MIPS: Extract schedule_mfi info from __schedule

schedule_mfi is supposed to be extracted from schedule(), and
is used in thread_saved_pc and get_wchan.

But, after optimization, schedule() is reduced to a sibling
call to __schedule(), and no real frame info can be extracted.

One solution is to compile schedule() with -fno-omit-frame-pointer
and -fno-optimize-sibling-calls, but that will incur performance
degradation.

Another solution is to extract info from the real scheduler,
__schedule, and this is the approache adopted here.

This patch reads the __schedule address by either following
the 'j' call in schedule if KALLSYMS is disabled or by using
kallsyms_lookup_name to lookup __schedule if KALLSYMS is
available, then, extracts schedule_mfi from __schedule frame info.

This patch also fixes the "Can't analyze schedule() prologue"
warning at boot time.

Signed-off-by: Tony Wu <tung7970@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5237/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/process.c

@@ -224,6 +224,9 @@ struct mips_frame_info {
 	int		pc_offset;
 };
 
+#define J_TARGET(pc,target)	\
+		(((unsigned long)(pc) & 0xf0000000) | ((target) << 2))
+
 static inline int is_ra_save_ins(union mips_instruction *ip)
 {
 #ifdef CONFIG_CPU_MICROMIPS
@@ -395,15 +398,42 @@ err:
 
 static struct mips_frame_info schedule_mfi __read_mostly;
 
+#ifdef CONFIG_KALLSYMS
+static unsigned long get___schedule_addr(void)
+{
+	return kallsyms_lookup_name("__schedule");
+}
+#else
+static unsigned long get___schedule_addr(void)
+{
+	union mips_instruction *ip = (void *)schedule;
+	int max_insns = 8;
+	int i;
+
+	for (i = 0; i < max_insns; i++, ip++) {
+		if (ip->j_format.opcode == j_op)
+			return J_TARGET(ip, ip->j_format.target);
+	}
+	return 0;
+}
+#endif
+
 static int __init frame_info_init(void)
 {
 	unsigned long size = 0;
 #ifdef CONFIG_KALLSYMS
 	unsigned long ofs;
+#endif
+	unsigned long addr;
 
-	kallsyms_lookup_size_offset((unsigned long)schedule, &size, &ofs);
+	addr = get___schedule_addr();
+	if (!addr)
+		addr = (unsigned long)schedule;
+
+#ifdef CONFIG_KALLSYMS
+	kallsyms_lookup_size_offset(addr, &size, &ofs);
 #endif
-	schedule_mfi.func = schedule;
+	schedule_mfi.func = (void *)addr;
 	schedule_mfi.func_size = size;
 
 	get_frame_info(&schedule_mfi);