microblaze: Fix msr instruction detection

Fix msr instructions detection. The current code
just use msrclr for loading msr content and compare
it with proper MSR content. If msrclr is not implemented
r8 contains pc address.
Previous code wanted to use MSR carry bit but if msrclr
wasn't implemented carry wasn't cleared.

Signed-off-by: Michal Simek <monstr@monstr.eu>

1 files changed, 7 insertions, 8 deletions

diff --git a/arch/microblaze/kernel/head.S b/arch/microblaze/kernel/head.S
index 0db20b5abb54..778a5ce2e4fc 100644
--- a/arch/microblaze/kernel/head.S
+++ b/arch/microblaze/kernel/head.S
@@ -62,15 +62,14 @@ real_start:
         andi    r1, r1, ~2
         mts     rmsr, r1
 /*
- * Here is checking mechanism which check if Microblaze has msr instructions
- * We load msr and compare it with previous r1 value - if is the same,
- * msr instructions works if not - cpu don't have them.
+ * According to Xilinx, msrclr instruction behaves like 'mfs rX,rpc'
+ * if the msrclr instruction is not enabled. We use this to detect
+ * if the opcode is available, by issuing msrclr and then testing the result.
+ * r8 == 0 - msr instructions are implemented
+ * r8 != 0 - msr instructions are not implemented
  */
-        /* r8=0 - I have msr instr, 1 - I don't have them */
-        rsubi   r0, r0, 1       /* set the carry bit */
-        msrclr  r0, 0x4         /* try to clear it */
-        /* read the carry bit, r8 will be '0' if msrclr exists */
-        addik   r8, r0, 0
+        msrclr  r8, 0 /* clear nothing - just read msr for test */
+        cmpu    r8, r8, r1 /* r1 must contain msr reg content */
 
 /* r7 may point to an FDT, or there may be one linked in.
    if it's in r7, we've got to save it away ASAP.