Re: [PATCH v3 09/47] arm64: mpam: Add cpu_pm notifier to restore MPAM sysregs

Gavin Shan
2026-01-19T06:50:34Z

Hi Ben,

On 1/13/26 12:58 AM, Ben Horgan wrote:
> From: James Morse <james.morse@arm.com>
>
> The MPAM system registers will be lost if the CPU is reset during PSCI's
> CPU_SUSPEND.
>
> Add a PM notifier to restore them.
>
> mpam_thread_switch(current) can't be used as this won't make any changes if
> the in-memory copy says the register already has the correct value. In
> reality the system register is UNKNOWN out of reset.
>
> Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
> Signed-off-by: James Morse <james.morse@arm.com>
> Signed-off-by: Ben Horgan <ben.horgan@arm.com>
> ---
> arch/arm64/kernel/mpam.c | 30 ++++++++++++++++++++++++++++++
> 1 file changed, 30 insertions(+)
>

One question below...

> diff --git a/arch/arm64/kernel/mpam.c b/arch/arm64/kernel/mpam.c
> index e6feff2324ac..dbe0a2d05abb 100644
> --- a/arch/arm64/kernel/mpam.c
> +++ b/arch/arm64/kernel/mpam.c
> @@ -4,6 +4,7 @@
> #include <asm/mpam.h>
>
> #include <linux/arm_mpam.h>
> +#include <linux/cpu_pm.h>
> #include <linux/jump_label.h>
> #include <linux/percpu.h>
>
> @@ -13,12 +14,41 @@ DEFINE_PER_CPU(u64, arm64_mpam_current);
>
> u64 arm64_mpam_global_default;
>
> +static int mpam_pm_notifier(struct notifier_block *self,
> + unsigned long cmd, void *v)
> +{
> + u64 regval;
> + int cpu = smp_processor_id();
> +
> + switch (cmd) {
> + case CPU_PM_EXIT:
> + /*
> + * Don't use mpam_thread_switch() as the system register
> + * value has changed under our feet.
> + */
> + regval = READ_ONCE(per_cpu(arm64_mpam_current, cpu));
> + write_sysreg_s(regval, SYS_MPAM1_EL1);
> + isb();
> +
> + write_sysreg_s(regval, SYS_MPAM0_EL1);
> +
> + return NOTIFY_OK;
> + default:
> + return NOTIFY_DONE;
> + }
> +}
> +
> +static struct notifier_block mpam_pm_nb = {
> + .notifier_call = mpam_pm_notifier,
> +};
> +
> static int __init arm64_mpam_register_cpus(void)
> {
> u64 mpamidr = read_sanitised_ftr_reg(SYS_MPAMIDR_EL1);
> u16 partid_max = FIELD_GET(MPAMIDR_EL1_PARTID_MAX, mpamidr);
> u8 pmg_max = FIELD_GET(MPAMIDR_EL1_PMG_MAX, mpamidr);
>
> + cpu_pm_register_notifier(&mpam_pm_nb);

Need we ensure MPAM capability exists in the hardware before the notifier is registerred?
Otherwise, mpam_pm_notifier() can accesses SYS_MPAM0_EL1 and SYS_MPAM1_EL1 system registers
which may not supported by the hardware.

> return mpam_register_requestor(partid_max, pmg_max);
> }
> /* Must occur before mpam_msc_driver_init() from subsys_initcall() */

Thanks,
Gavin