arm64: add indirection to C entry point

To allow setting the entry point for the secondary CPUs
provide a pointer, c_entry, which contains the location
to branc to after setting up the stack.

BUG=chrome-os-partner:31545
BRANCH=None
TEST=Built and booted to the kernel on ryu.

Change-Id: Ic2f6c79cde708b24c379345aed1e2cc0760ccad8
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/214771
Reviewed-by: Furquan Shaikh <furquan@chromium.org>

src/arch/arm64/c_entry.c

@@ -20,6 +20,14 @@
 #include <arch/stages.h>
 #include <arch/cpu.h>
 
+
+/*
+ * This variable holds entry point for CPUs starting up. Before the other
+ * CPUs are brought up this value will change to provide the secondary
+ * code path.
+ */
+void (*c_entry)(void) = &arm64_init;
+
 void __attribute__((weak)) arm64_soc_init(void)
 {
 	/* Default weak implementation does nothing. */

src/arch/arm64/stage_entry.S

@@ -99,7 +99,13 @@ ENTRY(arm64_c_environment)
 	bl	cpu_get_stack
 	mov	sp, x0
 
-	b	arm64_init
+	/* Get entry point by dereferencing c_entry. */
+	ldr	x0, 1f
+	ldr	x0, [x0]
+	br	x0
+.align 3
+	1:
+	.quad	c_entry
 ENDPROC(arm64_c_environment)
 
 CPU_RESET_ENTRY(arm64_cpu_startup)