Patch "drm/i915: Limit the number of loops for reading a split 64bit register" has been added to the 4.2-stable tree

Patch "drm/i915: Limit the number of loops for reading a split 64bit register" has been added to the 4.2-stable tree

[gregkh]

This is a note to let you know that I've just added the patch titled

    drm/i915: Limit the number of loops for reading a split 64bit register

to the 4.2-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     drm-i915-limit-the-number-of-loops-for-reading-a-split-64bit-register.patch
and it can be found in the queue-4.2 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From acd29f7b22262d9e848393b9b6ae13eb42d22514 Mon Sep 17 00:00:00 2001
From: Chris Wilson <chris@chris-wilson.co.uk>
Date: Tue, 8 Sep 2015 14:17:13 +0100
Subject: drm/i915: Limit the number of loops for reading a split 64bit register
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

From: Chris Wilson <chris@chris-wilson.co.uk>

commit acd29f7b22262d9e848393b9b6ae13eb42d22514 upstream.

In I915_READ64_2x32 we attempt to read a 64bit register using 2 32bit
reads. Due to the nature of the registers we try to read in this manner,
they may increment between the two instruction (e.g. a timestamp
counter). To keep the result accurate, we repeat the read if we detect
an overflow (i.e. the upper value varies). However, some hardware is just
plain flaky and may endless loop as the the upper 32bits are not stable.
Just give up after a couple of tries and report whatever we read last.

v2: Use the most recent values when erring out on an unstable register.

Reported-by: russianneuromancer@ya.ru
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=91906
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 drivers/gpu/drm/i915/i915_drv.h |   10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -3303,13 +3303,13 @@ int intel_freq_opcode(struct drm_i915_pr
 #define I915_READ64(reg)	dev_priv->uncore.funcs.mmio_readq(dev_priv, (reg), true)
 
 #define I915_READ64_2x32(lower_reg, upper_reg) ({			\
-	u32 upper, lower, tmp;						\
-	tmp = I915_READ(upper_reg);					\
+	u32 upper, lower, old_upper, loop = 0;				\
+	upper = I915_READ(upper_reg);					\
 	do {								\
-		upper = tmp;						\
+		old_upper = upper;					\
 		lower = I915_READ(lower_reg);				\
-		tmp = I915_READ(upper_reg);				\
-	} while (upper != tmp);						\
+		upper = I915_READ(upper_reg);				\
+	} while (upper != old_upper && loop++ < 2);			\
 	(u64)upper << 32 | lower; })
 
 #define POSTING_READ(reg)	(void)I915_READ_NOTRACE(reg)


Patches currently in stable-queue which might be from chris@chris-wilson.co.uk are

queue-4.2/drm-i915-always-mark-the-object-as-dirty-when-used-by-the-gpu.patch
queue-4.2/drm-i915-limit-the-number-of-loops-for-reading-a-split-64bit-register.patch