[PATCH] drm: Don't race connector registration

[Daniel Vetter]

On Tue, Jan 31, 2017 at 04:27:14PM -0800, Dave Hansen wrote:
> I added some printk()s all over and gathered a bit more information
> about what's going on.  It looks like the display doesn't work until the
> drm connector code cleans up the *old* connector.  For some reason, it
> isn't motivated to do that until I go to the console and back.
> 
> In this case, the display was connected to DP-4.
> intel_dp_destroy_mst_connector() got called on it when I switched away,
> but drm_connector_cleanup() did not get called.  Upon switching back
> DP-3/5/6 get created.  One of these *eventually* ends up being
> "enabled", but is not now.  When I switch over to the console,
> drm_connector_cleanup() finally gets called on the old connector: DP-4
> and I can switch back to X and I see one of DP-3/5/6 enabled and working.
> 
> Here are some snippets of dmesg interspersed with what I was doing:

Ok, so the delayed deleting seems to be involved in the bug (and we only
do that since we recently introduced refcounting for hotplugged
connectors). The question is who's getting confused, either kernel or X.
To figure this out, next time things are out of sync, please compare the
output of

$ xrandr

with what's reported in /sys/class/drm/*/status:

$ grep . /sys/class/drm/card0-DP-*/status

Another question: What desktop are you using, and if you unplug a screen,
does that general reconfigure the desktop size to disable that output? The
zombie connector only sticks around as long as someone is still using it
in the screen configuration. As soon as the reconfiguration has happened,
it should go away. You can test this by manually disabling the output when
it's stuck as on:

$ xrandr --output DP-4 --off

That should result in the delayed cleanup happening when you look at dmesg
afterwards.

Thanks, Daniel
> 
> Push DVI switch button to switch to other system:
> 
> > [ 6824.562838] drm_dp_destroy_port() kfree(ffff8801ade46800)
> > [ 6824.563164] drm_dp_destroy_connector_work() port: ffff8801ade42000 connector: ffff8801ade46000
> > [ 6824.563178] intel_dp_destroy_mst_connector() connector: ffff8801ade46000 name: DP-3 &name: ffff8801ade46048 intel_connector: ffff8801ade46000
> > [ 6824.563186] drm_sysfs_connector_remove() connector: ffff8801ade46000 kdev: ffff8801a941b400
> > [ 6824.571556] drm_connector_cleanup(ffff8801ade46000)::329 connector->registered: 0 cpu: 3
> > [ 6824.571570] drm_connector_cleanup() kfree() connector->name: 'DP-3' &name: ffff8801ade46048
> > [ 6824.571581] drm_dp_free_mst_port() kfree port: ffff8801ade42000
> > [ 6824.571587] drm_dp_destroy_connector_work() port: ffff8801ade42800 connector: ffff8801ade47000
> > [ 6824.571594] intel_dp_destroy_mst_connector() connector: ffff8801ade47000 name: DP-4 &name: ffff8801ade47048 intel_connector: ffff8801ade47000
> > [ 6824.571601] drm_sysfs_connector_remove() connector: ffff8801ade47000 kdev: ffff8801a941a000
> > [ 6824.571915] drm_dp_free_mst_port() kfree port: ffff8801ade42800
> > [ 6824.571925] drm_dp_destroy_connector_work() port: ffff8801ade40800 connector: ffff8801ade43000
> > [ 6824.571934] intel_dp_destroy_mst_connector() connector: ffff8801ade43000 name: DP-6 &name: ffff8801ade43048 intel_connector: ffff8801ade43000
> > [ 6824.571943] drm_sysfs_connector_remove() connector: ffff8801ade43000 kdev: ffff8801a9419800
> > [ 6824.572091] drm_connector_cleanup(ffff8801ade43000)::329 connector->registered: 0 cpu: 3
> > [ 6824.572101] drm_connector_cleanup() kfree() connector->name: 'DP-6' &name: ffff8801ade43048
> > [ 6824.572110] drm_dp_free_mst_branch_device() kfree mstb: ffff88030ac22600
> > [ 6824.572117] drm_dp_free_mst_port() kfree port: ffff8801ade40800
> 
> Push button to switch back:
> 
> > [ 6837.349693] drm_connector_init() connector->name: 'DP-3' &name: ffff88040231d848
> > [ 6837.349894] drm_sysfs_connector_add() connector: ffff88040231d800 kdev: ffff8801ae99f400
> > [ 6837.352786] drm_connector_init() connector->name: 'DP-5' &name: ffff880402318048
> > [ 6837.352951] drm_sysfs_connector_add() connector: ffff880402318000 kdev: ffff8801ae99c000
> > [ 6837.353036] drm_connector_init() connector->name: 'DP-6' &name: ffff88040d37f048
> > [ 6837.353154] drm_sysfs_connector_add() connector: ffff88040d37f000 kdev: ffff8801ae99ec00
> 
> I can type into the X session, but both screens are blank.  When I press
> Ctrl-Alt-F2, I get:
> 
> > [ 6850.494310] drm_connector_cleanup(ffff8801ade47000)::329 connector->registered: 0 cpu: 1
> > [ 6850.494314] drm_connector_cleanup() kfree() connector->name: 'DP-4' &name: ffff8801ade47048
> 
> Now I can switch back to X and everything is OK again.

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