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

[Will Deacon]

On Fri, Jul 01, 2016 at 05:41:37PM -0500, 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 at buserror.net>
> ---
> 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)

It's probably worth #undefing the macro here, since it really shouldn't
grow any users outside of this header file.

> +
>  /*
>   * 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;
> +}
> +
> +u64 arch_counter_get_cntvct_ool(void)
> +{
> +	return fsl_a008585_reread_counter(__arch_counter_get_cntvct);
> +}
> +
> +u64 arch_timer_get_vtval_ool(void)
> +{
> +	return fsl_a008585_reread_counter(__arch_timer_get_vtval);
> +}
> +
> +u64 arch_timer_get_ptval_ool(void)
> +{
> +	return fsl_a008585_reread_counter(__arch_timer_get_ptval);
> +}
> +
> +#endif /* CONFIG_FSL_ERRATUM_A008585 */
> +
>  static __always_inline
>  void arch_timer_reg_write(int access, enum arch_timer_reg reg, u32 val,
>  			  struct clock_event_device *clk)
> @@ -232,6 +277,35 @@ static __always_inline void set_next_event(const int access, unsigned long evt,
>  	arch_timer_reg_write(access, ARCH_TIMER_REG_CTRL, ctrl, clk);
>  }
>  
> +#ifdef CONFIG_FSL_ERRATUM_A008585
> +static __always_inline void fsl_a008585_set_next_event(const int access,
> +		unsigned long evt, struct clock_event_device *clk)
> +{
> +	unsigned long ctrl;
> +	u64 cval = evt + arch_counter_get_cntvct();
> +
> +	ctrl = arch_timer_reg_read(access, ARCH_TIMER_REG_CTRL, clk);
> +	ctrl |= ARCH_TIMER_CTRL_ENABLE;
> +	ctrl &= ~ARCH_TIMER_CTRL_IT_MASK;
> +	arch_timer_cval_write_cp15(access, cval);
> +	arch_timer_reg_write(access, ARCH_TIMER_REG_CTRL, ctrl, clk);
> +}
> +
> +static int fsl_a008585_set_next_event_virt(unsigned long evt,
> +					   struct clock_event_device *clk)
> +{
> +	fsl_a008585_set_next_event(ARCH_TIMER_VIRT_ACCESS, evt, clk);
> +	return 0;
> +}
> +
> +static int fsl_a008585_set_next_event_phys(unsigned long evt,
> +					   struct clock_event_device *clk)
> +{
> +	fsl_a008585_set_next_event(ARCH_TIMER_PHYS_ACCESS, evt, clk);
> +	return 0;
> +}
> +#endif /* CONFIG_FSL_ERRATUM_A008585 */
> +
>  static int arch_timer_set_next_event_virt(unsigned long evt,
>  					  struct clock_event_device *clk)
>  {
> @@ -260,6 +334,19 @@ static int arch_timer_set_next_event_phys_mem(unsigned long evt,
>  	return 0;
>  }
>  
> +static void fsl_a008585_set_sne(struct clock_event_device *clk)
> +{
> +#ifdef CONFIG_FSL_ERRATUM_A008585
> +	if (!static_branch_unlikely(&arch_timer_read_ool_enabled))
> +		return;
> +
> +	if (arch_timer_uses_ppi == VIRT_PPI)
> +		clk->set_next_event = fsl_a008585_set_next_event_virt;
> +	else
> +		clk->set_next_event = fsl_a008585_set_next_event_phys;
> +#endif
> +}
> +
>  static void __arch_timer_setup(unsigned type,
>  			       struct clock_event_device *clk)
>  {
> @@ -288,6 +375,8 @@ static void __arch_timer_setup(unsigned type,
>  		default:
>  			BUG();
>  		}
> +
> +		fsl_a008585_set_sne(clk);
>  	} else {
>  		clk->features |= CLOCK_EVT_FEAT_DYNIRQ;
>  		clk->name = "arch_mem_timer";
> @@ -485,6 +574,15 @@ static void __init arch_counter_register(unsigned type)
>  			arch_timer_read_counter = arch_counter_get_cntvct;
>  		else
>  			arch_timer_read_counter = arch_counter_get_cntpct;
> +
> +#ifdef CONFIG_FSL_ERRATUM_A008585
> +		/*
> +		 * Don't use the vdso fastpath if errata require using
> +		 * the out-of-line counter accessor.
> +		 */
> +		if (static_branch_unlikely(&arch_timer_read_ool_enabled))
> +			clocksource_counter.name = "arch_sys_counter_ool";

We could avoid this entirely by providing an arch_clocksource_data structure
like x86 does, and get rid of the ugly strcmp magic from update_vsyscall.

Will