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Backport A64 unstable timer patches from linux 5.1 Signed-off-by: Oskari Lemmela <oskari@lemmela.net> [Split the single patch into the two original patches] Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
245 lines
10 KiB
Diff
245 lines
10 KiB
Diff
From 7cd6dca3600d8d71328950216688ecd00015d1ce Mon Sep 17 00:00:00 2001
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From: Samuel Holland <samuel@sholland.org>
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Date: Sat, 12 Jan 2019 20:17:18 -0600
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Subject: [PATCH] clocksource/drivers/arch_timer: Workaround for Allwinner A64
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timer instability
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MIME-Version: 1.0
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Content-Type: text/plain; charset=UTF-8
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Content-Transfer-Encoding: 8bit
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The Allwinner A64 SoC is known[1] to have an unstable architectural
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timer, which manifests itself most obviously in the time jumping forward
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a multiple of 95 years[2][3]. This coincides with 2^56 cycles at a
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timer frequency of 24 MHz, implying that the time went slightly backward
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(and this was interpreted by the kernel as it jumping forward and
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wrapping around past the epoch).
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Investigation revealed instability in the low bits of CNTVCT at the
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point a high bit rolls over. This leads to power-of-two cycle forward
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and backward jumps. (Testing shows that forward jumps are about twice as
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likely as backward jumps.) Since the counter value returns to normal
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after an indeterminate read, each "jump" really consists of both a
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forward and backward jump from the software perspective.
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Unless the kernel is trapping CNTVCT reads, a userspace program is able
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to read the register in a loop faster than it changes. A test program
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running on all 4 CPU cores that reported jumps larger than 100 ms was
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run for 13.6 hours and reported the following:
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Count | Event
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-------+---------------------------
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9940 | jumped backward 699ms
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268 | jumped backward 1398ms
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1 | jumped backward 2097ms
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16020 | jumped forward 175ms
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6443 | jumped forward 699ms
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2976 | jumped forward 1398ms
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9 | jumped forward 356516ms
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9 | jumped forward 357215ms
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4 | jumped forward 714430ms
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1 | jumped forward 3578440ms
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This works out to a jump larger than 100 ms about every 5.5 seconds on
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each CPU core.
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The largest jump (almost an hour!) was the following sequence of reads:
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0x0000007fffffffff → 0x00000093feffffff → 0x0000008000000000
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Note that the middle bits don't necessarily all read as all zeroes or
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all ones during the anomalous behavior; however the low 10 bits checked
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by the function in this patch have never been observed with any other
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value.
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Also note that smaller jumps are much more common, with backward jumps
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of 2048 (2^11) cycles observed over 400 times per second on each core.
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(Of course, this is partially explained by lower bits rolling over more
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frequently.) Any one of these could have caused the 95 year time skip.
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Similar anomalies were observed while reading CNTPCT (after patching the
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kernel to allow reads from userspace). However, the CNTPCT jumps are
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much less frequent, and only small jumps were observed. The same program
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as before (except now reading CNTPCT) observed after 72 hours:
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Count | Event
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-------+---------------------------
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17 | jumped backward 699ms
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52 | jumped forward 175ms
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2831 | jumped forward 699ms
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5 | jumped forward 1398ms
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Further investigation showed that the instability in CNTPCT/CNTVCT also
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affected the respective timer's TVAL register. The following values were
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observed immediately after writing CNVT_TVAL to 0x10000000:
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CNTVCT | CNTV_TVAL | CNTV_CVAL | CNTV_TVAL Error
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--------------------+------------+--------------------+-----------------
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0x000000d4a2d8bfff | 0x10003fff | 0x000000d4b2d8bfff | +0x00004000
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0x000000d4a2d94000 | 0x0fffffff | 0x000000d4b2d97fff | -0x00004000
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0x000000d4a2d97fff | 0x10003fff | 0x000000d4b2d97fff | +0x00004000
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0x000000d4a2d9c000 | 0x0fffffff | 0x000000d4b2d9ffff | -0x00004000
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The pattern of errors in CNTV_TVAL seemed to depend on exactly which
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value was written to it. For example, after writing 0x10101010:
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CNTVCT | CNTV_TVAL | CNTV_CVAL | CNTV_TVAL Error
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--------------------+------------+--------------------+-----------------
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0x000001ac3effffff | 0x1110100f | 0x000001ac4f10100f | +0x1000000
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0x000001ac40000000 | 0x1010100f | 0x000001ac5110100f | -0x1000000
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0x000001ac58ffffff | 0x1110100f | 0x000001ac6910100f | +0x1000000
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0x000001ac66000000 | 0x1010100f | 0x000001ac7710100f | -0x1000000
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0x000001ac6affffff | 0x1110100f | 0x000001ac7b10100f | +0x1000000
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0x000001ac6e000000 | 0x1010100f | 0x000001ac7f10100f | -0x1000000
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I was also twice able to reproduce the issue covered by Allwinner's
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workaround[4], that writing to TVAL sometimes fails, and both CVAL and
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TVAL are left with entirely bogus values. One was the following values:
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CNTVCT | CNTV_TVAL | CNTV_CVAL
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--------------------+------------+--------------------------------------
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0x000000d4a2d6014c | 0x8fbd5721 | 0x000000d132935fff (615s in the past)
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Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
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========================================================================
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Because the CPU can read the CNTPCT/CNTVCT registers faster than they
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change, performing two reads of the register and comparing the high bits
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(like other workarounds) is not a workable solution. And because the
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timer can jump both forward and backward, no pair of reads can
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distinguish a good value from a bad one. The only way to guarantee a
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good value from consecutive reads would be to read _three_ times, and
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take the middle value only if the three values are 1) each unique and
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2) increasing. This takes at minimum 3 counter cycles (125 ns), or more
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if an anomaly is detected.
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However, since there is a distinct pattern to the bad values, we can
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optimize the common case (1022/1024 of the time) to a single read by
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simply ignoring values that match the error pattern. This still takes no
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more than 3 cycles in the worst case, and requires much less code. As an
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additional safety check, we still limit the loop iteration to the number
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of max-frequency (1.2 GHz) CPU cycles in three 24 MHz counter periods.
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For the TVAL registers, the simple solution is to not use them. Instead,
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read or write the CVAL and calculate the TVAL value in software.
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Although the manufacturer is aware of at least part of the erratum[4],
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there is no official name for it. For now, use the kernel-internal name
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"UNKNOWN1".
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[1]: https://github.com/armbian/build/commit/a08cd6fe7ae9
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[2]: https://forum.armbian.com/topic/3458-a64-datetime-clock-issue/
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[3]: https://irclog.whitequark.org/linux-sunxi/2018-01-26
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[4]: https://github.com/Allwinner-Homlet/H6-BSP4.9-linux/blob/master/drivers/clocksource/arm_arch_timer.c#L272
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Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
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Tested-by: Andre Przywara <andre.przywara@arm.com>
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Signed-off-by: Samuel Holland <samuel@sholland.org>
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Cc: stable@vger.kernel.org
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Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
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---
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Documentation/arm64/silicon-errata.txt | 2 +
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drivers/clocksource/Kconfig | 10 +++++
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drivers/clocksource/arm_arch_timer.c | 55 ++++++++++++++++++++++++++
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3 files changed, 67 insertions(+)
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--- a/Documentation/arm64/silicon-errata.txt
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+++ b/Documentation/arm64/silicon-errata.txt
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@@ -44,6 +44,8 @@ stable kernels.
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| Implementor | Component | Erratum ID | Kconfig |
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+----------------+-----------------+-----------------+-----------------------------+
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+| Allwinner | A64/R18 | UNKNOWN1 | SUN50I_ERRATUM_UNKNOWN1 |
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+| | | | |
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| ARM | Cortex-A53 | #826319 | ARM64_ERRATUM_826319 |
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| ARM | Cortex-A53 | #827319 | ARM64_ERRATUM_827319 |
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| ARM | Cortex-A53 | #824069 | ARM64_ERRATUM_824069 |
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--- a/drivers/clocksource/Kconfig
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+++ b/drivers/clocksource/Kconfig
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@@ -365,6 +365,16 @@ config ARM64_ERRATUM_858921
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The workaround will be dynamically enabled when an affected
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core is detected.
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+config SUN50I_ERRATUM_UNKNOWN1
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+ bool "Workaround for Allwinner A64 erratum UNKNOWN1"
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+ default y
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+ depends on ARM_ARCH_TIMER && ARM64 && ARCH_SUNXI
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+ select ARM_ARCH_TIMER_OOL_WORKAROUND
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+ help
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+ This option enables a workaround for instability in the timer on
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+ the Allwinner A64 SoC. The workaround will only be active if the
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+ allwinner,erratum-unknown1 property is found in the timer node.
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+
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config ARM_GLOBAL_TIMER
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bool "Support for the ARM global timer" if COMPILE_TEST
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select TIMER_OF if OF
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--- a/drivers/clocksource/arm_arch_timer.c
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+++ b/drivers/clocksource/arm_arch_timer.c
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@@ -319,6 +319,48 @@ static u64 notrace arm64_858921_read_cnt
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}
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#endif
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+#ifdef CONFIG_SUN50I_ERRATUM_UNKNOWN1
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+/*
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+ * The low bits of the counter registers are indeterminate while bit 10 or
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+ * greater is rolling over. Since the counter value can jump both backward
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+ * (7ff -> 000 -> 800) and forward (7ff -> fff -> 800), ignore register values
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+ * with all ones or all zeros in the low bits. Bound the loop by the maximum
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+ * number of CPU cycles in 3 consecutive 24 MHz counter periods.
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+ */
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+#define __sun50i_a64_read_reg(reg) ({ \
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+ u64 _val; \
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+ int _retries = 150; \
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+ \
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+ do { \
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+ _val = read_sysreg(reg); \
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+ _retries--; \
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+ } while (((_val + 1) & GENMASK(9, 0)) <= 1 && _retries); \
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+ \
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+ WARN_ON_ONCE(!_retries); \
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+ _val; \
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+})
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+
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+static u64 notrace sun50i_a64_read_cntpct_el0(void)
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+{
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+ return __sun50i_a64_read_reg(cntpct_el0);
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+}
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+
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+static u64 notrace sun50i_a64_read_cntvct_el0(void)
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+{
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+ return __sun50i_a64_read_reg(cntvct_el0);
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+}
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+
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+static u32 notrace sun50i_a64_read_cntp_tval_el0(void)
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+{
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+ return read_sysreg(cntp_cval_el0) - sun50i_a64_read_cntpct_el0();
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+}
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+
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+static u32 notrace sun50i_a64_read_cntv_tval_el0(void)
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+{
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+ return read_sysreg(cntv_cval_el0) - sun50i_a64_read_cntvct_el0();
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+}
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+#endif
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+
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#ifdef CONFIG_ARM_ARCH_TIMER_OOL_WORKAROUND
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DEFINE_PER_CPU(const struct arch_timer_erratum_workaround *, timer_unstable_counter_workaround);
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EXPORT_SYMBOL_GPL(timer_unstable_counter_workaround);
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@@ -408,6 +450,19 @@ static const struct arch_timer_erratum_w
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.read_cntvct_el0 = arm64_858921_read_cntvct_el0,
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},
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#endif
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+#ifdef CONFIG_SUN50I_ERRATUM_UNKNOWN1
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+ {
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+ .match_type = ate_match_dt,
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+ .id = "allwinner,erratum-unknown1",
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+ .desc = "Allwinner erratum UNKNOWN1",
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+ .read_cntp_tval_el0 = sun50i_a64_read_cntp_tval_el0,
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+ .read_cntv_tval_el0 = sun50i_a64_read_cntv_tval_el0,
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+ .read_cntpct_el0 = sun50i_a64_read_cntpct_el0,
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+ .read_cntvct_el0 = sun50i_a64_read_cntvct_el0,
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+ .set_next_event_phys = erratum_set_next_event_tval_phys,
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+ .set_next_event_virt = erratum_set_next_event_tval_virt,
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+ },
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+#endif
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};
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typedef bool (*ate_match_fn_t)(const struct arch_timer_erratum_workaround *,
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