ARM: convert all "mov.* pc, reg" to "bx reg" for ARMv6+

ARMv6 and greater introduced a new instruction ("bx") which can be used
to return from function calls.  Recent CPUs perform better when the
"bx lr" instruction is used rather than the "mov pc, lr" instruction,
and this sequence is strongly recommended to be used by the ARM
architecture manual (section A.4.1.1).

We provide a new macro "ret" with all its variants for the condition
code which will resolve to the appropriate instruction.

Rather than doing this piecemeal, and miss some instances, change all
the "mov pc" instances to use the new macro, with the exception of
the "movs" instruction and the kprobes code.  This allows us to detect
the "mov pc, lr" case and fix it up - and also gives us the possibility
of deploying this for other registers depending on the CPU selection.

Reported-by: Will Deacon <will.deacon@arm.com>
Tested-by: Stephen Warren <swarren@nvidia.com> # Tegra Jetson TK1
Tested-by: Robert Jarzmik <robert.jarzmik@free.fr> # mioa701_bootresume.S
Tested-by: Andrew Lunn <andrew@lunn.ch> # Kirkwood
Tested-by: Shawn Guo <shawn.guo@freescale.com>
Tested-by: Tony Lindgren <tony@atomide.com> # OMAPs
Tested-by: Gregory CLEMENT <gregory.clement@free-electrons.com> # Armada XP, 375, 385
Acked-by: Sekhar Nori <nsekhar@ti.com> # DaVinci
Acked-by: Christoffer Dall <christoffer.dall@linaro.org> # kvm/hyp
Acked-by: Haojian Zhuang <haojian.zhuang@gmail.com> # PXA3xx
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com> # Xen
Tested-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de> # ARMv7M
Tested-by: Simon Horman <horms+renesas@verge.net.au> # Shmobile
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/kernel/entry-common.S

@@ -8,6 +8,7 @@
  * published by the Free Software Foundation.
  */
 
+#include <asm/assembler.h>
 #include <asm/unistd.h>
 #include <asm/ftrace.h>
 #include <asm/unwind.h>
@@ -88,7 +89,7 @@ ENTRY(ret_from_fork)
 	cmp	r5, #0
 	movne	r0, r4
 	adrne	lr, BSYM(1f)
-	movne	pc, r5
+	retne	r5
 1:	get_thread_info tsk
 	b	ret_slow_syscall
 ENDPROC(ret_from_fork)
@@ -290,7 +291,7 @@ ENDPROC(ftrace_graph_caller_old)
 
 .macro mcount_exit
 	ldmia	sp!, {r0-r3, ip, lr}
-	mov	pc, ip
+	ret	ip
 .endm
 
 ENTRY(__gnu_mcount_nc)
@@ -298,7 +299,7 @@ UNWIND(.fnstart)
 #ifdef CONFIG_DYNAMIC_FTRACE
 	mov	ip, lr
 	ldmia	sp!, {lr}
-	mov	pc, ip
+	ret	ip
 #else
 	__mcount
 #endif
@@ -333,12 +334,12 @@ return_to_handler:
 	bl	ftrace_return_to_handler
 	mov	lr, r0			@ r0 has real ret addr
 	ldmia	sp!, {r0-r3}
-	mov	pc, lr
+	ret	lr
 #endif
 
 ENTRY(ftrace_stub)
 .Lftrace_stub:
-	mov	pc, lr
+	ret	lr
 ENDPROC(ftrace_stub)
 
 #endif /* CONFIG_FUNCTION_TRACER */
@@ -561,7 +562,7 @@ sys_mmap2:
 		streq	r5, [sp, #4]
 		beq	sys_mmap_pgoff
 		mov	r0, #-EINVAL
-		mov	pc, lr
+		ret	lr
 #else
 		str	r5, [sp, #4]
 		b	sys_mmap_pgoff