Add get_current_el_maybe_constant()

There are some cases where we want to run EL-dependent code in the
shared code.

We could use #ifdef, but it leaves slight possibility where we do not
know the exception level at the build-time (e.g. library code).

The counter approach is to use get_current_el(), but it is run-time
detection, so all EL code is linked, some of which might be unneeded.

This commit adds get_current_el_maybe_constant(). This is a static
inline function that returns a constant value if we know the exception
level at build-time. This is mostly the case.

    if (get_current_el_maybe_constant() == 1) {
            /* do something for EL1 */
    } else if (get_current_el_maybe_constant() == 3) {
            /* do something for EL3 */
    }

If get_current_el_maybe_constant() is build-time constant, the compiler
will optimize out the unreachable code.

If such code is included from the library code, it is not built-time
constant. In this case, it falls back to get_current_el(), so it still
works.

Change-Id: Idb03c20342a5b5173fe2d6b40e1fac7998675ad3
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

include/arch/aarch64/arch_helpers.h

@@ -531,6 +531,23 @@ static inline unsigned int get_current_el(void)
 	return GET_EL(read_CurrentEl());
 }
 
+static inline unsigned int get_current_el_maybe_constant(void)
+{
+#if defined(IMAGE_AT_EL1)
+	return 1;
+#elif defined(IMAGE_AT_EL2)
+	return 2;	/* no use-case in TF-A */
+#elif defined(IMAGE_AT_EL3)
+	return 3;
+#else
+	/*
+	 * If we do not know which exception level this is being built for
+	 * (e.g. built for library), fall back to run-time detection.
+	 */
+	return get_current_el();
+#endif
+}
+
 /*
  * Check if an EL is implemented from AA64PFR0 register fields.
  */