[Intel-gfx] [PATCH 4/4] fbdev: Debug knob to register without holding console_lock

Daniel Vetter daniel at ffwll.ch
Tue Dec 8 00:19:16 PST 2015

On Mon, Dec 07, 2015 at 07:32:42PM +0200, Tomi Valkeinen wrote:
> On 25/08/15 16:45, Daniel Vetter wrote:
> > When the usual fbcon legacy options are enabled we have
> > ->register_framebuffer
> >   ->fb notifier chain calls into fbcon
> >     ->fbcon sets up console on new fbi
> >       ->fbi->set_par
> >         ->drm_fb_helper_set_par exercises full kms api
> > 
> > And because of locking inversion hilarity all of register_framebuffer
> > is done with the console lock held. Which means that the first time on
> > driver load we exercise _all_ the kms code (all probe paths and
> > modeset paths for everything connected) is under the console lock.
> > That means if anything goes belly-up in that big pile of code nothing
> > ever reaches logfiles (and the machine is dead).
> > 
> > Usual tactic to debug that is to temporarily remove those console_lock
> > calls to be able to capture backtraces. I'm fed up writing this patch
> > and recompiling kernels. Hence this patch here to add an unsafe,
> > kernel-taining option to do this at runtime.
> I think this was never merged. This was part 4 of 4, were there
> dependencies or...? Should I apply this for 4.5?

Patches 1-3 have all already landed in drm, and patch 4 is free standing.
Would be great if you can pull it in.
> But then... I think my issues with console lock have been later at
> runtime, not at register. Maybe we need a module option to disable the
> console lock altogether. I wonder how much havoc that might create, though.

Hm, where in fbdev do you hold the console_lock outside of
registering/unregistering an fbdev (because of fbcon)? There's of course
general trouble with console_lock deadlocks and fun like that, but ime
that all got a lot more manageable since I added lockdep annotations to
Daniel Vetter
Software Engineer, Intel Corporation

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