linux-kernel - Re: [RFC PATCH 3/5] RISC-V: Add cpu

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Date:   Wed, 22 Aug 2018 20:54:51 +0530
From:   Anup Patel <anup@...infault.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Mark Rutland <mark.rutland@....com>,
        Damien Le Moal <Damien.LeMoal@....com>,
        "palmer@...ive.com" <palmer@...ive.com>,
        "linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>,
        Atish Patra <atish.patra@....com>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC PATCH 3/5] RISC-V: Add cpu_operatios structure

On Wed, Aug 22, 2018 at 11:33 AM, Christoph Hellwig <hch@...radead.org> wrote:
> On Tue, Aug 21, 2018 at 10:34:38PM +0530, Anup Patel wrote:
>> The cpu_operations is certainly required because SOC vendors will add
>> vendor-specific mechanism to selectively bringing-up CPUs/HARTs instead
>> of all CPUs entering Linux kernel simultaneously. In fact, we might also end-up
>> having CPU ON/OFF operations in SBI.
>
> Your forgot an essential part in your analysis:  Right now we only have
> one single way to deal with cpu on/offlining, and that is the dummy WFI
> kind.  Once other ways show up we can build proper infrastructure, but
> until then this is just a white elephant as we have no idea how these
> abstractions will look like.
>
> And my hope is that we'll just see new SBI calls, in which case we'll
> just need SBI and dummy version and can avoid all the indirect calls.

IMHO, rather than waiting for new CPU ON/OFF methods to come-up we
can keep the cpu_operations ready. Also, we are not re-inventing anything
here which we might have to discard later because cpu_operations are
already tried and hardened for Linux ARM64.

I agree with you that in long-term SBI-based CPU ON/OFF will be widely
used. Most likely we will have at-least two CPU ON/OFF methods:
1. Existing lottery based spinning
2. New SBI calls

Regards,
Anup