[PATCH 2/3] tracing: Add field parsing hist error for hist triggers

[Tom Zanussi]

If the user specifies a nonexistent field for a hist trigger, the
current code correctly flags that as an error, but doesn't tell the
user what happened.

Fix this by invoking hist_err() with an appropriate message when
nonexistent fields are specified.

Before:

  # echo 'hist:keys=pid:ts0=common_timestamp.usecs' >> 
/sys/kernel/debug/tracing/events/sched/sched_switch/trigger
  -su: echo: write error: Invalid argument
  # cat /sys/kernel/debug/tracing/events/sched/sched_switch/hist

After:

  # echo 'hist:keys=pid:ts0=common_timestamp.usecs' >> 
/sys/kernel/debug/tracing/events/sched/sched_switch/trigger
  -su: echo: write error: Invalid argument
  # cat /sys/kernel/debug/tracing/events/sched/sched_switch/hist
  ERROR: Couldn't find field: pid
    Last command: keys=pid:ts0=common_timestamp.usecs

Signed-off-by: Tom Zanussi <tom.zanu...@linux.intel.com>
Reported-by: Masami Hiramatsu <mhira...@kernel.org>
---
 kernel/trace/trace_events_hist.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
index 66c87be..f231fa2 100644
--- a/kernel/trace/trace_events_hist.c
+++ b/kernel/trace/trace_events_hist.c
@@ -2481,6 +2481,7 @@ static struct hist_field *parse_var_ref(struct 
hist_trigger_data *hist_data,
        else {
                field = trace_find_event_field(file->event_call, field_name);
                if (!field || !field->size) {
+                       hist_err("Couldn't find field: ", field_name);
                        field = ERR_PTR(-EINVAL);
                        goto out;
                }
-- 
1.9.3