MIPS: Loongson-2F: Flush the branch target history in BTB and RAS

As per chapter 15 "Errata: Issue of Out-of-order in loongson"[1] to work
around the Loongson 2F erratum we need to do:

"When switching from user mode to kernel mode, you should flush the
branch target history such as BTB and RAS."

[1] Chinese version: http://www.loongson.cn/uploadfile/file/200808211
[2] English version of chapter 15:
    http://groups.google.com.hk/group/loongson-dev/msg/e0d2e220958f10a6?dmode=source

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: Shinya Kuribayashi <shinya.kuribayashi@necel.com>
Patchwork: http://patchwork.linux-mips.org/patch/1066/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

1 files changed, 19 insertions, 0 deletions

diff --git a/arch/mips/include/asm/stackframe.h b/arch/mips/include/asm/stackframe.h
index 3b6da3330e32..c8419129e770 100644
--- a/arch/mips/include/asm/stackframe.h
+++ b/arch/mips/include/asm/stackframe.h
@@ -121,6 +121,25 @@
                 .endm
 #else
                 .macro  get_saved_sp    /* Uniprocessor variation */
+#ifdef CONFIG_CPU_LOONGSON2F
+                /*
+                 * Clear BTB (branch target buffer), forbid RAS (return address
+                 * stack) to workaround the Out-of-order Issue in Loongson2F
+                 * via its diagnostic register.
+                 */
+                move    k0, ra
+                jal     1f
+                 nop
+1:              jal     1f
+                 nop
+1:              jal     1f
+                 nop
+1:              jal     1f
+                 nop
+1:              move    ra, k0
+                li      k0, 3
+                mtc0    k0, $22
+#endif /* CONFIG_CPU_LOONGSON2F */
 #if defined(CONFIG_32BIT) || defined(KBUILD_64BIT_SYM32)
                 lui     k1, %hi(kernelsp)
 #else