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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>
This commit is contained in:
Daniel Vetter 2015-08-25 15:45:14 +02:00 committed by Tomi Valkeinen
parent 1c639baeaf
commit c3c296b069
1 changed files with 11 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

14
drivers/video/fbdev/core/fbmem.c
View File

@ -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;
@ -1675,15 +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);
unlock_fb_info(fb_info);
console_unlock();
if (!lockless_register_fb)
console_unlock();
return 0;
}