[PATCH v4] drm/trace: Buffer DRM logs in a ringbuffer accessible via debugfs

[Sean Paul]

On Wed, Jan 15, 2020 at 10:36:36AM +0000, Chris Wilson wrote:
> Quoting Sean Paul (2020-01-14 17:21:43)
> > From: Sean Paul <seanpaul at chromium.org>
> > 
> > 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.

Could you build such a thing with drm_trace underpinning it, just put the
pertinent information in the message?

> 
> 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.

I think that's essentially what we've got, I've just narrowly focused on
surfacing debug logs. If drm_trace_printf were exported, replacing
GEM_TRACE would be as simple as s/trace_printk/drm_trace_printf/. Initially I
thought exporting it to drivers would be a bad idea, but I'm open to changing my
mind on this as long as drivers are using it responsibly. 

Sean

> -Chris

-- 
Sean Paul, Software Engineer, Google / Chromium OS