[3.13.y-ckt stable] Patch "drm/i915: Don't call intel_prepare_page_flip() multiple times on gen2-4" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    drm/i915: Don't call intel_prepare_page_flip() multiple times on gen2-4

to the linux-3.13.y-queue branch of the 3.13.y-ckt extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.13.y-queue

This patch is scheduled to be released in version 3.13.11-ckt16.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.13.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From 85e86c51638ac5d48f15135472d8bb372647b137 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ville=20Syrj=C3=A4l=C3=A4?= <ville.syrjala at linux.intel.com>
Date: Wed, 17 Dec 2014 23:08:03 +0200
Subject: drm/i915: Don't call intel_prepare_page_flip() multiple times on
 gen2-4
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

commit 7d47559ee84b3ac206aa9e675606fafcd7c0b500 upstream.

The flip stall detector kicks in when pending>=INTEL_FLIP_COMPLETE. That
means if we first call intel_prepare_page_flip() but don't call
intel_finish_page_flip(), the next stall check will erroneosly think
the page flip was somehow stuck.

With enough debug spew emitted from the interrupt handler my 830 hangs
when this happens. My theory is that the previous vblank interrupt gets
sufficiently delayed that the handler will see the pending bit set in
IIR, but ISR still has the bit set as well (ie. the flip was processed
by CS but didn't complete yet). In this case the handler will proceed
to call intel_check_page_flip() immediately after
intel_prepare_page_flip(). It then tries to print a backtrace for the
stuck flip WARN, which apparetly results in way too much debug spew
delaying interrupt processing further. That then seems to cause an
endless loop in the interrupt handler, and the machine is dead until
the watchdog kicks in and reboots. At least limiting the number of
iterations of the loop in the interrupt handler also prevented the
hang.

So it seems better to not call intel_prepare_page_flip() without
immediately calling intel_finish_page_flip(). The IIR/ISR trickery
avoids races here so this is a perfectly safe thing to do.

v2: Fix typo in commit message (checkpatch)

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=88381
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=85888
Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula at intel.com>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 drivers/gpu/drm/i915/i915_irq.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
index 5e0b5e4..d72d737 100644
--- a/drivers/gpu/drm/i915/i915_irq.c
+++ b/drivers/gpu/drm/i915/i915_irq.c
@@ -3166,8 +3166,6 @@ static bool i8xx_handle_vblank(struct drm_device *dev,
 	if ((iir & flip_pending) == 0)
 		return false;

-	intel_prepare_page_flip(dev, plane);
-
 	/* We detect FlipDone by looking for the change in PendingFlip from '1'
 	 * to '0' on the following vblank, i.e. IIR has the Pendingflip
 	 * asserted following the MI_DISPLAY_FLIP, but ISR is deasserted, hence
@@ -3177,6 +3175,7 @@ static bool i8xx_handle_vblank(struct drm_device *dev,
 	if (I915_READ16(ISR) & flip_pending)
 		return false;

+	intel_prepare_page_flip(dev, plane);
 	intel_finish_page_flip(dev, pipe);

 	return true;
@@ -3353,8 +3352,6 @@ static bool i915_handle_vblank(struct drm_device *dev,
 	if ((iir & flip_pending) == 0)
 		return false;

-	intel_prepare_page_flip(dev, plane);
-
 	/* We detect FlipDone by looking for the change in PendingFlip from '1'
 	 * to '0' on the following vblank, i.e. IIR has the Pendingflip
 	 * asserted following the MI_DISPLAY_FLIP, but ISR is deasserted, hence
@@ -3364,6 +3361,7 @@ static bool i915_handle_vblank(struct drm_device *dev,
 	if (I915_READ(ISR) & flip_pending)
 		return false;

+	intel_prepare_page_flip(dev, plane);
 	intel_finish_page_flip(dev, pipe);

 	return true;
--
1.9.1