Re: [PATCH v2 1/2] ARM64: arch_timer: Work around QorIQ Erratum A-008585

On 29/06/16 09:11, Hanjun Guo wrote:

On 2016/5/13 18:24, Marc Zyngier wrote:

quoted

On Thu, 12 May 2016 23:37:39 -0500
Scott Wood [off-list ref] wrote:

Hi Scott,

quoted

Erratum A-008585 says that the ARM generic timer counter "has the
potential to contain an erroneous value for a small number of core
clock cycles every time 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 TVAL and count registers until successive reads
return the same value, and when writing TVAL to retry until counter
reads before and after the write return the same value.

This erratum can be found on LS1043A and LS2080A.

Signed-off-by: Scott Wood <redacted>
---
v2:
Significant rework based on feedback, including using static_key,
disabling VDSO counter access rather than adding the workaround to the
VDSO, and uninlining the loops.

Dropped the separate property for indicating that writes to TVAL are
affected, as I believe that's just a side effect of the implicit
counter read being corrupted, and thus a chip that is affected by one
will always be affected by the other.

Dropped the arm32 portion as it seems there was confusion about whether
LS1021A is affected.  Currently I am being told that it is not
affected.

I considered writing to CVAL rather than looping on TVAL writes, but
that would still have required separate set_next_event() code for the
erratum, and adding CVAL to the enum would have required a bunch of
extra handlers in switch statements (even where unused, due to compiler
warnings about unhandled enum values) including in an arm32 header.  It
seemed better to avoid the arm32 interaction and new untested
accessors.
---
 .../devicetree/bindings/arm/arch_timer.txt         |   6 ++
 arch/arm64/include/asm/arch_timer.h                |  37 +++++--
 drivers/clocksource/arm_arch_timer.c               | 110 +++++++++++++++++++++
 3 files changed, 144 insertions(+), 9 deletions(-)

diff --git a/Documentation/devicetree/bindings/arm/arch_timer.txt b/Documentation/devicetree/bindings/arm/arch_timer.txt
index e774128..ef5fbe9 100644
--- a/Documentation/devicetree/bindings/arm/arch_timer.txt
+++ b/Documentation/devicetree/bindings/arm/arch_timer.txt

@@ -25,6 +25,12 @@ to deliver its interrupts via SPIs.
 - always-on : a boolean property. If present, the timer is powered through an
   always-on power domain, therefore it never loses context.
 
+- fsl,erratum-a008585 : A boolean property. Indicates the presence of
+  QorIQ erratum A-008585, which says that reading the counter is
+  unreliable unless the same value is returned by back-to-back reads.
+  This also affects writes to the tval register, due to the implicit
+  counter read.
+
 ** Optional properties:
 
 - arm,cpu-registers-not-fw-configured : Firmware does not initialize

This should be part of a separate patch. Also, errata should be
documented in Documentation/arm64/silicon-errata.txt.

quoted

diff --git a/arch/arm64/include/asm/arch_timer.h b/arch/arm64/include/asm/arch_timer.h
index fbe0ca3..9715e85 100644
--- a/arch/arm64/include/asm/arch_timer.h
+++ b/arch/arm64/include/asm/arch_timer.h

@@ -23,10 +23,33 @@
 
 #include <linux/bug.h>
 #include <linux/init.h>
+#include <linux/jump_label.h>
 #include <linux/types.h>
 
 #include <clocksource/arm_arch_timer.h>
 
+extern struct static_key_false arch_timer_read_ool_enabled;
+
+#define ARCH_TIMER_REG_READ(reg, func) \
+extern u64 func##_ool(void); \
+static inline u64 __##func(void) \
+{ \
+	u64 val; \
+	asm volatile("mrs %0, " reg : "=r" (val)); \
+	return val; \
+} \
+static inline u64 _##func(void) \
+{ \
+	if (static_branch_unlikely(&arch_timer_read_ool_enabled)) \
+		return func##_ool(); \
+	else \
+		return __##func(); \
+}
+
+ARCH_TIMER_REG_READ("cntp_tval_el0", arch_timer_get_ptval)
+ARCH_TIMER_REG_READ("cntv_tval_el0", arch_timer_get_vtval)
+ARCH_TIMER_REG_READ("cntvct_el0", arch_counter_get_cntvct)
+

