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

On 2016/10/27 20:23, Marc Zyngier wrote:

On 27/10/16 13:17, Ding Tianhong wrote:

quoted

On 2016/10/27 18:58, Marc Zyngier wrote:

quoted

On 27/10/16 08:34, 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>
---
 Documentation/arm64/silicon-errata.txt |  1 +
 Documentation/kernel-parameters.txt    |  9 ++++
 arch/arm64/include/asm/arch_timer.h    | 28 ++++++++++-
 drivers/clocksource/Kconfig            | 14 +++++-
 drivers/clocksource/arm_arch_timer.c   | 88 ++++++++++++++++++++++++++--------
 5 files changed, 118 insertions(+), 22 deletions(-)

diff --git a/Documentation/arm64/silicon-errata.txt b/Documentation/arm64/silicon-errata.txt
index 405da11..70c5d5e 100644
--- a/Documentation/arm64/silicon-errata.txt
+++ b/Documentation/arm64/silicon-errata.txt

@@ -63,3 +63,4 @@ stable kernels.
 | Cavium         | ThunderX SMMUv2 | #27704          | N/A		       |
 |                |                 |                 |                         |
 | Freescale/NXP  | LS2080A/LS1043A | A-008585        | FSL_ERRATUM_A008585     |
+| Hisilicon      | Hip05/Hip06/Hip07 | #161601       | HISILICON_ERRATUM_161601|

I've already commented on the alignment. Please read my initial review.

It sees misunderstood, fix it this time.

quoted

quoted

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 6fa1d8a..735b4b6 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt

@@ -707,6 +707,15 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			erratum.  If unspecified, the workaround is
 			enabled based on the device tree.
 
+	clocksource.arm_arch_timer.hisilicon-161601=
+			[ARM64]
+			Format: <bool>
+			Enable/disable the workaround of Hisilicon
+			erratum 161601.  This can be useful for KVM
+			guests, if the guest device tree doesn't show the
+			erratum.  If unspecified, the workaround is
+			enabled based on the device tree.
+
 	clearcpuid=BITNUM [X86]
 			Disable CPUID feature X for the kernel. See
 			arch/x86/include/asm/cpufeatures.h for the valid bit

diff --git a/arch/arm64/include/asm/arch_timer.h b/arch/arm64/include/asm/arch_timer.h
index 118719d8..49b3041 100644
--- a/arch/arm64/include/asm/arch_timer.h
+++ b/arch/arm64/include/asm/arch_timer.h

@@ -29,7 +29,7 @@
 
 #include <clocksource/arm_arch_timer.h>
 
-#if IS_ENABLED(CONFIG_FSL_ERRATUM_A008585)
+#if IS_ENABLED(CONFIG_FSL_ERRATUM_A008585) || IS_ENABLED(CONFIG_HISILICON_ERRATUM_161601)
 extern struct static_key_false arch_timer_read_ool_enabled;
 #define needs_timer_erratum_workaround() \
 	static_branch_unlikely(&arch_timer_read_ool_enabled)

@@ -65,11 +65,35 @@ extern struct arch_timer_erratum_workaround *erratum_workaround;
 	_new;						\
 })
 
+
+
+/*
+ * The number of retries is an arbitrary value well beyond the highest number
+ * of iterations the loop has been observed to take.
+ * Verify whether the value of the second read is larger than the first by
+ * less than 32 is the only way to confirm the value is correct, 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.
+ */
+#define __hisi_161601_read_reg(reg) ({				\
+	u64 _old, _new;						\
+	int _retries = 200;					\

Please document how this value was found (either in the code or in the
commit message).

It really difficult to give the accurate standard, theoretically the error should not happened
twice together, so maybe 2 is enough here, I just give a arbitrary value.

quoted

quoted

+								\
+	do {							\
+		_old = read_sysreg(reg);			\
+		_new = read_sysreg(reg);			\
+		_retries--;					\
+	} while (unlikely((_new - _old) >> 5) && _retries);	\
+								\
+	WARN_ON_ONCE(!_retries);				\
+	_new;							\
+})

Same remark as in the previous patch.

I think the sentence *Verify whether the value of the second read is larger than the first by
less than 32 is the only way to confirm the value is correct* could explain why should *(_new - _old) >> 5*.
it is the same as (_new - _old)/32, also mean the _new should never bigger than _old more than 32.

This is not about the explanation of the erratum, but about the location
of the #define, which can be made private to the .c file instead of
being globally visible.

Ok, update it .

Thanks
Ding

Thanks,

	M.