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

[Tomi Valkeinen]

On 25/08/15 22:24, Rob Clark wrote:
> On Tue, Aug 25, 2015 at 9:45 AM, Daniel Vetter <daniel.vetter at ffwll.ch> 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 <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>
> 
> 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..

Any idea if anyone has ever looked at properly fixing this?

 Tomi

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