Re: [PATCH v12 14/25] irqchip/sifive-plic: Convert PLIC driver into a platform driver

[Thomas Gleixner <tglx@linutronix.de>]

On Fri, Feb 16 2024 at 22:41, Anup Patel wrote:
> On Fri, Feb 16, 2024 at 9:03 PM Thomas Gleixner <tglx@linutronix.de> wrote:
>> I don't think that removing the setup protection is correct.
>>
>> Assume you have maxcpus=N on the kernel command line, then the above
>> for_each_online_cpu() loop would result in cpuhp_setup == true when the
>> instances for the not onlined CPUs are set up, no?
>
> A platform can have multiple PLIC instances where each PLIC
> instance targets a subset of HARTs (or CPUs).
>
> Previously (before this patch), we were probing PLIC very early so on
> a platform with multiple PLIC instances, we need to ensure that cpuhp
> setup is done only after PLIC context associated with boot CPU is
> initialized hence the plic_cpuhp_setup_done check.
>
> This patch converts PLIC driver into a platform driver so now PLIC
> instances are probed after all available CPUs are brought-up. In this
> case, the cpuhp setup must be done only after PLIC context of all
> available CPUs are initialized otherwise some of the CPUs crash
> in plic_starting_cpu() due to lack of PLIC context initialization.

You're missing the point.

Assume you have 8 CPUs and 2 PLIC instances one for CPU0-3 and one for
CPU4-7.

Add "maxcpus=4" on the kernel command line, then only the first 4 CPUs
are brought up.

So at probe time cpu_online_mask has bit 0,1,2,3 set.

When the first PLIC it probed the loop which checks the context for each
online CPU will not clear cpuhp_setup and the hotplug state is installed.

Now the second PLIC is probed (the one for the offline CPUs 4-7) and the
loop will again not clear cpuhp_setup and it tries to install the state
again, no?

Thanks,

        tglx