This patch adds some documentation to 'patch_slb_encoding' function
explaining about how it clears the existing immediate value in the
given instruction and inserts a new one there.
Signed-off-by: Anshuman Khandual <khand...@linux.vnet.ibm.com>
---
arch/powerpc/mm/slb.c | 13 ++++++++++++-
1 file changed, 12 insertions(+), 1 deletion(-)
diff --git a/arch/powerpc/mm/slb.c b/arch/powerpc/mm/slb.c
index c87d5de..1962357 100644
--- a/arch/powerpc/mm/slb.c
+++ b/arch/powerpc/mm/slb.c
@@ -279,7 +279,18 @@ void switch_slb(struct task_struct *tsk, struct mm_struct
*mm)
static inline void patch_slb_encoding(unsigned int *insn_addr,
unsigned int immed)
{
- int insn = (*insn_addr & 0xffff0000) | immed;
+
+ /*
+ * This function patches either an li or a cmpldi instruction with
+ * a new immediate value. This relies on the fact that both li
+ * (which is actually addi) and cmpldi both take a 16-bit immediate
+ * value, and it is situated in the same location in the instruction,
+ * ie. bits 16-31 (Big endian bit order) or the lower 16 bits.
+ * To patch the value we read the existing instruction, clear the
+ * immediate value, and or in our new value, then write the instruction
+ * back.
+ */
+ unsigned int insn = (*insn_addr & 0xffff0000) | immed;
patch_instruction(insn_addr, insn);
}
--
2.1.0
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