On 13 December 2013 12:39, Russell King - ARM Linux
<li...@arm.linux.org.uk> wrote:
> I see what's causing this: the kuser helpers are using "bx lr" to return
> which will be undefined on non-Thumb CPUs. We generally cope fine with
> non-Thumb CPUs, conditionalising where necessary on HWCAP_THUMB or the
> T bit in the PSR being set.
>
> However, it looks like the kuser helpers got missed. As a check, please
> look at arch/arm/kernel/entry-armv.S, find the line with:
>
> .macro usr_ret, reg
>
> and ensure that the mov pc, \reg case always gets used. Please report
> back.
Uwe and Arnd came up with a solution except it doesn't work when I test it.
The suggested patch is:
diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
index 1879e8d..de15bfd 100644
--- a/arch/arm/kernel/entry-armv.S
+++ b/arch/arm/kernel/entry-armv.S
@@ -739,6 +739,18 @@ ENDPROC(__switch_to)
.macro usr_ret, reg
#ifdef CONFIG_ARM_THUMB
+ /*
+ * Having CONFIG_ARM_THUMB isn't a guarantee that the cpu has support
+ * for Thumb and so the bx instruction. Use a mov if the address to
+ * jump to is 32 bit aligned. (Note that this code is compiled in ARM
+ * mode, so this is the right test.)
+ */
+#if defined(CONFIG_CPU_32v4)
+ tst \reg, #3
+ moveq pc, \reg
+ b .
+#endif
+
bx \reg
#else
mov pc, \reg
With this applied, and the following two possibilities in next-20140403:
[1] ARCH_MULTI_V4 only (no CONFIG_ARM_THUMB)
CONFIG_CPU_32v4=y
[2] ARCH_MULTI_V4 and ARCH_MULTI_V4T
CONFIG_ARM_THUMB=y
CONFIG_CPU_32v4=y
CONFIG_CPU_32v4T=y
Booting a kernel with either option [1] or [2] yield the following results:
[1] works
[2] hangs after "[ 2.730000] Freeing unused kernel memory: 104K
(c02f5000 - c030f000)"
Any help why the moveq doesn't work would be much appreciated.
The commit where this was tested:
https://bitbucket.org/Kasreyn/linux-next/commits/48320cbe0d5a1fb6ada52cea9efd87c7712367cb
Regards,
Jonas
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