81
 #define	ACPI_BATTERY_BIX_CHANGE	ACPI_BATTERY_BIF_CHANGE
+#define ACPI_BATTERY_BTP_WARNING_LEVEL 20
 
 struct acpi_cmbat_softc {
     device_t	    dev;
@@ -70,6 +71,8 @@
     struct acpi_bix bix;
     struct acpi_bst bst;
     struct timespec bst_lastupdated;
+    BOOLEAN	acpi_btp_exists;
+    int btp_warning_level;
 };
 
 ACPI_SERIAL_DECL(cmbat, "ACPI cmbat");
@@ -87,6 +90,8 @@
 static void		acpi_cmbat_get_bix(void *arg);
 static int		acpi_cmbat_bst(device_t, struct acpi_bst *);
 static int		acpi_cmbat_bix(device_t, void *, size_t);
+static void		acpi_cmbat_btp_set(void *, int);
+static int 		acpi_cmbat_btp_sysctl(SYSCTL_HANDLER_ARGS);
 static void		acpi_cmbat_init_battery(void *arg);
 
 static device_method_t acpi_cmbat_methods[] = {
@@ -118,12 +123,12 @@
 {
     static char *cmbat_ids[] = { "PNP0C0A", NULL };
     int rv;
-    
+
     if (acpi_disabled("cmbat"))
-	return (ENXIO);
+    	return (ENXIO);
     rv = ACPI_ID_PROBE(device_get_parent(dev), dev, cmbat_ids, NULL);
-    if (rv <= 0)
-	device_set_desc(dev, "ACPI Control Method Battery");
+    if(rv <= 0)
+    	device_set_desc(dev, "ACPI Control Method Battery");
     return (rv);
 }
 
@@ -132,6 +137,7 @@
 {
     int		error;
     ACPI_HANDLE	handle;
+    ACPI_HANDLE tmp;
     struct acpi_cmbat_softc *sc;
 
     sc = device_get_softc(dev);
@@ -146,6 +152,23 @@
 	return (error);
     }
 
+     /* A potential battery warning level sysctl. */
+
+    ACPI_SERIAL_BEGIN(cmbat);
+    if(ACPI_SUCCESS(acpi_GetHandleInScope(handle, "_BTP", &tmp))) {
+
+		sc->acpi_btp_exists = TRUE;
+		sc->btp_warning_level = ACPI_BATTERY_BTP_WARNING_LEVEL;
+	
+		struct sysctl_oid *cmbat_oid = device_get_sysctl_tree(dev);
+		SYSCTL_ADD_PROC(NULL, SYSCTL_CHILDREN(cmbat_oid), OID_AUTO,
+		"warning_level", CTLTYPE_INT | CTLFLAG_RW, 0, 0,
+		acpi_cmbat_btp_sysctl, "I" ,"battery warning level");
+    }
+    else
+    	sc->acpi_btp_exists = FALSE;
+    ACPI_SERIAL_END(cmbat);
+
     /*
      * Install a system notify handler in addition to the device notify.
      * Toshiba notebook uses this alternate notify for its battery.
@@ -526,6 +549,75 @@
 }
 
 static void
+acpi_cmbat_btp_set(void *arg, int warning)
+{
+
+    struct acpi_cmbat_softc *sc;
+    device_t dev;
+    ACPI_HANDLE h;
+    ACPI_STATUS as;
+    uint64_t newtp;
+
+    ACPI_SERIAL_ASSERT(cmbat);
+    dev = arg;
+    sc = device_get_softc(dev);
+    h = acpi_get_handle(dev);
+
+	newtp = sc->bix.lfcap * warning / 100;
+	as = acpi_SetInteger(h, "_BTP", (uint32_t) newtp);
+
+	if (ACPI_FAILURE(as)) {
+		ACPI_VPRINT(dev, acpi_device_get_parent_softc(dev),
+		"error setting _BTP --%s\n", AcpiFormatException(as));
+		sc->btp_warning_level = 0;
+	}
+	else
+		sc->btp_warning_level = warning;
+}
+
+static int
+acpi_cmbat_btp_sysctl(SYSCTL_HANDLER_ARGS)
+{
+
+    struct sysctl_oid *parent;
+    struct acpi_cmbat_softc *sc;
+    device_t dev;
+    ACPI_HANDLE h;
+    long battery_index;
+
+
+    ACPI_SERIAL_BEGIN(cmbat);
+
+    parent = SYSCTL_PARENT(oidp); 
+    battery_index = strtol(parent->oid_name, NULL, 0);
+    dev = devclass_get_device(acpi_cmbat_devclass, (int) battery_index);
+
+    h = acpi_get_handle(dev);		
+    sc = device_get_softc(dev);
+
+    if(req->newptr && acpi_BatteryIsPresent(dev)) {
+	/* Write request */
+
+	SYSCTL_IN(req, &sc->btp_warning_level, sizeof(sc->btp_warning_level));
+	/* correct bogus writes */
+	if(sc->btp_warning_level < 0 || sc->btp_warning_level > 100)
+		sc->btp_warning_level = ACPI_BATTERY_BTP_WARNING_LEVEL;
+
+	acpi_cmbat_btp_set(dev, sc->btp_warning_level);
+    }
+
+	else if(req->newptr) /* write request w/o battery */
+		sc->btp_warning_level = 0;
+		
+    else /* read request */
+	SYSCTL_OUT(req, &sc->btp_warning_level, sizeof(sc->btp_warning_level));
+
+    ACPI_SERIAL_END(cmbat);
+    return (0);
+}
+
+
+static void
 acpi_cmbat_init_battery(void *arg)
 {
     struct acpi_cmbat_softc *sc;
@@ -578,6 +670,13 @@
 
 	valid = acpi_battery_bst_valid(&sc->bst) &&
 	    acpi_battery_bix_valid(&sc->bix);
+
+	/* set battery warning level */
+	if(valid && sc->acpi_btp_exists)
+    		acpi_cmbat_btp_set(dev, sc->btp_warning_level);
+	else
+		sc->btp_warning_level = 0;
+
 	ACPI_SERIAL_END(cmbat);
 
 	if (valid)