[PATCH] drm/i915: Discard previous atomic state on resume if connectors change

[Lyude]

If an MST device is disconnected while the machine is suspended, the
number of connectors will change as well after we call
intel_dp_mst_resume(). This means that any previous atomic state we had
before suspending is no longer valid, since it'll still be pointing to
missing connectors. We need to check for this before committing the
state, otherwise we'll kernel panic on resume whenever if any MST
display was disconnected before we started resuming:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
IP: [<ffffffffa01588ef>] drm_atomic_helper_check_modeset+0x29f/0xb40 [drm_kms_helper]
Call Trace:
 [<ffffffffa02354f4>] intel_atomic_check+0x34/0x1180 [i915]
 [<ffffffff810e6c3f>] ? mark_held_locks+0x6f/0xa0
 [<ffffffff810e6d99>] ? trace_hardirqs_on_caller+0x129/0x1b0
 [<ffffffffa00ff1d2>] drm_atomic_check_only+0x192/0x620 [drm]
 [<ffffffff813ee001>] ? pci_pm_thaw+0x21/0x90
 [<ffffffffa00ff677>] drm_atomic_commit+0x17/0x60 [drm]
 [<ffffffffa023e0ad>] intel_display_resume+0xbd/0x160 [i915]
 [<ffffffff813ee070>] ? pci_pm_thaw+0x90/0x90
 [<ffffffffa01b60d8>] i915_drm_resume+0xd8/0x160 [i915]
 [<ffffffffa01b6185>] i915_pm_resume+0x25/0x30 [i915]
 [<ffffffff813ee0d4>] pci_pm_resume+0x64/0xa0
 [<ffffffff814d9ea0>] dpm_run_callback+0x90/0x190
 [<ffffffff814da455>] device_resume+0xd5/0x1f0
 [<ffffffff814da58d>] async_resume+0x1d/0x50
 [<ffffffff810b6718>] async_run_entry_fn+0x48/0x150
 [<ffffffff810acc19>] process_one_work+0x1e9/0x5c0
 [<ffffffff810acb96>] ? process_one_work+0x166/0x5c0
 [<ffffffff810ad038>] worker_thread+0x48/0x4e0
 [<ffffffff810acff0>] ? process_one_work+0x5c0/0x5c0
 [<ffffffff810b3794>] kthread+0xe4/0x100
 [<ffffffff81742672>] ret_from_fork+0x22/0x50
 [<ffffffff810b36b0>] ? kthread_create_on_node+0x200/0x200

Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Lyude <cpaul@xxxxxxxxxx>
---
 drivers/gpu/drm/i915/intel_display.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 6e0d828..252c06c 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -15945,6 +15945,17 @@ void intel_display_resume(struct drm_device *dev)
 	dev_priv->modeset_restore_state = NULL;
 
 	/*
+	 * With MST, the number of connectors can change between suspend and
+	 * resume, which means that the state we want to restore might now be
+	 * impossible to use since it'll be pointing to non-existant
+	 * connectors.
+	 */
+	if (state->num_connector != dev->mode_config.num_connector) {
+		drm_atomic_state_free(state);
+		state = NULL;
+	}
+
+	/*
 	 * This is a cludge because with real atomic modeset mode_config.mutex
 	 * won't be taken. Unfortunately some probed state like
 	 * audio_codec_enable is still protected by mode_config.mutex, so lock
-- 
2.5.5