Given that this will have a (small) impact on non-affected platforms,
it'd be good to have this guarded by a erratum-specific config option
(CONFIG_FSL_ERRATUM_008585?) and turn it into trivial accessors when not
defined.

quoted

 /*
  * These register accessors are marked inline so the compiler can
  * nicely work out which register we want, and chuck away the rest of

@@ -66,19 +89,19 @@ u32 arch_timer_reg_read_cp15(int access, enum arch_timer_reg reg)
 	if (access == ARCH_TIMER_PHYS_ACCESS) {
 		switch (reg) {
 		case ARCH_TIMER_REG_CTRL:
-			asm volatile("mrs %0,  cntp_ctl_el0" : "=r" (val));
+			asm volatile("mrs %0, cntp_ctl_el0" : "=r" (val));

Spurious change?

quoted

 			break;
 		case ARCH_TIMER_REG_TVAL:
-			asm volatile("mrs %0, cntp_tval_el0" : "=r" (val));
+			val = _arch_timer_get_ptval();
 			break;
 		}
 	} else if (access == ARCH_TIMER_VIRT_ACCESS) {
 		switch (reg) {
 		case ARCH_TIMER_REG_CTRL:
-			asm volatile("mrs %0,  cntv_ctl_el0" : "=r" (val));
+			asm volatile("mrs %0, cntv_ctl_el0" : "=r" (val));

Same here.

quoted

 			break;
 		case ARCH_TIMER_REG_TVAL:
-			asm volatile("mrs %0, cntv_tval_el0" : "=r" (val));
+			val = _arch_timer_get_vtval();
 			break;
 		}
 	}

@@ -116,12 +139,8 @@ static inline u64 arch_counter_get_cntpct(void)
 
 static inline u64 arch_counter_get_cntvct(void)
 {
-	u64 cval;
-
 	isb();
-	asm volatile("mrs %0, cntvct_el0" : "=r" (cval));
-
-	return cval;
+	return _arch_counter_get_cntvct();
 }
 
 static inline int arch_timer_arch_init(void)

diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
index 5152b38..6f78831 100644
--- a/drivers/clocksource/arm_arch_timer.c
+++ b/drivers/clocksource/arm_arch_timer.c

@@ -79,10 +79,52 @@ static enum ppi_nr arch_timer_uses_ppi = VIRT_PPI;
 static bool arch_timer_c3stop;
 static bool arch_timer_mem_use_virtual;
 
+DEFINE_STATIC_KEY_FALSE(arch_timer_read_ool_enabled);
+EXPORT_SYMBOL_GPL(arch_timer_read_ool_enabled);
+

Once you have a config option, this can move to the guarded section
below...

quoted

 /*
  * Architected system timer support.
  */
 
+#ifdef CONFIG_ARM64

which can become CONFIG_FSL_ERRATUM_008585

quoted

+/*
+ * __always_inline is used to ensure that func() is not an actual function
+ * pointer, which would result in the register accesses potentially being too
+ * far apart for the loop to work.
+ */
+static __always_inline u64 arch_timer_reread(u64 (*func)(void))

This is quite a generic function name. I'd expect something that refers
to the erratum number.

This is a generic solution to reread the timer counter, how about using a generic one
here for more potential consumers in the future?

In that case, how about something like

"my_arch_timer_is_so_broken_that_I_have_to_poll_it"?

It should be generic enough, don't you think?

Joke aside, I want it to be clear that this is a workaround for an
erratum. This is not a general purpose function.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...
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