Loongson-2F-Flush-the-branch-target-history-such-as-.patch

From 9dc32aa94ab1857541789641e6b62f0209416b0b Mon Sep 17 00:00:00 2001
From: Wu Zhangjin <wuzhangjin@gmail.com>
Date: Mon, 6 Jul 2009 15:39:03 +0800
Subject: [PATCH] Loongson-2F: Flush the branch target history such as BTB and RAS
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commit 8235687ddfd6618f6257685f17280bf85db17595 in tip.

As the Chapter 15: "Errata: Issue of Out-of-order in loongson"[1] shows, to
workaround the Issue of Loongson-2F，We need to do:

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

This patch did clear BTB(branch target buffer), forbid RAS(row address strobe)
via Loongson-2F's 64bit diagnostic register.

[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>

diff --git a/arch/mips/include/asm/stackframe.h b/arch/mips/include/asm/stackframe.h
index c841912..0f4f73f 100644
--- a/arch/mips/include/asm/stackframe.h
+++ b/arch/mips/include/asm/stackframe.h
@@ -100,6 +100,25 @@
 #endif
 		.macro	get_saved_sp	/* SMP variation */
 		CPU_ID_MFC0	k0, CPU_ID_REG
+#ifdef CONFIG_CPU_LOONGSON2F
+		/*
+		 * Clear BTB(branch target buffer), forbid RAS(row address
+		 * strobe) to workaround the Out-of-oder Issue in Loongson2F
+		 * via it's 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
-- 
1.7.1.1