drm/i915: Hold mode_config->mutex during hotplug processing

drm/i915: Hold mode_config->mutex during hotplug processing

commit a65e34c79c88895766ab1f8a5afa451eed26622b upstream.

Hotplug detection is a mode setting operation and must hold the
struct_mutex or risk colliding with other mode setting operations.

In particular, the display port hotplug function attempts to re-train
the link if the monitor is supposed to be running when plugged back
in. If that happens while mode setting is underway, the link will get
scrambled, leaving it in an inconsistent state.

This is a special case -- usually the driver mode setting entry points
are covered by the upper level DRM code, but in this case the function
is invoked as a work function not under the control of DRM.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>