tracing: Deprecate auto-mounting tracefs in debugfs

tracing: Deprecate auto-mounting tracefs in debugfs

In January 2015, tracefs was created to allow access to the tracing
infrastructure without needing to compile in debugfs. When tracefs is
configured, the directory /sys/kernel/tracing will exist and tooling is
expected to use that path to access the tracing infrastructure.

To allow backward compatibility, when debugfs is mounted, it would
automount tracefs in its "tracing" directory so that tooling that had hard
coded /sys/kernel/debug/tracing would still work.

It has been over 10 years since the new interface was introduced, and all
tooling should now be using it. Start the process of deprecating the old
path so that it doesn't need to be maintained anymore.

A new config is added to allow distributions to disable automounting of
tracefs on debugfs.

If /sys/kernel/debug/tracing is accessed, a pr_warn() will trigger stating:

"NOTICE: Automounting of tracing to debugfs is deprecated and will be removed in 2030"

Expect to remove this feature in 5 years (2030).

Cc: <linux-trace-users@vger.kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/20250722170806.40c068c6@gandalf.local.home
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>