[Skiboot] [PATCH] head.S: document opal_naca

[Oliver O'Halloran]

On P7 machines OPAL can be loaded as a replacement for PHYP or in an
OPAL specific mode. In OPAL mode 0x0 contains a pointer to the OPAL
specific variant of the NACA which defines the entrypoint to OPAL, etc.
Later machines use the same load method for for PHYP and OPAL and do not
need the OPAL specific NACA, but this needs to be kept around for backwards
compatibility.

Signed-off-by: Oliver O'Halloran <oohall at gmail.com>
---
 asm/head.S | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/asm/head.S b/asm/head.S
index cd6623e20b3c..ddcfd46029fe 100644
--- a/asm/head.S
+++ b/asm/head.S
@@ -44,8 +44,9 @@
 	. = 0
 .global __head
 __head:
-	/* When booted as an OPAL LID, this is a pointer to the OPAL
-	 * variant of the NACA
+	/*
+	 * When booting a P7 machine in OPAL mode this pointer is used to
+	 * find the opal variant of the NACA. Unused on other machines.
 	 */
 	.llong	opal_naca
 
@@ -783,7 +784,7 @@ hv_lid_load_table:
 
 /*
  *
- * OPAL variant of NACA
+ * OPAL variant of NACA. This is only used when booting a P7 in OPAL mode.
  *
  */
 .global opal_naca
-- 
2.5.5