Re: [PATCH v3 3/3] arm64: arch_timer: Work around QorIQ Erratum A-008585

[Marc Zyngier <marc.zyngier-5wv7dgnIgG8@public.gmane.org>]

On 07/07/16 12:37, Ding Tianhong wrote:
> On 2016/7/7 17:49, Marc Zyngier wrote:
>> On 07/07/16 10:34, Ding Tianhong wrote:
>>> On 2016/7/2 6:41, Scott Wood wrote:
>>>> 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 <oss-fOR+EgIDQEHk1uMJSBkQmQ@public.gmane.org>
>>>> ---
>>>> v3:
>>>> - Used cval rather than a loop for the write side of the erratum
>>>> - Added a Kconfig control
>>>> - Moved the device tree binding into its own patch
>>>> - Added erratum to silicon-errata.txt
>>>> - Changed function names to contain the erratum name
>>>> - Factored out the setting of erratum versions of set_next_event
>>>>   to improve readability
>>>> - Added a comment clarifying that the timeout is arbitrary
>>>>
>>>> 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.
>>>> ---
>>>>  Documentation/arm64/silicon-errata.txt |   2 +
>>>>  arch/arm64/include/asm/arch_timer.h    |  48 ++++++++++++---
>>>>  drivers/clocksource/Kconfig            |  10 ++++
>>>>  drivers/clocksource/arm_arch_timer.c   | 103 +++++++++++++++++++++++++++++++++
>>>>  4 files changed, 154 insertions(+), 9 deletions(-)
>>>>
>>>> diff --git a/Documentation/arm64/silicon-errata.txt b/Documentation/arm64/silicon-errata.txt
>>>> index ba4b6ac..5778f62 100644
>>>> --- a/Documentation/arm64/silicon-errata.txt
>>>> +++ b/Documentation/arm64/silicon-errata.txt
>>>> @@ -57,3 +57,5 @@ stable kernels.
>>>>  | Cavium         | ThunderX ITS    | #22375, #24313  | CAVIUM_ERRATUM_22375    |
>>>>  | Cavium         | ThunderX GICv3  | #23154          | CAVIUM_ERRATUM_23154    |
>>>>  | Cavium         | ThunderX Core   | #27456          | CAVIUM_ERRATUM_27456    |
>>>> +|                |                 |                 |                         |
>>>> +| Freescale/NXP  | LS2080A/LS1043A | A-008585        | FSL_ERRATUM_A008585     |
>>>> diff --git a/arch/arm64/include/asm/arch_timer.h b/arch/arm64/include/asm/arch_timer.h
>>>> index fbe0ca3..70fbad9 100644
>>>> --- a/arch/arm64/include/asm/arch_timer.h
>>>> +++ b/arch/arm64/include/asm/arch_timer.h
>>>> @@ -23,10 +23,34 @@
>>>>  
>>>>  #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 (IS_ENABLED(CONFIG_FSL_ERRATUM_A008585) && \
>>>> +	    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)
>>>> +
>>>>  /*
>>>>   * These register accessors are marked inline so the compiler can
>>>>   * nicely work out which register we want, and chuck away the rest of
>>>> @@ -58,6 +82,16 @@ void arch_timer_reg_write_cp15(int access, enum arch_timer_reg reg, u32 val)
>>>>  	isb();
>>>>  }
>>>>  
>>>> +static __always_inline void arch_timer_cval_write_cp15(int access, u64 val)
>>>> +{
>>>> +	if (access == ARCH_TIMER_PHYS_ACCESS)
>>>> +		asm volatile("msr cntp_cval_el0, %0" : : "r" (val));
>>>> +	else if (access == ARCH_TIMER_VIRT_ACCESS)
>>>> +		asm volatile("msr cntv_cval_el0, %0" : : "r" (val));
>>>> +
>>>> +	isb();
>>>> +}
>>>> +
>>>>  static __always_inline
>>>>  u32 arch_timer_reg_read_cp15(int access, enum arch_timer_reg reg)
>>>>  {
>>>> @@ -66,19 +100,19 @@ 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));
>>>>  			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));
>>>>  			break;
>>>>  		case ARCH_TIMER_REG_TVAL:
>>>> -			asm volatile("mrs %0, cntv_tval_el0" : "=r" (val));
>>>> +			val = _arch_timer_get_vtval();
>>>>  			break;
>>>>  		}
>>>>  	}
>>>> @@ -116,12 +150,8 @@ 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/Kconfig b/drivers/clocksource/Kconfig
