[PATCH] armv8: implement workaround for broken CNTFRQ_EL0 value

[Kaustabh Chakraborty]

On 2025-10-15 07:48, Ahmad Fatoum wrote:
> 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/

In barebox, the timer is a device driver, like in linux. In U-Boot 
however, it is
instead part of the ARM architecture specific code. And it is being 
accessed by
a single function, not dependent on others.

However, in U-Boot i found gd->arch.timer_rate_hz, which can be used, 
like so:

	if (gd->arch.timer_rate_hz)
		return gd->arch.timer_rate_hz;

This property needs to be set in board file – in timer_init(). So I will 
do
that instead.

> 
> 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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