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

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Date:   Thu, 23 Aug 2018 09:55:45 +0530
From:   Atish Patra <atish.patra@....com>
To:     Anup Patel <anup@...infault.org>,
        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>,
        "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 8/22/18 8:54 PM, Anup Patel wrote:
> 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
> 

I am fine with either keeping the cpu_ops infrastructure for now or 
reintroducing again along with better smp enablement methods.

Anyways, there were concerns about all existing booting method (all cpu 
thrown to Linux at the same time). I was thinking to adopt spin table 
boot method for RISC-V as well. I can drop this patch now and 
reintroduce with spin table boot method.

Any thoughts ?

Regards,
Atish