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

[Chris Wilson]

Quoting Sean Paul (2020-01-15 13:41:58)
> 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?

Not as is. The global has to go, and there's no use for debugfs. So we
are just left with a sprintf() around a ring_buffer. I am left in the
same position as just wanting to generalise tracek to take the ringbuffer
as a parameter.

> > 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. 

I definitely can't make the mistake of flooding kms tracing with
overwhelming execution traces -- we can't go back to mixing kms traces
with execution traces.
-Chris