Quoting Sean Paul (2020-01-14 17:21:43)

From: Sean Paul <redacted>

This patch uses a ring_buffer to keep a "flight recorder" (name credit Weston)
of DRM logs for a specified set of debug categories. The user writes a
bitmask of debug categories to the "trace_mask" node and can read log
messages from the "trace" node.

These nodes currently exist in debugfs under the dri directory. I
intended on exposing all of this through tracefs originally, but the
tracefs entry points are not exposed, so there's no way to create
tracefs files from drivers at the moment. I think it would be a
worthwhile endeavour, but one requiring more time and conversation to
ensure the drm traces fit somewhere sensible.

Fwiw, I have a need for client orientated debug message store, with
the primary purpose of figuring out -EINVAL. We need per-client so we can
put sensitive information about the potentially buggy client behaviour,
and of course it needs to be accessible by the non-privileged client.

On the execution side, it's easy to keep track of the client so we could
trace execution flow per client, within reason. And we could do
similarly for kms clients.

Just chiming to say, I don't think a duplicate of dmesg hidden inside
debugfs achieves much. But a generic tracek-esque ringbuf would be very
useful -- even if only so we can separate our GEM_TRACE from the global
tracek.
-Chris