========================================================================

Because the CPU can read the CNTPCT/CNTVCT registers faster than they
change, performing two reads of the register and comparing the high bits
(like other workarounds) is not a workable solution. And because the
timer can jump both forward and backward, no pair of reads can
distinguish a good value from a bad one. The only way to guarantee a
good value from consecutive reads would be to read _three_ times, and
take the middle value iff the three values are 1) individually unique
and 2) increasing. This takes at minimum 3 cycles (125 ns), or more if
an anomaly is detected.

However, since there is a distinct pattern to the bad values, we can
optimize the common case (2046/2048 of the time) to a single read by
simply ignoring values that match the pattern. This still takes no more
than 3 cycles in the worst case, and requires much less code.

As an additional safety check, limit the loop iteration based on the
number of maximum-frequency CPU cycles in three 24 MHz counter periods.

[1]: https://github.com/armbian/build/commit/a08cd6fe7ae9
[2]: https://forum.armbian.com/topic/3458-a64-datetime-clock-issue/
[3]: https://irclog.whitequark.org/linux-sunxi/2018-01-26

Tested-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Samuel Holland <samuel@sholland.org>
---
 drivers/clocksource/Kconfig          | 11 +++++++++
 drivers/clocksource/arm_arch_timer.c | 43 ++++++++++++++++++++++++++++++++++++
 2 files changed, 54 insertions(+)

diff --git a/drivers/clocksource/Kconfig b/drivers/clocksource/Kconfig
index 8e8a09755d10..7a5d434dd30b 100644
--- a/drivers/clocksource/Kconfig
+++ b/drivers/clocksource/Kconfig
@@ -364,6 +364,17 @@ config ARM64_ERRATUM_858921
 	  The workaround will be dynamically enabled when an affected
 	  core is detected.
 
+config SUN50I_A64_UNSTABLE_TIMER
+	bool "Workaround for Allwinner A64 timer instability"
+	default y
+	depends on ARM_ARCH_TIMER && ARM64 && ARCH_SUNXI
+	select ARM_ARCH_TIMER_OOL_WORKAROUND
+	help
+	  This option enables a workaround for instability in the timer on
+	  the Allwinner A64 SoC. The workaround will only be active if the
+	  allwinner,sun50i-a64-unstable-timer property is found in the
+	  timer node.
+
 config ARM_GLOBAL_TIMER
 	bool "Support for the ARM global timer" if COMPILE_TEST
 	select TIMER_OF if OF
diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
index 57cb2f00fc07..4fba50716bda 100644
--- a/drivers/clocksource/arm_arch_timer.c
+++ b/drivers/clocksource/arm_arch_timer.c
@@ -319,6 +319,40 @@ static u64 notrace arm64_858921_read_cntvct_el0(void)
 }
 #endif
 
+#ifdef CONFIG_SUN50I_A64_UNSTABLE_TIMER
+/*
+ * The low bits of each register can transiently read as all ones or all zeroes
+ * when bit 11 or greater rolls over. Since the value can jump both backward
+ * (7ff -> 000 -> 800) and forward (7ff -> fff -> 800), it is simplest to just
+ * ignore register values with all ones or zeros in the low bits.
+ *
+ * Bound the loop by the worst-case number of CPU cycles that can occur during
+ * three distinct counter periods.
+ */
+#define __sun50i_a64_read_reg(reg) ({					\
+	u64 _val;							\
+	int _retries = 150;						\
+									\
+	do {								\
+		_val = read_sysreg(reg);				\
+		_retries--;						\
+	} while (((_val + 1) & GENMASK(10, 0)) <= 1 && _retries);	\
+									\
+	WARN_ON_ONCE(!_retries);					\
+	_val;								\
+})
+
+static u64 notrace sun50i_a64_read_cntpct_el0(void)
+{
+	return __sun50i_a64_read_reg(cntpct_el0);
+}
+
+static u64 notrace sun50i_a64_read_cntvct_el0(void)
+{
+	return __sun50i_a64_read_reg(cntvct_el0);
+}
+#endif
+
 #ifdef CONFIG_ARM_ARCH_TIMER_OOL_WORKAROUND
 DEFINE_PER_CPU(const struct arch_timer_erratum_workaround *, timer_unstable_counter_workaround);
 EXPORT_SYMBOL_GPL(timer_unstable_counter_workaround);
@@ -408,6 +442,15 @@ static const struct arch_timer_erratum_workaround ool_workarounds[] = {
 		.read_cntvct_el0 = arm64_858921_read_cntvct_el0,
 	},
 #endif
+#ifdef CONFIG_SUN50I_A64_UNSTABLE_TIMER
+	{
+		.match_type = ate_match_dt,
+		.id = "allwinner,sun50i-a64-unstable-timer",
+		.desc = "Allwinner A64 timer instability",
+		.read_cntpct_el0 = sun50i_a64_read_cntpct_el0,
+		.read_cntvct_el0 = sun50i_a64_read_cntvct_el0,
+	},
+#endif
 };
 
 typedef bool (*ate_match_fn_t)(const struct arch_timer_erratum_workaround *,
