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

[Sean Paul <sean@poorly.run>]

On Wed, Jan 15, 2020 at 05:38:13PM +0000, Chris Wilson wrote:
> 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@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).

Agree, that should have been 

struct file_operations *drm_trace_buffer_file_ops(struct drm_trace_buffer *buf);

or something like that..

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

I think if we wanted to share this more broadly, we'd probably look at adding
it in kernel/trace/ and enabling subsystems to add their own traces to tracefs.

Sean


> -Chris

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