MIPS: genex: Don't reload address unnecessarily

In ejtag_debug_handler() we must reload the address of
ejtag_debug_buffer_spinlock if an sc fails, since the address in k0 will
have been clobbered by the result of the sc instruction. In the case
where we simply load a non-zero value (ie. there's contention for the
lock) the address will not be clobbered & we can simply branch back to
repeat the load from memory without reloading the address into k0.

The primary motivation for this change is that it moves the target of
the bnez instruction to an instruction within the LL/SC loop (the LL
itself), which we know contains no other memory accesses & therefore
isn't affected by Loongson3 LL/SC errata.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org
Cc: Huacai Chen <chenhc@lemote.com>
Cc: Jiaxun Yang <jiaxun.yang@flygoat.com>
Cc: linux-kernel@vger.kernel.org

arch/mips/kernel/genex.S

@@ -355,8 +355,8 @@ NESTED(ejtag_debug_handler, PT_SIZE, sp)
 #ifdef CONFIG_SMP
 1:	PTR_LA	k0, ejtag_debug_buffer_spinlock
 	__SYNC(full, loongson3_war)
-	ll	k0, 0(k0)
-	bnez	k0, 1b
+2:	ll	k0, 0(k0)
+	bnez	k0, 2b
 	PTR_LA	k0, ejtag_debug_buffer_spinlock
 	sc	k0, 0(k0)
 	beqz	k0, 1b