Patch "powerpc: Fix bad inline asm constraint in create_zero_mask()" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

powerpc: Fix bad inline asm constraint in create_zero_mask()

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
powerpc-fix-bad-inline-asm-constraint-in-create_zero_mask.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Anton Blanchard <[email protected]>
Date: Sat, 30 Apr 2016 08:29:27 +1000
Subject: powerpc: Fix bad inline asm constraint in create_zero_mask()

From: Anton Blanchard <[email protected]>

commit b4c112114aab9aff5ed4568ca5e662bb02cdfe74 upstream.

In create_zero_mask() we have:

	addi	%1,%2,-1
	andc	%1,%1,%2
	popcntd	%0,%1

using the "r" constraint for %2. r0 is a valid register in the "r" set, but addi X,r0,X turns it into an li:

	li	r7,-1
	andc	r7,r7,r0
	popcntd	r4,r7

Fix this by using the "b" constraint, for which r0 is not a valid register.

This was found with a kernel build using gcc trunk, narrowed down to when -frename-registers was enabled at -O2. It is just luck however that we aren't seeing this on older toolchains.

Thanks to Segher for working with me to find this issue.

---
 arch/powerpc/include/asm/word-at-a-time.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

@@ -82,7 +82,7 @@ static inline unsigned long create_zero_
 	    "andc	%1,%1,%2\n\t"
 	    "popcntd	%0,%1"
 		: "=r" (leading_zero_bits), "=&r" (trailing_zero_bit_mask)
-		: "r" (bits));
+		: "b" (bits));
 
 	return leading_zero_bits;
 }

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/powerpc-fix-bad-inline-asm-constraint-in-create_zero_mask.patch