linux-kernel - Re: [PATCH RFC 0/3] riscv: Add DMA

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Date:   Wed, 19 May 2021 12:44:55 +0530
From:   Anup Patel <anup@...infault.org>
To:     Drew Fustini <drew@...gleboard.org>
Cc:     Christoph Hellwig <hch@....de>, Guo Ren <guoren@...nel.org>,
        Anup Patel <anup.patel@....com>,
        Palmer Dabbelt <palmerdabbelt@...gle.com>, wefu@...hat.com,
        Wei Wu (吴伟) <lazyparser@...il.com>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-sunxi@...ts.linux.dev, Guo Ren <guoren@...ux.alibaba.com>,
        Paul Walmsley <paul.walmsley@...ive.com>
Subject: Re: [PATCH RFC 0/3] riscv: Add DMA_COHERENT support

On Wed, May 19, 2021 at 12:24 PM Drew Fustini <drew@...gleboard.org> wrote:
>
> On Wed, May 19, 2021 at 08:06:17AM +0200, Christoph Hellwig wrote:
> > On Wed, May 19, 2021 at 02:05:00PM +0800, Guo Ren wrote:
> > > Since the existing RISC-V ISA cannot solve this problem, it is better
> > > to provide some configuration for the SOC vendor to customize.
> >
> > We've been talking about this problem for close to five years.  So no,
> > if you don't manage to get the feature into the ISA it can't be
> > supported.
>
> Isn't it a good goal for Linux to support the capabilities present in
> the SoC that a currently being fab'd?
>
> I believe the CMO group only started last year [1] so the RV64GC SoCs
> that are going into mass production this year would not have had the
> opporuntiy of utilizing any RISC-V ISA extension for handling cache
> management.

The current Linux RISC-V policy is to only accept patches for frozen or
ratified ISA specs.
(Refer, Documentation/riscv/patch-acceptance.rst)

This means even if emulate CMO instructions in OpenSBI, the Linux
patches won't be taken by Palmer because CMO specification is
still in draft stage.

Also, we all know how much time it takes for RISCV international
to freeze some spec. Judging by that we are looking at another
3-4 years at minimum.

Regards,
Anup