linux-kernel - Re: [PATCH] fbdev: wait for references go away

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Message-ID: <20200121055348.s4anrveo2z6avin6@sirius.home.kraxel.org>
Date:   Tue, 21 Jan 2020 06:53:48 +0100
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
Cc:     dri-devel@...ts.freedesktop.org, marmarek@...isiblethingslab.com,
        "open list:FRAMEBUFFER LAYER" <linux-fbdev@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] fbdev: wait for references go away

  Hi,

> > open.  Which can result in drm driver not being able to grab resources
> > (and fail initialization) because the firmware framebuffer still holds
> > them.  Reportedly plymouth can trigger this.
> 
> Could you please describe issue some more?
> 
> I guess that a problem is happening during DRM driver load while fbdev
> driver is loaded? I assume do_unregister_framebuffer() is called inside
> do_remove_conflicting_framebuffers()?

Yes.  Specifically bochs-drm.ko and efifb in virtual machines.

> At first glance it seems to be an user-space issue as it should not be
> holding references on /dev/fb0 while DRM driver is being loaded.

Well, the drm driver is loaded by udev like everything else.

Dunno what plymouth (graphical boot screen tool) does to handle the
situation.  I guess listening to udev events.  So it should notice efifb
going away and drop the /dev/fb0 reference, but this races against
bochs-drm initializing.

> > Fix this by trying to wait until all references are gone.  Don't wait
> > forever though given that userspace might keep the file handle open.
> 
> Where does the 1s maximum delay come from?

Pulled out something out of thin air which I expect being on the safe
side.  plymouth responding on the udev event should need only a small
fraction of that.

cheers,
  Gerd