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

commit acd29f7b 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=91906Signed-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: Luis Henriques <luis.henriques@canonical.com>

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)