Re: [RFC PATCH v2 09/35] LoongArch: Switch over to GENERIC_CPU_DEVICES

[Jonathan Cameron]

On Wed, 13 Sep 2023 16:37:57 +0000
James Morse <james.morse@xxxxxxx> wrote:

> Now that GENERIC_CPU_DEVICES calls arch_register_cpu(), which can be
> overridden by the arch code, switch over to this to allow common code
> to choose when the register_cpu() call is made.
> 
> This allows topology_init() to be removed.
> 
> This is an intermediate step to the logic being moved to drivers/acpi,
> where GENERIC_CPU_DEVICES will do the work when booting with acpi=off.
> 
> Signed-off-by: James Morse <james.morse@xxxxxxx>
> ---
>  arch/loongarch/Kconfig           |  1 +
>  arch/loongarch/kernel/topology.c | 29 ++---------------------------
>  2 files changed, 3 insertions(+), 27 deletions(-)
> 
> diff --git a/arch/loongarch/Kconfig b/arch/loongarch/Kconfig
> index 2bddd202470e..5bed51adc68c 100644
> --- a/arch/loongarch/Kconfig
> +++ b/arch/loongarch/Kconfig
> @@ -72,6 +72,7 @@ config LOONGARCH
>  	select GENERIC_CLOCKEVENTS
>  	select GENERIC_CMOS_UPDATE
>  	select GENERIC_CPU_AUTOPROBE
> +	select GENERIC_CPU_DEVICES
>  	select GENERIC_ENTRY
>  	select GENERIC_GETTIMEOFDAY
>  	select GENERIC_IOREMAP if !ARCH_IOREMAP
> diff --git a/arch/loongarch/kernel/topology.c b/arch/loongarch/kernel/topology.c
> index caa7cd859078..8e4441c1ff39 100644
> --- a/arch/loongarch/kernel/topology.c
> +++ b/arch/loongarch/kernel/topology.c
> @@ -7,20 +7,13 @@
>  #include <linux/percpu.h>
>  #include <asm/bootinfo.h>
>  
> -static DEFINE_PER_CPU(struct cpu, cpu_devices);
> -
>  #ifdef CONFIG_HOTPLUG_CPU
>  int arch_register_cpu(int cpu)
>  {
> -	int ret;
>  	struct cpu *c = &per_cpu(cpu_devices, cpu);
>  
> -	c->hotpluggable = 1;

This is a bit subtle.  Can loongarch hotplug a CPU that
is also io_master(cpu)?  I have no idea if there is a subtle difference
between.

1) CPUs present at boot where if they are an io_master they are not allowed
   to be hot removed.
2) CPUs that turn up (hotplugged) later which are an io_master and by original code
   can be removed.

My guess is that no io_master CPU can be hotplugged in making this irrelevant
and your code correct as the =1 is just a micro optimizatoin.

If we can confirm that, a one line addition to the patch description would be
great. 

Otherwise LGTM

> -	ret = register_cpu(c, cpu);
> -	if (ret < 0)
> -		pr_warn("register_cpu %d failed (%d)\n", cpu, ret);
> -
> -	return ret;
> +	c->hotpluggable = !io_master(cpu);
> +	return register_cpu(c, cpu);
>  }
>  EXPORT_SYMBOL(arch_register_cpu);
>  
> @@ -33,21 +26,3 @@ void arch_unregister_cpu(int cpu)
>  }
>  EXPORT_SYMBOL(arch_unregister_cpu);
>  #endif
> -
> -static int __init topology_init(void)
> -{
> -	int i, ret;
> -
> -	for_each_present_cpu(i) {
> -		struct cpu *c = &per_cpu(cpu_devices, i);
> -
> -		c->hotpluggable = !io_master(i);
> -		ret = register_cpu(c, i);
> -		if (ret < 0)
> -			pr_warn("topology_init: register_cpu %d failed (%d)\n", i, ret);
> -	}
> -
> -	return 0;
> -}
> -
> -subsys_initcall(topology_init);