Add "trace-cmd list -s" that will show the systems of events that are available. Basically, just look at all the directories in tracefs/events, besides "ftrace" and list them.
Fix submitted: https://patchwork.kernel.org/patch/11525167/
Fixed by 59a79f0f4f5e ("trace-cmd: Add option "-s" to list subcommand, to display available event systems")