[PATCH 1/2] arm64: arch_timer: Workaround for Allwinner A64 timer instability

[Samuel Holland]

The Allwinner A64 SoC is known [1] to have an unstable architectural
timer, which manifests itself most obviously in the time jumping forward
a multiple of 95 years [2][3]. This coincides with 2^56 cycles at a
timer frequency of 24 MHz, implying that the time went slightly backward
(and this was interpreted by the kernel as it jumping forward and
wrapping around past the epoch).

Further investigation revealed instability in the low bits of CNTVCT at
the point a high bit rolls over. This leads to power-of-two cycle
forward and backward jumps. (Testing shows that forward jumps are about
twice as likely as backward jumps.)

Without trapping reads to CNTVCT, a userspace program is able to read it
in a loop faster than it changes. A test program running on all 4 CPU
cores that reported jumps larger than 100 ms was run for 13.6 hours and
reported the following:

 Count | Event
-------+---------------------------
  9940 | jumped backward      699ms
   268 | jumped backward     1398ms
     1 | jumped backward     2097ms
 16020 | jumped forward       175ms
  6443 | jumped forward       699ms
  2976 | jumped forward      1398ms
     9 | jumped forward    356516ms
     9 | jumped forward    357215ms
     4 | jumped forward    714430ms
     1 | jumped forward   3578440ms

This works out to a jump larger than 100 ms about every 5.5 seconds on
each CPU core.

The largest jump (almost an hour!) was the following sequence of reads:
      0x0000007fffffffff → 0x00000093feffffff → 0x0000008000000000

Note that the middle bits don't necessarily all read as all zeroes or
all ones during the anomalous behavior; however the low 11 bits checked
by the function in this patch have never been observed with any other
value.

Also note that smaller jumps are much more common, with the smallest
backward jumps of 2048 cycles observed over 400 times per second on each
core. (Of course, this is partially due to lower bits rolling over more
frequently.) Any one of these could have caused the 95 year time skip.

Similar anomalies were observed while reading CNTPCT (after patching the
kernel to allow reads from userspace). However, the jumps are much less
frequent, and only small jumps were observed. The same program as before
(except now reading CNTPCT) observed after 72 hours:

 Count | Event
-------+---------------------------
    17 | jumped backward      699ms
    52 | jumped forward       175ms
  2831 | jumped forward       699ms
     5 | jumped forward      1398ms

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

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.

[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

Signed-off-by: Samuel Holland <samuel at sholland.org>
---
 drivers/clocksource/Kconfig          | 11 ++++++++++
 drivers/clocksource/arm_arch_timer.c | 39 ++++++++++++++++++++++++++++++++++++
 2 files changed, 50 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..66ce13578c52 100644
--- a/drivers/clocksource/arm_arch_timer.c
+++ b/drivers/clocksource/arm_arch_timer.c
@@ -319,6 +319,36 @@ 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.
+ */
+static u64 notrace sun50i_a64_read_cntpct_el0(void)
+{
+	u64 val;
+
+	do {
+		val = read_sysreg(cntpct_el0);
+	} while (((val + 1) & GENMASK(10, 0)) <= 1);
+
+	return val;
+}
+
+static u64 notrace sun50i_a64_read_cntvct_el0(void)
+{
+	u64 val;
+
+	do {
+		val = read_sysreg(cntvct_el0);
+	} while (((val + 1) & GENMASK(10, 0)) <= 1);
+
+	return val;
+}
+#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 +438,15 @@ 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 *,
-- 
2.16.1