fix(drivers/tzc400): never disable filter 0

Disabling filter 0 causes inability to access DRAM.
An attempt leads to an abort.
ARM manual disallows to disable filter 0, but if we do
that from SRAM, nothing bad happens.

This patch prevents disabling of a filter 0, allowing to
reconfigure other filters from DRAM.

Note: this patch doesn't change the logic after reset.
It is only needed in case someone wants to reconfigure the
previously configured TZASC.

Change-Id: I196a0cb110a89afbde97f64a94df3101f28708a4
Signed-off-by: stsp@users.sourceforge.net

drivers/arm/tzc/tzc400.c

@@ -291,6 +291,11 @@ void tzc400_enable_filters(void)
 	for (filter = 0U; filter < tzc400.num_filters; filter++) {
 		state = _tzc400_get_gate_keeper(tzc400.base, filter);
 		if (state != 0U) {
+			/* Filter 0 is special and cannot be disabled.
+			 * So here we allow it being already enabled. */
+			if (filter == 0U) {
+				continue;
+			}
 			/*
 			 * The TZC filter is already configured. Changing the
 			 * programmer's view in an active system can cause
@@ -312,14 +317,17 @@ void tzc400_enable_filters(void)
 void tzc400_disable_filters(void)
 {
 	unsigned int filter;
+	unsigned int state;
+	unsigned int start = 0U;
 
 	assert(tzc400.base != 0U);
 
-	/*
-	 * We don't do the same state check as above as the Gatekeepers are
-	 * disabled after reset.
-	 */
-	for (filter = 0; filter < tzc400.num_filters; filter++)
+	/* Filter 0 is special and cannot be disabled. */
+	state = _tzc400_get_gate_keeper(tzc400.base, 0);
+	if (state != 0U) {
+		start++;
+	}
+	for (filter = start; filter < tzc400.num_filters; filter++)
 		_tzc400_set_gate_keeper(tzc400.base, filter, 0);
 }