linux-kernel - Re: [PATCH 1/1] riscv: default to CONFIG_RISCV_SBI

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Date:   Fri, 17 Dec 2021 17:39:22 +0530
From:   Anup Patel <anup@...infault.org>
To:     Heinrich Schuchardt <heinrich.schuchardt@...onical.com>,
        Atish Patra <atishp@...shpatra.org>,
        Palmer Dabbelt <palmer@...belt.com>
Cc:     Jessica Clarke <jrtc27@...c27.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] riscv: default to CONFIG_RISCV_SBI_V01=n

+Atish

On Fri, Dec 17, 2021 at 4:12 PM Heinrich Schuchardt
<heinrich.schuchardt@...onical.com> wrote:
>
>
>
> On 12/16/21 17:51, Jessica Clarke wrote:
> > On 16 Dec 2021, at 14:17, Heinrich Schuchardt <heinrich.schuchardt@...onical.com> wrote:
> >>
> >> On 12/16/21 14:49, Jessica Clarke wrote:
> >>> On 16 Dec 2021, at 12:35, Heinrich Schuchardt <heinrich.schuchardt@...onical.com> wrote:
> >>>>
> >>>> The SBI 0.1 specification is obsolete. The current version is 0.3.
> >>>> Hence we should not rely by default on SBI 0.1 being implemented.
> >>> It’s what BBL implements, and some people are still using it,
> >>> especially given early hardware shipped before OpenSBI grew in
> >>> popularity.
> >>> Jess
> >>
> >> Do you mean BBL is not developed anymore?
> >>
> >> Some people may still be using a 0.1 SBI. But that minority stuck on an outdated software stack does not justify defaulting to deprecated settings in future Linux releases.
> >
> > BBL is still actively maintained; its most recent commit was 24 days
> > ago. Also, the amount of code CONFIG_RISCV_SBI_V01 affects is tiny, so
> > I see no tangible benefit from making this change, just unnecessary
> > breakage of perfectly functional systems.
>
> Only the default is changed. How could this break any existing system?
> You can still compile with the deprecated setting.
>
> I can not see why we should keep a default that will cause issues on
> systems complying to the current SBI specification.

I agree with Heinrich.

Almost all SBI implementations (OpenSBI, EDK2, KVM, Xvisor, etc) are
providing at least SBI v0.2 and we can't endlessly wait for BBL to move
away from SBI v0.1. We can't totally remove SBI v0.1 but we should
at least disable it by default.

Same rationale applies to the spinwait CPU operations which are only
required for systems using BBL. The sparse HART id series from Atish
can include a patch to have spinwait CPU operations disabled by
default.

@Atish/Palmer, what do you think ?

Regards,
Anup