mips: add support to restore exception vector base before booting linux

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 patch adds an option to enable reset exception vector base to its
previous value, or a user configured value before booting linux kernel.

Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>

4 files changed, 54 insertions, 0 deletions

diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index a3ae603..5e20fee 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 88438b9..8b37cc4 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 8c0d767..f1db6d2 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 b8568c0..8fff754 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();
+}