On Tue, Aug 25, 2015 at 9:45 AM, Daniel Vetter [off-list ref] 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.
Cc: Jean-Christophe Plagniol-Villard <redacted>
Cc: Tomi Valkeinen <redacted>
Cc: <redacted>
Signed-off-by: Daniel Vetter <redacted>
This one was causing me some problems, if I tried to enable
lockless_register_fb. It *looks* like it should work, so I'm not
quite sure what the deal is. But I'm 110% fan of getting something
like this working, because console_lock is pretty much the bane of kms
developer's existence..
I'll have to debug further on a system where I can see more than the
bottom three lines of the second to last backtrace..
BR,
-R
quoted hunk ↗ jump to hunk
---
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);
+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;
}
--1.8.3.1
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