[Xenomai] ARM/BeagleBone Thumb2 Kernels

[Charles Steinkuehler <charles@steinkuehler.net>]

So when I try to build a Xenomai patched kernel for the BeagleBone using
Thumb2 instructions, I get the following linker errors:

>   LD      init/built-in.o
> arch/arm/kernel/built-in.o: In function `__irq_usr':
> :(.kprobes.text+0x3c2): relocation truncated to fit: R_ARM_THM_JUMP19
> against symbol `__ipipe_ret_to_user_irqs_disabled' defined in .text
> section in arch/arm/kernel/built-in.o
> arch/arm/kernel/built-in.o: In function `ret_from_exception':
> :(.kprobes.text+0x600): relocation truncated to fit: R_ARM_THM_JUMP19
> against symbol `__ipipe_ret_to_user_irqs_disabled' defined in .text
> section in arch/arm/kernel/built-in.o
> make: *** [vmlinux] Error 1
> 
> The link error goes away if I disable CONFIG_THUMB2_KERNEL, which wasn't
> set in the original kernel config.

This has recently become more of a problem, as the default BeagleBone
kernel has switched to enabling CONFIG_THUMB2_KERNEL.

A bit of Googling turned up the following patch to fix a similar problem
on the Tegra (not Xenomai related):

  http://patchwork.ozlabs.org/patch/236760/

I applied a similar if-then patch (see below) to the two Xenomai calls
that generate the linker errors, and the result is a working kernel.

Can someone more familiar with iPipeand ARM assembly review the attached
minor patch and advise on whether or not it seems OK?

As I understand things, the two bne calls in the default ipipe patch
don't work when building a kernel using the thumb instructions due to
the limited range of the conditional branch.  Using the if-then
construct when THUMB2 is enabled allows for a larger addressing range in
the branch, thus avoiding the linker errors.  If this seems OK, perhaps
it should be applied to the master ipipe patch set?  If not, is there a
better way to avoid the linker errors when building with the thumb
instructions?

-- 
Charles Steinkuehler
charles@steinkuehler.net
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diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
index e2bc263..6f4d9f0 100644
--- a/arch/arm/kernel/entry-armv.S
+++ b/arch/arm/kernel/entry-armv.S
@@ -469,6 +469,7 @@ __irq_usr:
 	kuser_cmpxchg_check
 	irq_handler
 #ifdef CONFIG_IPIPE
+THUMB(	it	ne )
 	bne	__ipipe_ret_to_user_irqs_disabled
 #endif	/* CONFIG_IPIPE */
 	get_thread_info tsk
@@ -767,6 +768,7 @@ ENTRY(ret_from_exception)
 	disable_irq
 	bl	__ipipe_check_root
 	cmp     r0, #1
+THUMB(	it	ne )
 	bne	__ipipe_ret_to_user_irqs_disabled  @ Fast exit path over non-root domains
 #endif /* CONFIG_IPIPE */
 	get_thread_info tsk
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