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

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Message-ID: <CAAhSdy2E_H-qiyPbpGn68WftTU=2rA6VL7g0PGwyorwQM3kmqw@mail.gmail.com>
Date:   Thu, 23 Aug 2018 11:15:29 -0400
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 Thu, Aug 23, 2018 at 9:37 AM, Christoph Hellwig <hch@...radead.org> wrote:
> On Wed, Aug 22, 2018 at 08:54:51PM +0530, Anup Patel wrote:
>> 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.
>
> Which is a different cpu architecture, and has shown to actually need
> it.  IFF we end up needing it on riscv we can still copy and paste
> it from AMD64.
>
>
>> 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
>
> And in this most likely case there is no need for an ops vector,
> a simple if/else will be much simpler and cleaner.

Like Atish mentioned, there is a possibility of existing HW going for
spin-table method instead of lottery based spinning.

This means in future we will have three or more CPU ON/OFF methods:
1. Existing lottery based spinning
2. spin-table
3. New SBI calls

I am fine dropping cpu_operations now but I am sure we will end-up
adding it back eventually.

Regards,
Anup