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

[Tom Rini]

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 <kauschl...@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-b63f9c69f...@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;
>  }

If we do a size comparison on say imx8mm_evk_fspi, we see:
               u-boot: add: 1/0, grow: 5/0 bytes: 68/0 (68)
                 function                                   old     new   delta
                 get_tbclk                                    -      32     +32
                 timer_get_us                                44      56     +12
                 static.get_timer_us                         44      56     +12
                 udelay                                     116     120      +4
                 static.efi_stall                            88      92      +4
                 cli_readline_into_buffer                  2452    2456      +4
               spl-u-boot-spl: add: 1/0, grow: 3/0 bytes: 52/0 (52)
                 function                                   old     new   delta
                 get_tbclk                                    -      32     +32
                 static.get_timer_us                         44      56     +12
                 timer_get_us                                44      48      +4
                 __udelay                                    64      68      +4

So I think the CONFIG option from before was part of what we want too.
I think this happens late enough that we can do:
#ifdef CONFIG_WHATEVER
        return gd->arch.timer_rate_hz;
#else
        asm(...);
        return cntfrq;
#endif

Yes? Thanks.

-- 
Tom

signature.asc
Description: PGP signature