feat(trbe): introduce trbe_disable() function

This patch adds trbe_disable() which disables Trace buffer access
from lower ELs in all security state. This function makes Secure
state the owner of Trace buffer and access from EL2/EL1 generate
trap exceptions to EL3.

Signed-off-by: Arvind Ram Prakash <arvind.ramprakash@arm.com>
Change-Id: If3e3bd621684b3c28f44c3ed2fe3df30b143f8cd

include/lib/extensions/trbe.h

@@ -10,9 +10,13 @@
 #include <context.h>
 
 #if ENABLE_TRBE_FOR_NS
+void trbe_disable(cpu_context_t *ctx);
 void trbe_enable(cpu_context_t *ctx);
 void trbe_init_el2_unused(void);
 #else
+static inline void trbe_disable(cpu_context_t *ctx)
+{
+}
 static inline void trbe_enable(cpu_context_t *ctx)
 {
 }

lib/extensions/trbe/trbe.c

@@ -39,6 +39,25 @@ void trbe_enable(cpu_context_t *ctx)
 	write_ctx_reg(state, CTX_MDCR_EL3, mdcr_el3_val);
 }
 
+void trbe_disable(cpu_context_t *ctx)
+{
+	el3_state_t *state = get_el3state_ctx(ctx);
+	u_register_t mdcr_el3_val = read_ctx_reg(state, CTX_MDCR_EL3);
+
+	/*
+	 * MDCR_EL3.NSTBE = 0b0
+	 *  Trace Buffer owning Security state is secure state. If FEAT_RME
+	 *  is not implemented, this field is RES0.
+	 *
+	 * MDCR_EL3.NSTB = 0b00
+	 *  Clear these bits to disable access of trace buffer control registers
+	 *  from lower ELs in any security state.
+	 */
+	mdcr_el3_val &= ~(MDCR_NSTB(MDCR_NSTB_EL1));
+	mdcr_el3_val &= ~(MDCR_NSTBE_BIT);
+	write_ctx_reg(state, CTX_MDCR_EL3, mdcr_el3_val);
+}
+
 void trbe_init_el2_unused(void)
 {
 	/*