[3.16.y-ckt stable] Patch "drm/i915: Limit the number of loops for reading a split 64bit register" has been added to staging queue

Luis Henriques luis.henriques at canonical.com
Mon Sep 28 17:31:08 UTC 2015


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

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

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.16.y-queue

This patch is scheduled to be released in version 3.16.7-ckt18.

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

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From d669338106431a853a98e88eb662d531df9da269 Mon Sep 17 00:00:00 2001
From: Chris Wilson <chris at 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

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 at ya.ru
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=91906
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: MichaƂ Winiarski <michal.winiarski at intel.com>
Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
Cc: Jani Nikula <jani.nikula at linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula at intel.com>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 drivers/gpu/drm/i915/i915_drv.h | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 67f72fa5a769..1378dadc618e 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -2682,13 +2682,13 @@ int vlv_freq_opcode(struct drm_i915_private *dev_priv, int val);
 #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)




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