[PATCH V2 2/4] powerpc: add helper to check if offset is within rel branch range

[Anju T Sudhakar]

From: "Naveen N. Rao" <naveen.n....@linux.vnet.ibm.com>

Signed-off-by: Naveen N. Rao <naveen.n....@linux.vnet.ibm.com>
Signed-off-by: Anju T Sudhakar <a...@linux.vnet.ibm.com>
---
 arch/powerpc/include/asm/code-patching.h |  1 +
 arch/powerpc/lib/code-patching.c         | 24 +++++++++++++++++++++++-
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/code-patching.h 
b/arch/powerpc/include/asm/code-patching.h
index 2015b07..75ee4f4 100644
--- a/arch/powerpc/include/asm/code-patching.h
+++ b/arch/powerpc/include/asm/code-patching.h
@@ -22,6 +22,7 @@
 #define BRANCH_SET_LINK        0x1
 #define BRANCH_ABSOLUTE        0x2
 
+bool is_offset_in_branch_range(long offset);
 unsigned int create_branch(const unsigned int *addr,
                           unsigned long target, int flags);
 unsigned int create_cond_branch(const unsigned int *addr,
diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c
index d5edbeb..f643451 100644
--- a/arch/powerpc/lib/code-patching.c
+++ b/arch/powerpc/lib/code-patching.c
@@ -32,6 +32,28 @@ int patch_branch(unsigned int *addr, unsigned long target, 
int flags)
        return patch_instruction(addr, create_branch(addr, target, flags));
 }
 
+bool is_offset_in_branch_range(long offset)
+{
+       /*
+        * Powerpc branch instruction is :
+        *
+        *  0         6                 30   31
+        *  +---------+----------------+---+---+
+        *  | opcode  |     LI         |AA |LK |
+        *  +---------+----------------+---+---+
+        *  Where AA = 0 and LK = 0
+        *
+        * LI is a signed 24 bits integer. The real branch offset is computed
+        * by: imm32 = SignExtend(LI:'0b00', 32);
+        *
+        * So the maximum forward branch should be:
+        *   (0x007fffff << 2) = 0x01fffffc =  0x1fffffc
+        * The maximum backward branch should be:
+        *   (0xff800000 << 2) = 0xfe000000 = -0x2000000
+        */
+       return (offset >= -0x2000000 && offset <= 0x1fffffc && !(offset & 0x3));
+}
+
 unsigned int create_branch(const unsigned int *addr,
                           unsigned long target, int flags)
 {
@@ -43,7 +65,7 @@ unsigned int create_branch(const unsigned int *addr,
                offset = offset - (unsigned long)addr;
 
        /* Check we can represent the target in the instruction format */
-       if (offset < -0x2000000 || offset > 0x1fffffc || offset & 0x3)
+       if (!is_offset_in_branch_range(offset))
                return 0;
 
        /* Mask out the flags and target, so they don't step on each other. */
-- 
2.7.4