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

[Chris Wilson]

Quoting Sean Paul (2020-01-15 14:21:18)
> On Wed, Jan 15, 2020 at 02:01:19PM +0000, Chris Wilson wrote:
> > 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.
> > 
> 
> Ah, I think I see what you're getting at now. I think it would be reasonable to
> split out a drm_trace_buffer from the current code for this purpose. We could
> have an interface like:
> 
> struct drm_trace_buffer *drm_trace_buffer_init(unsigned int num_pages);
> int drm_trace_buffer_resize(struct drm_trace_buffer *buf, unsigned int num_pages);
> int drm_trace_buffer_printf(struct drm_trace_buffer *buf, const char *format, ...);
> int drm_trace_buffer_output(struct seq_file *seq);
> void drm_trace_buffer_cleanup(struct drm_trace_buffer *buf);
> 
> Then to Joonas' point, we could have drm_trace_log which uses this interface to
> mirror the logs with a debugfs interface.
> 
> Would that work for your purpose?

The seq_file doesn't marry with the anticipated uAPI, I'll probably need
a raw file_ops (thinking along the lines of return an fd to userspace,
that is read ala /dev/kmsg).

I would be tempted to drop the drm_ and put it straight in lib/
-Chris