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clocksource/drivers/arch_timer: Workaround for Allwinner A64 timer in…


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).

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.) Since the counter value returns to normal
after an indeterminate read, each "jump" really consists of both a
forward and backward jump from the software perspective.

Unless the kernel is trapping CNTVCT reads, a userspace program is able
to read the register 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 10 bits checked
by the function in this patch have never been observed with any other

Also note that smaller jumps are much more common, with backward jumps
of 2048 (2^11) cycles observed over 400 times per second on each core.
(Of course, this is partially explained by 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

Further investigation showed that the instability in CNTPCT/CNTVCT also
affected the respective timer's TVAL register. The following values were
observed immediately after writing CNVT_TVAL to 0x10000000:

 CNTVCT             | CNTV_TVAL  | CNTV_CVAL          | CNTV_TVAL Error
 0x000000d4a2d8bfff | 0x10003fff | 0x000000d4b2d8bfff | +0x00004000
 0x000000d4a2d94000 | 0x0fffffff | 0x000000d4b2d97fff | -0x00004000
 0x000000d4a2d97fff | 0x10003fff | 0x000000d4b2d97fff | +0x00004000
 0x000000d4a2d9c000 | 0x0fffffff | 0x000000d4b2d9ffff | -0x00004000

The pattern of errors in CNTV_TVAL seemed to depend on exactly which
value was written to it. For example, after writing 0x10101010:

 CNTVCT             | CNTV_TVAL  | CNTV_CVAL          | CNTV_TVAL Error
 0x000001ac3effffff | 0x1110100f | 0x000001ac4f10100f | +0x1000000
 0x000001ac40000000 | 0x1010100f | 0x000001ac5110100f | -0x1000000
 0x000001ac58ffffff | 0x1110100f | 0x000001ac6910100f | +0x1000000
 0x000001ac66000000 | 0x1010100f | 0x000001ac7710100f | -0x1000000
 0x000001ac6affffff | 0x1110100f | 0x000001ac7b10100f | +0x1000000
 0x000001ac6e000000 | 0x1010100f | 0x000001ac7f10100f | -0x1000000

I was also twice able to reproduce the issue covered by Allwinner's
workaround[4], that writing to TVAL sometimes fails, and both CVAL and
TVAL are left with entirely bogus values. One was the following values:

 CNTVCT             | CNTV_TVAL  | CNTV_CVAL
 0x000000d4a2d6014c | 0x8fbd5721 | 0x000000d132935fff (615s in the past)
Reviewed-by: Marc Zyngier <>


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 only if the three values are 1) each unique and
2) increasing. This takes at minimum 3 counter 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 (1022/1024 of the time) to a single read by
simply ignoring values that match the error pattern. This still takes no
more than 3 cycles in the worst case, and requires much less code. As an
additional safety check, we still limit the loop iteration to the number
of max-frequency (1.2 GHz) CPU cycles in three 24 MHz counter periods.

For the TVAL registers, the simple solution is to not use them. Instead,
read or write the CVAL and calculate the TVAL value in software.

Although the manufacturer is aware of at least part of the erratum[4],
there is no official name for it. For now, use the kernel-internal name

[1]: armbian/build@a08cd6f

Acked-by: Maxime Ripard <>
Tested-by: Andre Przywara <>
Signed-off-by: Samuel Holland <>
Signed-off-by: Daniel Lezcano <>
  • Loading branch information...
smaeul authored and dlezcano committed Jan 13, 2019
1 parent e7e7e0d commit c950ca8c35eeb32224a63adc47e12f9e226da241
Showing with 67 additions and 0 deletions.
  1. +2 −0 Documentation/arm64/silicon-errata.txt
  2. +10 −0 drivers/clocksource/Kconfig
  3. +55 −0 drivers/clocksource/arm_arch_timer.c
@@ -44,6 +44,8 @@ stable kernels.

| Implementor | Component | Erratum ID | Kconfig |
| Allwinner | A64/R18 | UNKNOWN1 | SUN50I_ERRATUM_UNKNOWN1 |
| | | | |
| ARM | Cortex-A53 | #826319 | ARM64_ERRATUM_826319 |
| ARM | Cortex-A53 | #827319 | ARM64_ERRATUM_827319 |
| ARM | Cortex-A53 | #824069 | ARM64_ERRATUM_824069 |
@@ -360,6 +360,16 @@ config ARM64_ERRATUM_858921
The workaround will be dynamically enabled when an affected
core is detected.

bool "Workaround for Allwinner A64 erratum UNKNOWN1"
default y
depends on ARM_ARCH_TIMER && ARM64 && ARCH_SUNXI
This option enables a workaround for instability in the timer on
the Allwinner A64 SoC. The workaround will only be active if the
allwinner,erratum-unknown1 property is found in the timer node.

bool "Support for the ARM global timer" if COMPILE_TEST
select TIMER_OF if OF
@@ -326,6 +326,48 @@ static u64 notrace arm64_1188873_read_cntvct_el0(void)

* The low bits of the counter registers are indeterminate while bit 10 or
* greater is rolling over. Since the counter value can jump both backward
* (7ff -> 000 -> 800) and forward (7ff -> fff -> 800), ignore register values
* with all ones or all zeros in the low bits. Bound the loop by the maximum
* number of CPU cycles in 3 consecutive 24 MHz counter periods.
#define __sun50i_a64_read_reg(reg) ({ \
u64 _val; \
int _retries = 150; \
do { \
_val = read_sysreg(reg); \
_retries--; \
} while (((_val + 1) & GENMASK(9, 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);

static u32 notrace sun50i_a64_read_cntp_tval_el0(void)
return read_sysreg(cntp_cval_el0) - sun50i_a64_read_cntpct_el0();

static u32 notrace sun50i_a64_read_cntv_tval_el0(void)
return read_sysreg(cntv_cval_el0) - sun50i_a64_read_cntvct_el0();

DEFINE_PER_CPU(const struct arch_timer_erratum_workaround *, timer_unstable_counter_workaround);
@@ -423,6 +465,19 @@ static const struct arch_timer_erratum_workaround ool_workarounds[] = {
.read_cntvct_el0 = arm64_1188873_read_cntvct_el0,
.match_type = ate_match_dt,
.id = "allwinner,erratum-unknown1",
.desc = "Allwinner erratum UNKNOWN1",
.read_cntp_tval_el0 = sun50i_a64_read_cntp_tval_el0,
.read_cntv_tval_el0 = sun50i_a64_read_cntv_tval_el0,
.read_cntpct_el0 = sun50i_a64_read_cntpct_el0,
.read_cntvct_el0 = sun50i_a64_read_cntvct_el0,
.set_next_event_phys = erratum_set_next_event_tval_phys,
.set_next_event_virt = erratum_set_next_event_tval_virt,

typedef bool (*ate_match_fn_t)(const struct arch_timer_erratum_workaround *,

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