Merge pull request #540 from antonio-nino-diaz-arm/an/porting_guide
Porting guide: Clarify API that don't follow AAPCS
diff --git a/docs/porting-guide.md b/docs/porting-guide.md
index 004f70d..741ee20 100644
--- a/docs/porting-guide.md
+++ b/docs/porting-guide.md
@@ -654,7 +654,8 @@
per-CPU stacks). This function will be invoked very early in the
initialization sequence which mandates that this function should be
implemented in assembly and should not rely on the avalability of a C
-runtime environment.
+runtime environment. This function can clobber x0 - x8 and must preserve
+x9 - x29.
This function plays a crucial role in the power domain topology framework in
PSCI and details of this can be found in [Power Domain Topology Design].