linux-kernel - [PATCH] MIPS: cavium_octeon: Fix syncw generation.

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Date:   Wed, 12 Feb 2020 10:24:55 +1300
From:   Mark Tomlinson <mark.tomlinson@...iedtelesis.co.nz>
To:     paulburton@...nel.org, linux-mips@...r.kernel.org
Cc:     chris.packham@...iedtelesis.co.nz, linux-kernel@...r.kernel.org,
        Mark Tomlinson <mark.tomlinson@...iedtelesis.co.nz>
Subject: [PATCH] MIPS: cavium_octeon: Fix syncw generation.

The Cavium Octeon CPU uses a special sync instruction for implementing
wmb, and due to a CPU bug, the instruction must appear twice. A macro
had been defined to hide this:

 #define __SYNC_rpt(type)     (1 + (type == __SYNC_wmb))

which was intended to evaluate to 2 for __SYNC_wmb, and 1 for any other
type of sync. However, this expression is evaluated by the assembler,
and not the compiler, and the result of '==' in the assembler is 0 or
-1, not 0 or 1 as it is in C. The net result was wmb() producing no code
at all. The simple fix in this patch is to change the '+' to '-'.

Fixes: bf92927251b3 ("MIPS: barrier: Add __SYNC() infrastructure")
Signed-off-by: Mark Tomlinson <mark.tomlinson@...iedtelesis.co.nz>
---
 arch/mips/include/asm/sync.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/mips/include/asm/sync.h b/arch/mips/include/asm/sync.h
index 7c6a1095f5..aabd097933 100644
--- a/arch/mips/include/asm/sync.h
+++ b/arch/mips/include/asm/sync.h
@@ -155,9 +155,11 @@
  * effective barrier as noted by commit 6b07d38aaa52 ("MIPS: Octeon: Use
  * optimized memory barrier primitives."). Here we specify that the affected
  * sync instructions should be emitted twice.
+ * Note that this expression is evaluated by the assembler (not the compiler),
+ * and that the assembler evaluates '==' as 0 or -1, not 0 or 1.
  */
 #ifdef CONFIG_CPU_CAVIUM_OCTEON
-# define __SYNC_rpt(type)	(1 + (type == __SYNC_wmb))
+# define __SYNC_rpt(type)	(1 - (type == __SYNC_wmb))
 #else
 # define __SYNC_rpt(type)	1
 #endif
-- 
2.25.0