>>>> index c346be6..672ddc3 100644
>>>> --- a/drivers/clocksource/Kconfig
>>>> +++ b/drivers/clocksource/Kconfig
>>>> @@ -207,6 +207,16 @@ config ARM_ARCH_TIMER_EVTSTREAM
>>>>  	  This must be disabled for hardware validation purposes to detect any
>>>>  	  hardware anomalies of missing events.
>>>>  
>>>> +config FSL_ERRATUM_A008585
>>>> +	bool "Workaround for Freescale/NXP Erratum A-008585"
>>>> +	default y
>>>> +	depends on ARM_ARCH_TIMER && ARM64
>>>> +	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
>>>> +	  fsl,erratum-a008585 property is found in the timer node.
>>>> +
>>>>  config ARM_GLOBAL_TIMER
>>>>  	bool
>>>>  	select CLKSRC_OF if OF
>>>> diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
>>>> index 5152b38..7ead4eb 100644
>>>> --- a/drivers/clocksource/arm_arch_timer.c
>>>> +++ b/drivers/clocksource/arm_arch_timer.c
>>>> @@ -83,6 +83,51 @@ static bool arch_timer_mem_use_virtual;
>>>>   * Architected system timer support.
>>>>   */
>>>>  
>>>> +#ifdef CONFIG_FSL_ERRATUM_A008585
>>>> +DEFINE_STATIC_KEY_FALSE(arch_timer_read_ool_enabled);
>>>> +EXPORT_SYMBOL_GPL(arch_timer_read_ool_enabled);
>>>> +
>>>> +/*
>>>> + * __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.
>>>> + *
>>>> + * The timeout is an arbitrary value well beyond the highest number
>>>> + * of iterations the loop has been observed to take.
>>>> + */
>>>> +static __always_inline u64 fsl_a008585_reread_counter(u64 (*func)(void))
>>>> +{
>>>> +	u64 cval_old, cval_new;
>>>> +	int timeout = 200;
>>>> +
>>>> +	do {
>>>> +		isb();
>>>> +		cval_old = func();
>>>> +		cval_new = func();
>>>> +		timeout--;
>>>> +	} while (unlikely(cval_old != cval_new) && timeout);
>>>> +
>>>> +	WARN_ON_ONCE(!timeout);
>>>> +	return cval_new;
>>>> +}
>>> Hi Scott:
>>>
>>> I have test this patch, this solution looks will break the performance a little more than I expected.
>>> it will have more than 10% that the cval will read again, we could sure that the cval_old always equal to the
>>> cval_new in the normal circumstances, so I prefer this way:
>>>
>>> 	do {
>>> 		isb();
>>> 		cval_old = func();
>>> 		cval_new = func();
>>> 		timeout--;	
>>> 	} while (unlikely((cval_new - cval_old) >> 2) && timeout);
>>
>> What makes you think that ignoring the two bottom bits is a safe thing
>> to do? Talking about performance when the HW has such a dramatic bug is
>> like putting a bigger engine on a car that has no brakes: you just hit
>> the wall quicker.
>>
>> Thanks,
>>
> 
> I have a chip which has the same problem like Scott's chip, and I
> wish to solve this problem in the same way, our chip designer told me
> that if you got a wrong value from the cntvct_el0, you would not get
> a wrong value until 8 cycles later, so I could ignoring the lowest 3
> bits if I reading twice together.

Is that CPU cycles? Or timer cycles? What guarantees do you have that
the two reads are *always* done in the right timing window?

> The key problem is the probability of this bug, my chip has 1/100000
> chance to met this bug, so use 10% performance to fix this bug looks
> more expensive.

You care about performance, I care about correctness. If 10% of your CPU
is wasted on a correct workaround, tough. Next time, your HW guy will
spend more time getting it right.

Anyway, what you are describing is a different bug from what FSL has, so
don't try and shoehorn your own constraints in another workaround.
Please implement your own.

Thanks,

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