[ARM] 3309/1: disable the pre-ARMv5 NPTL kernel helper in the non MMU case

Patch from Nicolas Pitre

The cmpxchg emulation on pre-ARMv5 relies on user code executed from a
kernel address.  If the operation cannot complete atomically, it is
aborted from the usr_entry macro by clearing the Z flag.  This clearing
of the Z flag is done whenever the user pc is above TASK_SIZE.

However this "pc >= TASK_SIZE" test cannot work in the non MMU case.
Worse: the current code will corrupt the Z flag on every entry to the
kernel.

Let's disable it in the non MMU case for now.  Using NPTL on non MMU
targets needs to be worked out anyway.
Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/kernel/entry-armv.S

@@ -333,9 +333,13 @@ __pabt_svc:
 					@ from the exception stack
 
 #if __LINUX_ARM_ARCH__ < 6 && !defined(CONFIG_NEEDS_SYSCALL_FOR_CMPXCHG)
+#ifndef CONFIG_MMU
+#warning "NPTL on non MMU needs fixing"
+#else
 	@ make sure our user space atomic helper is aborted
 	cmp	r2, #TASK_SIZE
 	bichs	r3, r3, #PSR_Z_BIT
+#endif
 #endif
 
 	@
@@ -756,12 +760,18 @@ __kuser_cmpxchg:				@ 0xffff0fc0
 	 * exception happening just after the str instruction which would
 	 * clear the Z flag although the exchange was done.
 	 */
+#ifdef CONFIG_MMU
 	teq	ip, ip			@ set Z flag
 	ldr	ip, [r2]		@ load current val
 	add	r3, r2, #1		@ prepare store ptr
 	teqeq	ip, r0			@ compare with oldval if still allowed
 	streq	r1, [r3, #-1]!		@ store newval if still allowed
 	subs	r0, r2, r3		@ if r2 == r3 the str occured
+#else
+#warning "NPTL on non MMU needs fixing"
+	mov	r0, #-1
+	adds	r0, r0, #0
+#endif
 	mov	pc, lr
 
 #else