[PATCH v3] tracing: Ignore mmiotrace from kernel commandline

["Ziqian SUN (Zamir)" <zsun@redhat.com>]

From: "Ziqian SUN (Zamir)" <zsun@redhat.com>

The mmiotrace tracer cannot be enabled with ftrace=mmiotrace in kernel
commandline. With this patch, noboot is added to the tracer struct,
and when system boot with a tracer that has noboot=true, it will print
out a warning message and continue booting.

Signed-off-by: Ziqian SUN (Zamir) <zsun@redhat.com>
--
v1 -> v2 : remove unessential comment
v2 -> v3 : Use tracer struct instead of a separate list to store noboot
---
 kernel/trace/trace.c           | 7 +++++++
 kernel/trace/trace.h           | 2 ++
 kernel/trace/trace_mmiotrace.c | 1 +
 3 files changed, 10 insertions(+)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 5360b7a..48c474f 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -5358,6 +5358,13 @@ static int tracing_set_tracer(struct trace_array *tr, const char *buf)
 	if (t == tr->current_trace)
 		goto out;
 
+	/* Some tracers won't work on kernel command line */
+	if (system_state < SYSTEM_RUNNING && t->noboot) {
+		pr_warn("Tracer '%s' is not allowed on command line, ignored\n",
+			t->name);
+		goto out;
+	}
+
 	/* Some tracers are only allowed for the top level buffer */
 	if (!trace_ok_for_array(t, tr)) {
 		ret = -EINVAL;
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index fb5d54d..652c682 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -444,6 +444,8 @@ struct tracer {
 #ifdef CONFIG_TRACER_MAX_TRACE
 	bool			use_max_tr;
 #endif
+	/* True if tracer cannot be enabled in kernel param */
+	bool			noboot;
 };
 
 
diff --git a/kernel/trace/trace_mmiotrace.c b/kernel/trace/trace_mmiotrace.c
index cd7480d..dca78fc 100644
--- a/kernel/trace/trace_mmiotrace.c
+++ b/kernel/trace/trace_mmiotrace.c
@@ -282,6 +282,7 @@ static enum print_line_t mmio_print_line(struct trace_iterator *iter)
 	.close		= mmio_close,
 	.read		= mmio_read,
 	.print_line	= mmio_print_line,
+	.noboot		= true,
 };
 
 __init static int init_mmio_trace(void)
-- 
1.8.3.1