[PATCH 2/2] armv8: generic_timer: Use event stream for udelay

[Quentin Schulz]

Hi Peter,

On 4/23/24 10:10, Peter Hoyes wrote:
> From: Peter Hoyes <Peter.Hoyes at arm.com>
> 
> Polling cntpct_el0 in a tight loop for delays is inefficient.
> This is particularly apparent on Arm FVPs, which do not simulate
> real time, meaning that a 1s sleep can take a couple of orders
> of magnitude longer to execute in wall time.
> 
> If running at EL2 or above (where CNTHCTL_EL2 is available), enable
> the cntpct_el0 event stream temporarily and use wfe to implement
> the delay more efficiently. The event period is chosen as a
> trade-off between efficiency and the fact that Arm FVPs do not
> typically simulate real time.
> 
> This is only implemented for Armv8 boards, where an architectural
> timer exists.
> 
> Some mach-socfpga AArch64 boards already override __udelay to make
> it always inline, so guard the functionality with a new
> ARMV8_UDELAY_EVENT_STREAM Kconfig, enabled by default.
> 
> Signed-off-by: Peter Hoyes <Peter.Hoyes at arm.com>
> ---
>   arch/arm/cpu/armv8/Kconfig         |  8 ++++++++
>   arch/arm/cpu/armv8/generic_timer.c | 27 +++++++++++++++++++++++++++
>   arch/arm/include/asm/system.h      |  6 ++++--
>   3 files changed, 39 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm/cpu/armv8/Kconfig b/arch/arm/cpu/armv8/Kconfig
> index 9f0fb369f7..544c5e2d74 100644
> --- a/arch/arm/cpu/armv8/Kconfig
> +++ b/arch/arm/cpu/armv8/Kconfig
> @@ -191,6 +191,14 @@ config ARMV8_EA_EL3_FIRST
>   	  Exception handling at all exception levels for External Abort and
>   	  SError interrupt exception are taken in EL3.
>   
> +config ARMV8_UDELAY_EVENT_STREAM
> +	bool "Use the event stream for udelay"
> +	default y if !ARCH_SOCFPGA
> +	help
> +	  Use the event stream provided by the AArch64 architectural timer for
> +	  delays. This is more efficient than the default polling
> +	  implementation.
> +
>   menuconfig ARMV8_CRYPTO
>   	bool "ARM64 Accelerated Cryptographic Algorithms"
>   
> diff --git a/arch/arm/cpu/armv8/generic_timer.c b/arch/arm/cpu/armv8/generic_timer.c
> index 8f83372cbc..e18b5c8187 100644
> --- a/arch/arm/cpu/armv8/generic_timer.c
> +++ b/arch/arm/cpu/armv8/generic_timer.c
> @@ -115,3 +115,30 @@ ulong timer_get_boot_us(void)
>   
>   	return val / get_tbclk();
>   }
> +
> +#if CONFIG_IS_ENABLED(ARMV8_UDELAY_EVENT_STREAM)
> +void __udelay(unsigned long usec)
> +{
> +	u64 target = get_ticks() + usec_to_tick(usec);
> +

This can theoretically overflow, do we have any guarantee this cannot 
happen in real life, like... we would need U-Boot to be running for 100 
years without being powered-down/reset or something like that? Can we 
document this assumption? Does this make sense?

> +	/* At EL2 or above, use the event stream to avoid polling CNTPCT_EL0 so often */
> +	if (current_el() >= 2) {
> +		u32 cnthctl_val;
> +		const u8 event_period = 0x7;
> +
> +		asm volatile("mrs %0, cnthctl_el2" : "=r" (cnthctl_val));
> +		asm volatile("msr cnthctl_el2, %0" : : "r"
> +			(cnthctl_val | CNTHCTL_EL2_EVNT_EN | CNTHCTL_EL2_EVNT_I(event_period)));
> +
> +		while (get_ticks() + (1ULL << event_period) <= target)

This could be an overflow as well.

> +			wfe();
> +
> +		/* Reset the event stream */
> +		asm volatile("msr cnthctl_el2, %0" : : "r" (cnthctl_val));
> +	}
> +
> +	/* Fall back to polling CNTPCT_EL0 */
> +	while (get_ticks() <= target)

get_ticks() could wrap around here maybe?

Cheers,
Quentin