[Intel-gfx] [PATCH] drm/i915: fix hpd work vs. flush_work in the pageflip code deadlock

[Daniel Vetter]

Historically we've run our own driver hotplug handling in our own
work-queue, which then launched the drm core hotplug handling in the
system workqueue. This is important since we flush our own driver
workqueue in the pageflip code while hodling modeset locks, and only
the drm hotplug code grabbed these locks. But with

commit 69787f7da6b2adc4054357a661aaa1701a9ca76f
Author: Daniel Vetter <daniel.vetter at ffwll.ch>
Date:   Tue Oct 23 18:23:34 2012 +0000

    drm: run the hpd irq event code directly

this was changed and now we could deadlock in our flip handler if
there's a hotplug work blocking the progress of the crucial unpin
works. So this broke the careful deadlock avoidance implemented in

commit b4a98e57fc27854b5938fc8b08b68e5e68b91e1f
Author: Chris Wilson <chris at chris-wilson.co.uk>
Date:   Thu Nov 1 09:26:26 2012 +0000

    drm/i915: Flush outstanding unpin tasks before pageflipping

Since the rule thus far has been that work items on our own workqueue
may never grab modeset locks simply restore that rule again.

v2: Add a comment to the declaration of dev_priv->wq to warn readers
about the tricky implications of using it. Suggested by Chris Wilson.

Cc: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Stuart Abercrombie <sabercrombie at chromium.org>
Reported-by: Stuart Abercrombie <sabercrombie at chromium.org>
References: http://permalink.gmane.org/gmane.comp.freedesktop.xorg.drivers.intel/26239
Cc: stable at vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>
---
 drivers/gpu/drm/i915/i915_drv.h | 7 +++++++
 drivers/gpu/drm/i915/i915_irq.c | 3 +--
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 4da1f86..7bd1370 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -1248,6 +1248,13 @@ typedef struct drm_i915_private {
 
 	unsigned int fsb_freq, mem_freq, is_ddr3;
 
+	/**
+	 * wq - Driver workqueue for GEM.
+	 *
+	 * NOTE: Work items scheduled here are not allowed to grab any modeset
+	 * locks, for otherwise the flushing done in the pageflip code will
+	 * result in deadlocks.
+	 */
 	struct workqueue_struct *wq;
 
 	/* Display functions */
diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
index 4c6853f..39e4dd0 100644
--- a/drivers/gpu/drm/i915/i915_irq.c
+++ b/drivers/gpu/drm/i915/i915_irq.c
@@ -1027,8 +1027,7 @@ static inline void intel_hpd_irq_handler(struct drm_device *dev,
 		dev_priv->display.hpd_irq_setup(dev);
 	spin_unlock(&dev_priv->irq_lock);
 
-	queue_work(dev_priv->wq,
-		   &dev_priv->hotplug_work);
+	schedule_work(&dev_priv->hotplug_work);
 }
 
 static void gmbus_irq_handler(struct drm_device *dev)
-- 
1.8.4.rc3