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

[Sean Paul]

On Wed, Jan 22, 2020 at 3:06 AM Daniel Vetter <daniel at ffwll.ch> wrote:
>
> On Mon, Jan 20, 2020 at 01:56:21PM -0500, Steven Rostedt wrote:
> > On Thu, 16 Jan 2020 07:27:22 +0100
> > Daniel Vetter <daniel at ffwll.ch> wrote:
> >
> > > On Tue, Jan 14, 2020 at 12:21:43PM -0500, Sean Paul wrote:
> > > > 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.
> > >
> > > Hm, since the idea is to ship this in production environments debugfs is
> > > out. sysfs is also kinda the wrong thing, so maybe trying to get this
> > > stuffed into tracefs is actually the way to go?
> > >
> >
> > Why not use normal tracepoints and the tracing infrastructure? You can
> > add your own instance as rasdaemon does, which isn't affected by other
> > tracing. There's code now to even create these instances and enable and
> > disable events from within the kernel.
> >
> >   https://lore.kernel.org/lkml/1574276919-11119-1-git-send-email-divya.indi@oracle.com/
>
> Hm, without looking at the details this indeed seems like the thing we
> want ... Sean?

Ohh indeed, I think we could make this work. Thanks for the pointer, Steven!

The only item that needs sorting is: how does userspace select which
debug events are traced. I think we could solve both with another
module parameter to sit beside drm.debug with the same semantics (call
it drm.trace)?


Sean

> -Daniel
>
> >
> > As this is tracefs, you can mount it without even compiling in debugfs.
> >
> > -- Steve
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch