[PATCH v2 3/4] arm64: arch_timer: Work around Erratum Hisilicon-161601

On 2016/10/29 0:00, Will Deacon wrote:

Hi Ding,

On Thu, Oct 27, 2016 at 03:34:10PM +0800, Ding Tianhong wrote:

quoted

Erratum Hisilicon-161601 says that the ARM generic timer counter "has the
potential to contain an erroneous value when the timer value changes".
Accesses to TVAL (both read and write) are also affected due to the implicit counter
read.  Accesses to CVAL are not affected.

The workaround is to reread the system count registers until the value of the second
read is larger than the first one by less than 32, the system counter can be guaranteed
not to return wrong value twice by back-to-back read and the error value is always larger
than the correct one by 32. Writes to TVAL are replaced with an equivalent write to CVAL.

The workaround is enabled if the hisilicon,erratum-161601 property is found in
the timer node in the device tree.  This can be overridden with the
clocksource.arm_arch_timer.hisilicon-161601 boot parameter, which allows KVM
users to enable the workaround until a mechanism is implemented to
automatically communicate this information.

Fix some description for fsl erratum a008585.

v2: Significant rework based on feedback, including seperate the fsl erratum a008585
    to another patch, update the erratum name and remove unwanted code.

Signed-off-by: Ding Tianhong <redacted>
---

[...]

quoted

diff --git a/drivers/clocksource/Kconfig b/drivers/clocksource/Kconfig
index 8a753fd..4aafb6a 100644
--- a/drivers/clocksource/Kconfig
+++ b/drivers/clocksource/Kconfig

@@ -312,8 +312,20 @@ config FSL_ERRATUM_A008585
 	help
 	  This option enables a workaround for Freescale/NXP Erratum
 	  A-008585 ("ARM generic timer may contain an erroneous
-	  value").  The workaround will only be active if the
+	  value").  The workaround will be active if the
 	  fsl,erratum-a008585 property is found in the timer node.
+	  This can be overridden with the clocksource.arm_arch_timer.fsl-a008585
+	  boot parameter.
+
+config HISILICON_ERRATUM_161601
+	bool "Workaround for Hisilicon Erratum 161601"
+	default y
+	depends on ARM_ARCH_TIMER && ARM64
+	help
+	  This option enables a workaround for Hisilicon Erratum
+	  161601. The workaround will be active if the hisilicon,erratum-161601
+	  property is found in the timer node. This can be overridden with
+	  the clocksource.arm_arch_timer.hisilicon-161601 boot parameter.

I'm really not keen on having a kernel commandline parameter for this.
It's not something we've done for other, similar errata (e.g. CNTFRQ
reporting the wrong value) and I think it's a slippery slope to having
more of these workarounds controlled at boot-time. If you have a board
that is affected by this, it's always going to need the workaround. Why
would you turn it off?

OK, miss it.

Will

.