========================================================================
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(+)
@@ -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
@@ -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 *,
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 (2^11) 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 CNTPCT 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