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

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Date:   Wed, 19 May 2021 08:25:51 +0000
From:   Damien Le Moal <Damien.LeMoal@....com>
To:     Anup Patel <anup@...infault.org>,
        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" <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" <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 2021/05/19 16:16, Anup Patel wrote:
> 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.

Which is the root cause of most problems with riscv extension support in Linux.
All RISC-V foundation members need to apply pressure on the foundation and these
standard groups to deliver frozen specifications with an acceptable schedule.
c.f. the H extensions specs which are not yet frozen despite not having been
changed for months if not years.


-- 
Damien Le Moal
Western Digital Research