[PATCH v2 4/4] arm64: Document values for system registers on boot

[Mark Brown]

When booting require that the system registers available at or below the
exception level the kernel is entered be initialised but do not specify
what values should be used in the general case, creating some potential
for issues if the kernel does not subsequently configure those registers
explicitly (for example if they are not yet used by the kernel) or where
their effects may create issues during early configuration.

Specify that where the architecture provides a reset value that value
must be used.

Signed-off-by: Mark Brown <broonie at kernel.org>
---
 Documentation/arm64/booting.rst | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/Documentation/arm64/booting.rst b/Documentation/arm64/booting.rst
index 4fcc00add117..d49f568eb79d 100644
--- a/Documentation/arm64/booting.rst
+++ b/Documentation/arm64/booting.rst
@@ -205,7 +205,9 @@ Before jumping into the kernel, the following conditions must be met:
   All writable architected system registers at or below the exception
   level where the kernel image will be entered must be initialised by
   software at a higher exception level to prevent execution in an UNKNOWN
-  state.
+  state.  Where these architected system registers have reset values
+  specified by the architecture they must be initialised to those values
+  unless specified more specifically.
 
   - SCR_EL3.FIQ must have the same value across all CPUs the kernel is
     executing on.
-- 
2.20.1