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

Tomi Valkeinen
2015-09-01T14:41:48Z

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On 01/09/15 17:34, Rob Clark wrote:
> On Tue, Sep 1, 2015 at 6:32 AM, Tomi Valkeinen <tomi.valkeinen@ti.com> wrote:
>>
>>
>> On 25/08/15 22:24, Rob Clark wrote:
>>> On Tue, Aug 25, 2015 at 9:45 AM, Daniel Vetter <daniel.vetter@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@jcrosoft.com>
>>>> Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
>>>> Cc: linux-fbdev@vger.kernel.org
>>>> Signed-off-by: Daniel Vetter <daniel.vetter@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?
>
> I hadn't had a chance to look at it further yet.. I think Daniel
> claimed it worked for him, but he was probably on intel-next, where I
> was on drm-next at the time which seemed to be having some unrelated
> i915 issues (when I was trying to debug atomic fb-helper patches). So
> can't really say that the issue I had was actually related to this
> patch. I'll try again later this week or next, when hopefully i915 in
> drm-next is in better shape..

Oh, I didn't mean this patch, but the whole console lock in general.
I've also banged my head to a wall because of it =).

Tomi

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