Re: [PATCH v2] armv8: implement workaround for broken CNTFRQ_EL0 value

[Kaustabh Chakraborty <kauschluss@disroot.org>]

On 2025-10-17 20:40, Tom Rini wrote:
> On Fri, Oct 17, 2025 at 08:49:47PM +0530, 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.
>> 
>> Use gd->arch.timer_rate_hz to override the queried value if set. This
>> setting needs to be done in the board file, preferrably in 
>> timer_init().
>> 
>> Signed-off-by: Kaustabh Chakraborty <kauschluss@disroot.org>
>> ---
>> Changes in v2:
>> - Switched to using gd->arch.timer_rate_hz instead of a new config
>>   option (Ahmad Fatoum)
>> - Link to v1: 
>> https://lore.kernel.org/r/20251014-armv8-broken-cntfrq-v1-1-b63f9c69ffcb@disroot.org
>> ---
>>  arch/arm/cpu/armv8/generic_timer.c | 4 ++++
>>  1 file changed, 4 insertions(+)
>> 
>> diff --git a/arch/arm/cpu/armv8/generic_timer.c 
>> b/arch/arm/cpu/armv8/generic_timer.c
>> index 
>> 1de7ec596fc7cbbc3e78a241f163bc0a4fcad6b6..1bc72ed26ca31af8e8e72008ed7885cbde23b076 
>> 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 (gd->arch.timer_rate_hz)
>> +		return gd->arch.timer_rate_hz;
>> +
>>  	asm volatile("mrs %0, cntfrq_el0" : "=r" (cntfrq));
>>  	return cntfrq;
>>  }
> 
> This would impact coreprimevelte (as it's the only ARM64 platform to 
> set
> gd->arch.timer_rate_hz) but as it's also a Samsung platform it's likely
> in the same broken situation your platforms are.

I guess coreprimevelte is a a bit too old to have that issue. Either 
way,
it would be fine as the frequency is set to 26MHz [1], which is the 
timer
frequency of that device [2].

[1] 
https://elixir.bootlin.com/u-boot/v2025.10/source/arch/arm/mach-mmp/board.c#L31
[2] 
https://github.com/LegoLivesMatter/android_kernel_samsung_coreprimevelte/blob/android-5.1/arch/arm64/boot/dts/pxa1908.dtsi#L91

> 
> --
> Tom