Implement perf_callchain_user for o32 ABI (on mipsel)

index a824485..623d63a 100644
@@ -536,12 +536,105 @@ handle_associated_event(struct cpu_hw_events *cpuc,
 /* Callchain handling code. */
 
 /*
+ * Userspace code
+ */
+
+/*
  * Leave userspace callchain empty for now. When we find a way to trace
  * the user stack callchains, we add here.
  */
 void perf_callchain_user(struct perf_callchain_entry *entry,
 		    struct pt_regs *regs)
 {
+#define ADDIU_SP_INSTR		0x27bd0000
+#define SW_RA_INSTR		0xafbf0000
+	/*
+	 * Find the first RA and FrameSize.
+	 */
+	unsigned long ra_addr = regs->regs[31];
+	unsigned long sp_addr = regs->regs[29];
+	unsigned long pc_addr = regs->cp0_epc;
+	unsigned long ra = 0;
+	unsigned int limit;
+	size_t stack_size = 0;
+	size_t ra_offset = 0;
+
+	/*
+	 * Try to find the initial stack size and the return address. It
+	 * is possible that the code is still in the prologue and the
+	 * return address might not have been stored to the stack yet
+	 */
+	for (limit = 0, pc_addr = regs->cp0_epc;
+	     limit < (PAGE_SIZE / 4) && (!ra_offset || !stack_size);
+	     --pc_addr, ++limit) {
+		int instr;
+		if (!access_ok(VERIFY_READ, pc_addr, sizeof(instr)))
+			return;
+		if (__copy_from_user_inatomic(&instr, (void *) pc_addr, sizeof(instr)))
+			return;
+		switch (instr & 0xffff0000) {
+		case ADDIU_SP_INSTR:
+			stack_size = abs((short)(instr & 0xffff));
+			goto __out_of_loop;
+			break;
+		case SW_RA_INSTR:
+			ra_offset = instr & 0xffff;
+			break;
+		}
+	}
+
+__out_of_loop:
+	if (stack_size == 0)
+		return;
+
+	if (ra_offset) {
+		ra_addr = sp_addr + ra_offset;
+		if (!access_ok(VERIFY_READ, ra_addr, sizeof(ra)))
+			return;
+		if (__copy_from_user_inatomic(&ra, (void *) ra_addr, sizeof(ra)))
+			return;
+	}
+
+	sp_addr = sp_addr + stack_size;
+
+
+	/* now try to walk from the return address we found */
+	limit = 0;
+	while (entry->nr < PERF_MAX_STACK_DEPTH && ra != 0) {
+		perf_callchain_store(entry, ra);
+		ra_offset = stack_size = 0;
+
+		for (pc_addr = ra;
+		     (!ra_offset || !stack_size) && limit < PAGE_SIZE/4/2;
+		     --pc_addr, ++limit) {
+			int instr;
+			if (!access_ok(VERIFY_READ, pc_addr, sizeof(instr)))
+				return;
+			if (__copy_from_user_inatomic(&instr, (void *) pc_addr, sizeof(instr)))
+				return;
+			switch (instr & 0xffff0000) {
+			case ADDIU_SP_INSTR:
+				stack_size = abs((short)(instr & 0xffff));
+				break;
+			case SW_RA_INSTR:
+				ra_offset = instr & 0xffff;
+				break;
+			}
+		}
+
+		/* must have hit a limit */
+		if (!ra_offset || !stack_size)
+			return;
+
+		ra_addr = sp_addr + ra_offset;
+		if (!access_ok(VERIFY_READ, ra_addr, sizeof(ra)))
+			return;
+		if (__copy_from_user_inatomic(&ra, (void *) ra_addr, sizeof(ra)))
+			return;
+		sp_addr = sp_addr + stack_size;
+	}
+
+	return;
 }
 
 static void save_raw_perf_callchain(struct perf_callchain_entry *entry,