Workaround the Out-of-order Issue of Loongson-2F

From: Wu Zhangjin <[email protected]>

Changes from old version:

o Cleanup some comments and align some instructions.

----

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:

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

o Doing some tricks to the indirect branch target to make sure that the indirect branch target can not be in the I/O region.

This patchset applied the above methods and for the binutils patch[3] have been merged into binutils 2.20.1, so, it's time to upstream this patchset now. without this patch, the machines will hang when the instruction sequence hit the Out-of-order Issue of Loongson-2F, therefore, this patchset is very urgent for both 2.6.33 and 2.6.34.

[1] Chinese Version: http://www.loongson.cn/uploadfile/file/20080821113149.pdf
[2] English Version of Chapter 15:
http://groups.google.com.hk/group/loongson-dev/msg/e0d2e220958f10a6?dmode=source
[3] http://sourceware.org/ml/binutils/2009-11/msg00387.html

Regards,
Wu Zhangjin

Wu Zhangjin (3):
Loongson-2F: Flush the branch target history such as BTB and RAS
Loongson-2F: Enable fixups of binutils 2.20.1
Loongson-2F: Fixup of problems introduced by -mfix-loongson2f-jump of

    binutils 2.20.1

 arch/mips/Makefile                 |    4 +++-
 arch/mips/include/asm/stackframe.h |   19 +++++++++++++++++++
 arch/mips/loongson/common/reset.c  |   11 ++++++++++-
 3 files changed, 32 insertions(+), 2 deletions(-)

From: Wu Zhangjin <[email protected]>

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

---
 arch/mips/include/asm/stackframe.h |   19 +++++++++++++++++++
 1 files changed, 19 insertions(+), 0 deletions(-)

index 3b6da33..52a62f5 100644
@@ -121,6 +121,25 @@
 		.endm
 #else
 		.macro	get_saved_sp	/* Uniprocessor variation */
+#ifdef CONFIG_CPU_LOONGSON2F
+		/*
+		 * Clear BTB (branch target buffer), forbid RAS (row address
+		 * strobe) 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

1.7.0.1

From: Wu Zhangjin <[email protected]>

As the "Fixups of Loongson2F" patch[1] to binutils have been applied into binutils 2.20.1. It's time to enable the options provided by the patch to compile the kernel.

Without these fixups, the system will hang unexpectedly for the bug of processor.

To learn more about these fixups, please refer to the following references.

[1] "Fixups of Loongson2F" patch for binutils(actually for gas)
http://sourceware.org/ml/binutils/2009-11/msg00387.html
[2] Chapter 15 of "Loongson2F User Manual"(Chinese Version)
http://www.loongson.cn/uploadfile/file/200808211
[3] English Version of the above chapter 15
http://groups.google.com.hk/group/loongson-dev/msg/e0d2e220958f10a6?dmode=source

---
 arch/mips/Makefile |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)

index 2f2eac2..5ae342e 100644
@@ -135,7 +135,9 @@ cflags-$(CONFIG_CPU_LOONGSON2)	+= -Wa,--trap
 cflags-$(CONFIG_CPU_LOONGSON2E) += \
 	$(call cc-option,-march=loongson2e,-march=r4600)
 cflags-$(CONFIG_CPU_LOONGSON2F) += \
-	$(call cc-option,-march=loongson2f,-march=r4600)
+	$(call cc-option,-march=loongson2f,-march=r4600) \
+	$(call as-option,-Wa$(comma)-mfix-loongson2f-nop,) \
+	$(call as-option,-Wa$(comma)-mfix-loongson2f-jump,)
 
 cflags-$(CONFIG_CPU_MIPS32_R1)	+= $(call cc-option,-march=mips32,-mips32 -U_MIPS_ISA -D_MIPS_ISA=_MIPS_ISA_MIPS32) \
 			-Wa,-mips32 -Wa,--trap

1.7.0.1

From: Wu Zhangjin <[email protected]>

The -mfix-loongson2f-jump option provided by the binutils 2.20.1 have fixed the Out-of-order Issue of Loongson-2F described in Chapter 15 of "Loongson2F User Manual"[1,2], but introduced some problems.

The option changes all of the jumping target to "addr & 0xcfffffff" through the at($1) register, but for the REBOOT address of loongson-2F: 0xbfc00000, this is totally wrong, so, this patch try to avoid the problem via telling the assembler not to use at($1) register.

[1] Loongson2F User Manual(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

---
 arch/mips/loongson/common/reset.c |   11 ++++++++++-
 1 files changed, 10 insertions(+), 1 deletions(-)

index 4bd9c18..dc311f2 100644
@@ -21,8 +21,17 @@ static void loongson_restart(char *command)
 	/* do preparation for reboot */
 	mach_prepare_reboot();
 
-	/* reboot via jumping to boot base address */
+	/* reboot via jumping to boot base address
+	 *
+	 * ".set noat" and ".set at" are used to ensure the address not break
+	 * by the -mfix-loongson2f-jump option provided by binutils 2.20.1 (or
+	 * higher version) which try to change the jumping address to "addr &
+	 * 0xcfffffff" via the at($1) register, this is totally wrong for
+	 * 0xbfc00000 (LOONGSON_BOOT_BASE).
+	 */
+	__asm__ __volatile__(".set noat\n");
 	((void (*)(void))ioremap_nocache(LOONGSON_BOOT_BASE, 4)) ();
+	__asm__ __volatile__(".set at\n");
 }
 
 static void loongson_poweroff(void)

1.7.0.1