[PATCH] ARM: Enable unaligned accesses on armv6 and later

Sascha Hauer s.hauer at pengutronix.de
Mon Jul 9 03:09:39 EDT 2012

We have the following in the tree:

|commit af42feb9d255bc3ea3b514180f265479ea8834f9
|Author: Enrico Scholz <enrico.scholz at sigma-chemnitz.de>
|Date:   Mon Jan 2 11:49:17 2012 +0100
|    ARM: set SCTRL[A] only when architecture does not support unaligned access
|    Recent gcc generates code with unaligned access when architecture
|    supports it. Setting A bit unconditionally causes data-aborts on such
|    code rendering barebox unusable.
|    Signed-off-by: Enrico Scholz <enrico.scholz at sigma-chemnitz.de>
|    Signed-off-by: Sascha Hauer <s.hauer at pengutronix.de>

What the patch tried is correct: We should set the A bit only when the architecture
does not support unaligned accesses. To figure out whether the architecture supports
unaligned accesses the patch tested for the U bit which is wrong. The U bit may be
0 after a reset, so instead of testing for the U bit we have to set it. This can
be done on armv6 and later. All others have the A bit set to trap unaligned accesses.

Signed-off-by: Sascha Hauer <s.hauer at pengutronix.de>

Enrico, can you confirm this is correct?


 arch/arm/cpu/start.c |   12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/arch/arm/cpu/start.c b/arch/arm/cpu/start.c
index 3c282ee..523179d 100644
--- a/arch/arm/cpu/start.c
+++ b/arch/arm/cpu/start.c
@@ -81,13 +81,11 @@ void __naked __bare_init reset(void)
 	r &= ~(CR_M | CR_C | CR_B | CR_S | CR_R | CR_V);
 	r |= CR_I;
-	if (!(r & CR_U))
-		/* catch unaligned access on architectures which do not
-		 * support unaligned access */
-		r |= CR_A;
-	else
-		r &= ~CR_A;
+#if __LINUX_ARM_ARCH__ >= 6
+	r |= CR_U;
+	r |= CR_A;
 #ifdef __ARMEB__
 	r |= CR_B;

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