linux-kernel - Re: [PATCH 1/1] irqchip/gicv3: iterate over possible CPUs by for_each_possible

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Date:   Fri, 15 Sep 2017 11:05:25 +0800
From:   zijun_hu <zijun_hu@...o.com>
To:     Marc Zyngier <marc.zyngier@....com>
Cc:     linux-kernel@...r.kernel.org, zijun_hu@....com, tglx@...utronix.de,
        jason@...edaemon.net, james.morse@....com, sudeep.holla@....com
Subject: Re: [PATCH 1/1] irqchip/gicv3: iterate over possible CPUs by
 for_each_possible_cpu()

On 09/15/2017 03:20 AM, Marc Zyngier wrote:
> On Thu, Sep 14 2017 at  1:15:14 pm BST, zijun_hu <zijun_hu@...o.com> wrote:
>> From: zijun_hu <zijun_hu@....com>
>>
>> get_cpu_number() doesn't use existing helper to iterate over possible
>> CPUs, so error happens in case of discontinuous @cpu_possible_mask
>> such as 0b11110001.
> 
> Do you have an example of such a situation? Your patch is definitely an
> improvement, but I'd like to understand how you get there...
> 
> Thanks,
> 
> 	M.
> 
a few conditions which maybe result in discontiguous @cpu_possible_mask
are noticed and considered of by ARM64 init code as indicated by bellow code
segments:
in arch/arm64/kernel/smp.c :
void __init smp_init_cpus(void) {
......
	/*
	 * We need to set the cpu_logical_map entries before enabling
	 * the cpus so that cpu processor description entries (DT cpu nodes
	 * and ACPI MADT entries) can be retrieved by matching the cpu hwid
	 * with entries in cpu_logical_map while initializing the cpus.
	 * If the cpu set-up fails, invalidate the cpu_logical_map entry.
	 */
	for (i = 1; i < nr_cpu_ids; i++) {
		if (cpu_logical_map(i) != INVALID_HWID) {
			if (smp_cpu_setup(i))
				cpu_logical_map(i) = INVALID_HWID;
		}
	}
......
}

/*
 * Initialize cpu operations for a logical cpu and
 * set it in the possible mask on success
 */
static int __init smp_cpu_setup(int cpu)
{
	if (cpu_read_ops(cpu))
		return -ENODEV;

	if (cpu_ops[cpu]->cpu_init(cpu))
		return -ENODEV;

	set_cpu_possible(cpu, true);

	return 0;
}

i browses GICv3 drivers code and notice that a little weird code segments