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

Tomi Valkeinen
2015-12-07T17:32:42Z

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On 25/08/15 16:45, Daniel Vetter 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.

I think this was never merged. This was part 4 of 4, were there
dependencies or...? Should I apply this for 4.5?

But then... I think my issues with console lock have been later at
runtime, not at register. Maybe we need a module option to disable the
console lock altogether. I wonder how much havoc that might create, though.

Tomi

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