On Mon, Dec 07, 2015 at 07:32:42PM +0200, Tomi Valkeinen wrote:

On 25/08/15 16:45, Daniel Vetter wrote:

quoted

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
console_lock.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch