linux-kernel - [PATCH AUTOSEL 5.5 13/67] MIPS: cavium

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Date:   Thu,  5 Mar 2020 12:12:14 -0500
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Mark Tomlinson <mark.tomlinson@...iedtelesis.co.nz>,
        Chris Packham <chris.packham@...iedtelesis.co.nz>,
        Paul Burton <paulburton@...nel.org>,
        linux-mips@...r.kernel.org, Sasha Levin <sashal@...nel.org>
Subject: [PATCH AUTOSEL 5.5 13/67] MIPS: cavium_octeon: Fix syncw generation.

From: Mark Tomlinson <mark.tomlinson@...iedtelesis.co.nz>

[ Upstream commit 97e914b7de3c943011779b979b8093fdc0d85722 ]

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>
Tested-by: Chris Packham <chris.packham@...iedtelesis.co.nz>
Signed-off-by: Paul Burton <paulburton@...nel.org>
Cc: linux-mips@...r.kernel.org
Cc: linux-kernel@...r.kernel.org
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 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 7c6a1095f5562..aabd097933fe9 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
-- 
2.20.1