feat(cpus): make revision procedure call optional

For runtime errata, we should avoid generating calls to
`cpu_get_rev_var` unless its necessary. Make the path that generates
this call parameterized, and cache the result in a temporary register to
allow future calls that go down the alternate path to retrieve the cache
CPU revision.

Change-Id: I9882ede76568fbd9a7ccd4caa74eff0c66a7b20e
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>

include/lib/cpus/aarch64/cpu_macros.S

@@ -472,15 +472,25 @@
  * _chosen:
  *	Compile time flag on whether the erratum is included
  *
- * clobbers: x0-x9 (PCS compliant)
+ * _get_rev:
+ *	Optional parameter that determines whether to insert a call to the CPU revision fetching
+ *	procedure. Stores the result of this in the temporary register x10.
+ *
+ * clobbers: x0-x10 (PCS compliant)
  */
-.macro apply_erratum _cpu:req, _cve:req, _id:req, _chosen:req
-	.if \_chosen
+.macro apply_erratum _cpu:req, _cve:req, _id:req, _chosen:req, _get_rev=GET_CPU_REV
+	.if (\_chosen & \_get_rev)
 		mov	x9, x30
 		bl	cpu_get_rev_var
+		mov	x10, x0
+	.elseif (\_chosen)
+		mov	x9, x30
+		mov	x0, x10
+	.endif
+
+	.if \_chosen
 		bl	erratum_\_cpu\()_\_id\()_wa
 		mov	x30, x9
-
 	.endif
 .endm
 

include/lib/cpus/errata.h

@@ -66,6 +66,9 @@ CASSERT(sizeof(struct erratum_entry) == ERRATUM_ENTRY_SIZE,
 #define NO_ASSERT		0
 #define NO_APPLY_AT_RESET	0
 #define APPLY_AT_RESET		1
+#define GET_CPU_REV		1
+#define NO_GET_CPU_REV		0
+
 /* useful for errata that end up always being worked around */
 #define ERRATUM_ALWAYS_CHOSEN	1