Re: [PATCH] clocksource/drivers/arm_arch_timer: Workaround bcm2712 broken EL2 virtual timer

[Gary Guo]

On Fri Jul 10, 2026 at 9:09 AM BST, Marc Zyngier wrote:
> It appears that the bcm2712 SoC found in the relatively popular
> RPi5 has a broken EL2 virtual timer.
>
> We do not know the reason why the timer isn't working (the timer
> is ticking, but the interrupt never fires), and the SoC vendor
> doesn't communicate on the reason why this isn't working, leaving
> users and maintainers in the dark.
>
> Paper over the issue by detecting the broken HW, falling back to
> the physical timer instead, and let the user know about it.
> Also taint the kernel as the machine is definitely not compliant
> with the spec, and we don't know what else is wrong with it.
>
> Reported-by: John <therealgraysky@xxxxxxxxx>
> Reported-by: Daniel Drake <dan@xxxxxxxxxxxxxxx>
> Reported-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
> Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
> Cc: Florian Fainelli <florian.fainelli@xxxxxxxxxxxx>
> Cc: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>

Tested on my RPi 5 and this does make it boot again

Tested-by: Gary Guo <gary@xxxxxxxxxxx>

Best,
Gary

> ---
>  drivers/clocksource/arm_arch_timer.c | 24 +++++++++++++++++++++++-
>  1 file changed, 23 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
> index 4adf756423de9..7b4a98df6962b 100644
> --- a/drivers/clocksource/arm_arch_timer.c
> +++ b/drivers/clocksource/arm_arch_timer.c
> @@ -1090,6 +1090,27 @@ static int __init arch_timer_common_init(void)
>  	return arch_timer_arch_init();
>  }
>  
> +static bool __init has_broken_el2_vtimer(void)
> +{
> +	/*
> +	 * SoCs described here have been found to be broken, though no
> +	 * explanation has been volunteered by the vendor. Let the user know
> +	 * we're papering over the vendor's lack of communication.
> +	 */
> +	static const char * const broken_el2_vtimer[] __initconst = {
> +		"brcm,bcm2712",
> +		NULL
> +	};
> +
> +	if (of_machine_compatible_match(broken_el2_vtimer)) {
> +		add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK);
> +		pr_warn_once(HW_ERR "Known broken EL2 virtual timer, ignoring it\n");
> +		return true;
> +	}
> +
> +	return false;
> +}
> +
>  /**
>   * arch_timer_select_ppi() - Select suitable PPI for the current system.
>   *
> @@ -1115,7 +1136,8 @@ static int __init arch_timer_common_init(void)
>  static enum arch_timer_ppi_nr __init arch_timer_select_ppi(void)
>  {
>  	if (is_kernel_in_hyp_mode()) {
> -		if (arch_timer_ppi[ARCH_TIMER_HYP_VIRT_PPI])
> +		if (arch_timer_ppi[ARCH_TIMER_HYP_VIRT_PPI] &&
> +		    !has_broken_el2_vtimer())
>  			return ARCH_TIMER_HYP_VIRT_PPI;
>  
>  		pr_warn_once(FW_BUG "VHE-capable CPU without EL2 virtual timer interrupt\n");