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

Daniel Vetter daniel.vetter at ffwll.ch
Tue Aug 25 06:45:14 PDT 2015

When the usual fbcon legacy options are enabled we have
  ->fb notifier chain calls into fbcon
    ->fbcon sets up console on new fbi
        ->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 at jcrosoft.com>
Cc: Tomi Valkeinen <tomi.valkeinen at ti.com>
Cc: linux-fbdev at vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>
 drivers/video/fbdev/core/fbmem.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
index 0705d8883ede..4e73b6f6b1c0 100644
--- a/drivers/video/fbdev/core/fbmem.c
+++ b/drivers/video/fbdev/core/fbmem.c
@@ -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);
+	"Lockless framebuffer registration for debugging [default=off]");
 static int do_register_framebuffer(struct fb_info *fb_info)
 	int i, ret;
@@ -1675,15 +1680,18 @@ static int do_register_framebuffer(struct fb_info *fb_info)
 	registered_fb[i] = fb_info;
 	event.info = fb_info;
-	console_lock();
+	if (!lockless_register_fb)
+		console_lock();
 	if (!lock_fb_info(fb_info)) {
-		console_unlock();
+		if (!lockless_register_fb)
+			console_unlock();
 		return -ENODEV;
 	fb_notifier_call_chain(FB_EVENT_FB_REGISTERED, &event);
-	console_unlock();
+	if (!lockless_register_fb)
+		console_unlock();
 	return 0;

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