linux-kernel - Re: [PATCH] drm: assure aux_dev is nonzero before using it

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Date:   Tue, 18 Aug 2020 10:58:01 -0700 (PDT)
From:   Zwane Mwaikambo <zwanem@...il.com>
To:     Lyude Paul <lyude@...hat.com>
cc:     Daniel Vetter <daniel@...ll.ch>, tcamuso@...hat.com,
        dkwon@...hat.com, Linux Kernel <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH] drm: assure aux_dev is nonzero before using it

On Wed, 12 Aug 2020, Lyude Paul wrote:

> On Wed, 2020-08-12 at 16:10 +0200, Daniel Vetter wrote:
> > On Wed, Aug 12, 2020 at 12:16 AM Zwane Mwaikambo <zwanem@...il.com> wrote:
> > > On Tue, 11 Aug 2020, Daniel Vetter wrote:
> > > 
> > > > On Mon, Aug 10, 2020 at 10:11:50AM -0700, Zwane Mwaikambo wrote:
> > > > > Hi Folks,
> > > > >     I know this thread eventually dropped off due to not identifying
> > > > > the underlying issue. It's still occuring on 5.8 and in my case it
> > > > > happened because the udev device nodes for the DP aux devices were not
> > > > > cleaned up whereas the kernel had no association with them. I can
> > > > > reproduce the bug just by creating a device node for a non-existent
> > > > > minor
> > > > > device and calling open().
> > > > 
> > > > Hm I don't have that thread anymore, but generally these bugs are solved
> > > > by not registering the device before it's ready for use. We do have
> > > > drm_connector->late_register for that stuff. Just a guess since I'm not
> > > > seeing full details here.
> > > 
> > > In this particular case, the physical device disappeared before the nodes
> > > were cleaned up. It involves putting a computer to sleep with a monitor
> > > plugged in and then waking it up with the monitor unplugged.
> > 
> > We also have early_unregister for the reverse, but yes this sounds
> > more tricky ... Adding Lyude who's been working on way too much
> > lifetime fun around dp recently.
> > -Daniel
> > 
> Hi-I think just checking whether the auxdev is NULL or not is a reasonable
> fix, although I am curious as to how exactly the aux dev's parent is getting
> destroyed before it's child, which I would have thought would be the only way
> you could hit this?

Hi, If this is acceptable, would you consider an updated patch against 
5.8?

Thanks,
	Zwane