Commit c3c296b0 authored Aug 25, 2015 by Daniel Vetter Committed by Tomi Valkeinen Dec 15, 2015

fbdev: Debug knob to register without holding console_lock



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.

Cc: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: linux-fbdev@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>

parent 1c639bae

drivers/video/fbdev/core/fbmem.c

+11 −3

Original line number	Diff line number	Diff line
		@@ -1608,6 +1608,11 @@ static int do_remove_conflicting_framebuffers(struct apertures_struct *a,
		return 0;
		}

		static bool lockless_register_fb;
		module_param_named_unsafe(lockless_register_fb, lockless_register_fb, bool, 0400);
		MODULE_PARM_DESC(lockless_register_fb,
		"Lockless framebuffer registration for debugging [default=off]");

		static int do_register_framebuffer(struct fb_info *fb_info)
		{
		int i, ret;
		@@ -1675,14 +1680,17 @@ static int do_register_framebuffer(struct fb_info *fb_info)
		registered_fb[i] = fb_info;

		event.info = fb_info;
		if (!lockless_register_fb)
		console_lock();
		if (!lock_fb_info(fb_info)) {
		if (!lockless_register_fb)
		console_unlock();
		return -ENODEV;
		}

		fb_notifier_call_chain(FB_EVENT_FB_REGISTERED, &event);
		unlock_fb_info(fb_info);
		if (!lockless_register_fb)
		console_unlock();
		return 0;
		}