[01/26,v7] mips: add support to restore exception vector base before booting linux

diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index a3ae603044..5e20feeefb 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -287,6 +287,36 @@  config MIPS_RELOCATION_TABLE_SIZE
 
 	  If unsure, leave at the default value.
 
+config RESTORE_EXCEPTION_VECTOR_BASE
+	bool "Restore exception vector base before booting linux kernel"
+	default n
+	help
+	  In U-Boot the exception vector base will be moved to top of memory,
+	  to be used to display register dump when exception occurs.
+	  But some old linux kernel does not honor the base set in CP0_EBASE.
+	  A modified exception vector base will cause kernel crash.
+
+	  This option will restore the exception vector base to its previous
+	  value.
+
+	  If unsure, say N.
+
+config OVERRIDE_EXCEPTION_VECTOR_BASE
+	bool "Override the exception vector base to be restored"
+	depends on RESTORE_EXCEPTION_VECTOR_BASE
+	default n
+	help
+	  Enable this option if you want to use a different exception vector
+	  base rather than the previously saved one.
+
+config NEW_EXCEPTION_VECTOR_BASE
+	hex "New exception vector base"
+	depends on OVERRIDE_EXCEPTION_VECTOR_BASE
+	range 0x80000000 0xbffff000
+	default 0x80000000
+	help
+	  The exception vector base to be restored before booting linux kernel
+
 endmenu
 
 menu "OS boot interface"
diff --git a/arch/mips/include/asm/u-boot-mips.h b/arch/mips/include/asm/u-boot-mips.h
index 88438b9576..8b37cc4029 100644
--- a/arch/mips/include/asm/u-boot-mips.h
+++ b/arch/mips/include/asm/u-boot-mips.h
@@ -9,4 +9,6 @@  void except_vec_ejtag_debug(void);
 
 int arch_misc_init(void);
 
+void trap_restore(void);
+
 #endif /* _U_BOOT_MIPS_H_ */
diff --git a/arch/mips/lib/bootm.c b/arch/mips/lib/bootm.c
index 8c0d7672f2..f1db6d23b8 100644
--- a/arch/mips/lib/bootm.c
+++ b/arch/mips/lib/bootm.c
@@ -294,6 +294,9 @@  static void boot_jump_linux(bootm_headers_t *images)
 	bootstage_report();
 #endif
 
+	if (CONFIG_IS_ENABLED(RESTORE_EXCEPTION_VECTOR_BASE))
+		trap_restore();
+
 	if (images->ft_len)
 		kernel(-2, (ulong)images->ft_addr, 0, 0);
 	else
diff --git a/arch/mips/lib/traps.c b/arch/mips/lib/traps.c
index b8568c00fe..8fff7541e3 100644
--- a/arch/mips/lib/traps.c
+++ b/arch/mips/lib/traps.c
@@ -20,6 +20,8 @@ 
 
 DECLARE_GLOBAL_DATA_PTR;
 
+static unsigned long saved_ebase;
+
 static void show_regs(const struct pt_regs *regs)
 {
 	const int field = 2 * sizeof(unsigned long);
@@ -102,7 +104,24 @@  void trap_init(ulong reloc_addr)
 	set_handler(0x180, &except_vec3_generic, 0x80);
 	set_handler(0x280, &except_vec_ejtag_debug, 0x80);
 
+	saved_ebase = read_c0_ebase() & 0xfffff000;
+
 	write_c0_ebase(ebase);
 	clear_c0_status(ST0_BEV);
 	execution_hazard_barrier();
 }
+
+void trap_restore(void)
+{
+	set_c0_status(ST0_BEV);
+	execution_hazard_barrier();
+
+#ifdef CONFIG_OVERRIDE_EXCEPTION_VECTOR_BASE
+	write_c0_ebase(CONFIG_NEW_EXCEPTION_VECTOR_BASE & 0xfffff000);
+#else
+	write_c0_ebase(saved_ebase);
+#endif
+
+	clear_c0_status(ST0_BEV);
+	execution_hazard_barrier();
+}