[PATCH] armv8: implement workaround for broken CNTFRQ_EL0 value

[Ahmad Fatoum]

Hello Kaustabh,

On 14.10.25 09:22, Kaustabh Chakraborty wrote:
> In devices where the U-Boot is used as a secondary bootloader, we rely
> on the device's primary bootloader to implement CNTFRQ_EL0. However,
> this reliance may lead to a non-functional timer in broken firmware.
> 
> For instance, some versions of Samsung's S-Boot don't implement it. It's
> also not possible to set it in the U-Boot, because it's booted in a lower
> exception level. CNTFRQ_EL0 is reported to be 0.
> 
> Thus, the only solution is to add a config option which, when enabled,
> returns a hard coded value (CONFIG_COUNTER_FREQUENCY) instead of querying
> CNTFRQ_EL0.

Why not read it from the clock-frequency property in the device tree?

This is what barebox is now doing, incidentally also for Samsung support, see:
https://lore.kernel.org/barebox/20250729203659.1858575-3-ivo.ivanov.ivanov1@gmail.com/

Cheers,
Ahmad

> 
> Signed-off-by: Kaustabh Chakraborty <kauschluss at disroot.org>
> ---
>  arch/arm/cpu/armv8/Kconfig         | 9 +++++++++
>  arch/arm/cpu/armv8/generic_timer.c | 4 ++++
>  2 files changed, 13 insertions(+)
> 
> diff --git a/arch/arm/cpu/armv8/Kconfig b/arch/arm/cpu/armv8/Kconfig
> index 199335cd6040d30a355a7fd5448a449fa7f550fb..d54d874b7caea200e1c05f522d8affe6d239a102 100644
> --- a/arch/arm/cpu/armv8/Kconfig
> +++ b/arch/arm/cpu/armv8/Kconfig
> @@ -4,6 +4,15 @@ config CMO_BY_VA_ONLY
>  	bool "Force cache maintenance to be exclusively by VA"
>  	depends on !SYS_DISABLE_DCACHE_OPS
>  
> +config ARMV8_CNTFRQ_BROKEN
> +	bool "Fix broken ARMv8 generic timer"
> +	depends on COUNTER_FREQUENCY > 0
> +	depends on SYS_ARCH_TIMER
> +	help
> +	  Say Y here if there is no trustable firmware present which sets
> +	  CNTFRQ_EL0 frequency before U-Boot. U-Boot will use the
> +	  CONFIG_COUNTER_FREQUENCY value as the reported frequency.
> +
>  config ARMV8_SPL_EXCEPTION_VECTORS
>  	bool "Install crash dump exception vectors"
>  	depends on SPL
> diff --git a/arch/arm/cpu/armv8/generic_timer.c b/arch/arm/cpu/armv8/generic_timer.c
> index 1de7ec596fc7cbbc3e78a241f163bc0a4fcad6b6..409570cf6fda79527e296b958368b552900524b1 100644
> --- a/arch/arm/cpu/armv8/generic_timer.c
> +++ b/arch/arm/cpu/armv8/generic_timer.c
> @@ -19,6 +19,10 @@ DECLARE_GLOBAL_DATA_PTR;
>  unsigned long notrace get_tbclk(void)
>  {
>  	unsigned long cntfrq;
> +
> +	if (IS_ENABLED(CONFIG_ARMV8_CNTFRQ_BROKEN))
> +		return CONFIG_COUNTER_FREQUENCY;
> +
>  	asm volatile("mrs %0, cntfrq_el0" : "=r" (cntfrq));
>  	return cntfrq;
>  }
> 
> ---
> base-commit: a5b3be1a37e19646b9d36c91f11b23f568be4f21
> change-id: 20251014-armv8-broken-cntfrq-f350f68fbf6e
> 
> Best regards,


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