linux-kernel - Re: [PATCH] drm/vkms: add missing drm_crtc_vblank_put to the get/put pair on flush

Open Source and information security mailing list archives

Message-ID: <20200728215515.GG6419@phenom.ffwll.local>
Date:   Tue, 28 Jul 2020 23:55:15 +0200
From:   daniel@...ll.ch
To:     unlisted-recipients:; (no To-header on input)
Cc:     Daniel Vetter <daniel@...ll.ch>,
        Melissa Wen <melissa.srw@...il.com>,
        Haneen Mohammed <hamohammed.sa@...il.com>,
        Rodrigo Siqueira <rodrigosiqueiramelo@...il.com>,
        David Airlie <airlied@...ux.ie>,
        Rodrigo Siqueira <Rodrigo.Siqueira@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        kernel-usp@...glegroups.com
Subject: Re: [PATCH] drm/vkms: add missing drm_crtc_vblank_put to the get/put
 pair on flush

On Tue, Jul 28, 2020 at 04:16:34PM +0000, Sidong Yang wrote:
> On Sun, Jul 26, 2020 at 12:26:08PM +0200, Daniel Vetter wrote:
> > On Sat, Jul 25, 2020 at 9:29 PM Melissa Wen <melissa.srw@...il.com> wrote:
> > >
> > > On Sat, Jul 25, 2020 at 4:19 PM Melissa Wen <melissa.srw@...il.com> wrote:
> > > >
> > > > > No, this very first warning continues (only once) :(
> > > > > From here (drm_crtc_vblank_on):
> > > > >         if (atomic_read(&vblank->refcount) != 0 || drm_vblank_offdelay == 0)
> > > > >                 drm_WARN_ON(dev, drm_vblank_enable(dev, pipe));
> > > >
> > > > Sorry, not sure when this warning is triggered.
> > >
> > > Again, I just had to look at the trace:
> > > [   52.299388]  drm_get_last_vbltimestamp+0xaa/0xc0 [drm]
> > > [   52.299389]  drm_reset_vblank_timestamp+0x5b/0xd0 [drm]
> > > [   52.299389]  drm_crtc_vblank_on.cold+0x37/0x103 [drm]
> > > [   52.299390]  drm_atomic_helper_commit_modeset_enable
> > 
> > Yeah I think vkms can't generate a reasonable timestamp when the
> > hrtimer is off. I thought the warning comes from a different
> > callchain, but seems to be a general problem.
> > 
> > I guess in the vkms timestamp function we should check whether the
> > timer is running, and if it's not running, then we just grab the
> > current time and done.
> 
> I tried some test about this scenario that commit_tail calls in sequence disable 
> - enable - commit.
> In a first test. there was a warning and found out that it raised from 
> vkms_get_vblank_timestamp() the code checking vblank_hrtimer's expire time and 
> vblank_time. In first run, vblank_time and hrtimer's expire time was both zero.  
> because vblank wasn't happened yet. this warning wasn't happend since second run 
> that vblank time was set from first run. 
> 
> I don't know it's good way to solve the problem. Is there no problem in other 
> drm modules?

Generally real hw drivers always have working clocks, not like the fake
ones we have here :-) The idea behind the timestamp callback is that when
vblank interrupts aren't enabled, the timestamp will help us keep track of
how many vblanks have happened.

So I think (but might be wrong) correct fix for this issue would be to
check whether vblanks are enabled, and if not, simply pass back the
current system time. That's a lie, but much better than whatever value was
set last time around the hrtimer fired- e.g. similar problem can happen
later on when the vblank interrupt was off for a very long time.
-Daniel

> 
> -Sidong
> 
> > -Daniel
> > 
> > > >
> > > > >
> > > > > > But I'm still wondering why after step 3 we don't get -EINVAL from
> > > > > > vblank_get() - after vblank_off() vblank->enabled should be false
> > > > > > again, getting us back to the same state as after 1. Is that not
> > > > > > happening?
> > > > >
> > > > > Yes (sorry if it got confused), we got -EINVAL after setp 3:
> > > > >
> > > > > In step 3, at the end of the 2nd running, we have:
> > > > > atomic_disable
> > > > > --> vblank_off [!vblank->inmodeset + refcount going 0->1 + inmodeset=1]
> > > > > and then in next vblank_get: -EINVAL (!vblank->enabled + refcount ends 1)
> > > > > as in the first step.
> > > > >
> > > > > Melissa
> > > > >
> > > > > > -Daniel
> > > > > >
> > > > > > >
> > > > > > > > >
> > > > > > > > > Thanks
> > > > > > > > > -Sidong
> > > > > > > > >
> > > > > > > > > > > >
> > > > > > > > > > > > >             crtc->state->event = NULL;
> > > > > > > > > > > > >     }
> > > > > > > > > > > > >
> > > > > > > > > > > > > --
> > > > > > > > > > > > > 2.27.0
> > > > > > > > > > > > >
> > > > > > > > > > > >
> > > > > > > > > > > > --
> > > > > > > > > > > > Daniel Vetter
> > > > > > > > > > > > Software Engineer, Intel Corporation
> > > > > > > > > > > > http://blog.ffwll.ch
> > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > > --
> > > > > > > > > > Daniel Vetter
> > > > > > > > > > Software Engineer, Intel Corporation
> > > > > > > > > > http://blog.ffwll.ch
> > > > > > > > > > _______________________________________________
> > > > > > > > > > dri-devel mailing list
> > > > > > > > > > dri-devel@...ts.freedesktop.org
> > > > > > > > > > https://lists.freedesktop.org/mailman/listinfo/dri-devel
> > > > > > > >
> > > > > > > >
> > > > > > > >
> > > > > > > > --
> > > > > > > > Daniel Vetter
> > > > > > > > Software Engineer, Intel Corporation
> > > > > > > > http://blog.ffwll.ch
> > > > > >
> > > > > >
> > > > > >
> > > > > > --
> > > > > > Daniel Vetter
> > > > > > Software Engineer, Intel Corporation
> > > > > > http://blog.ffwll.ch
> > 
> > 
> > 
> > -- 
> > Daniel Vetter
> > Software Engineer, Intel Corporation
> > http://blog.ffwll.ch

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives