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

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Message-ID: <20210519064435.GA3076809@x1>
Date:   Tue, 18 May 2021 23:44:35 -0700
From:   Drew Fustini <drew@...gleboard.org>
To:     Guo Ren <guoren@...nel.org>
Cc:     Christoph Hellwig <hch@....de>, Anup Patel <anup.patel@....com>,
        Palmer Dabbelt <palmerdabbelt@...gle.com>, wefu@...hat.com,
        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@...ive.com, Nick Kossifidis <mick@....forth.gr>,
        Benjamin Koch <snowball@...b.de>,
        Matteo Croce <mcroce@...ux.microsoft.com>,
        Wei Fu <tekkamanninja@...il.com>
Subject: Re: [PATCH RFC 0/3] riscv: Add DMA_COHERENT support

On Wed, May 19, 2021 at 01:48:23PM +0800, Guo Ren wrote:
> On Wed, May 19, 2021 at 1:20 PM Christoph Hellwig <hch@....de> wrote:
> >
> > On Wed, May 19, 2021 at 05:04:13AM +0000, guoren@...nel.org wrote:
> > > From: Guo Ren <guoren@...ux.alibaba.com>
> > >
> > > The RISC-V ISA doesn't yet specify how to query or modify PMAs, so let
> > > vendors define the custom properties of memory regions in PTE.
> >
> > Err, hell no.   The ISA needs to gets this fixed first.  Then we can
> > talk about alternatives patching things in or trapping in the SBI.
> > But if the RISC-V ISA can't get these basic done after years we can't
> > support it in Linux at all.
> 
> The patchset just leaves a configuration chance for vendors. Before
> RISC-V ISA fixes it, we should give the chance to let vendor solve
> their real chip issues.

This patch series looks like it might be useful for the StarFive JH7100
[1] [2] too as it has peripherals on a non-coherent interconnect. GMAC,
USB and SDIO require that the L2 cache must be manually flushed after
DMA operations if the data is intended to be shared with U74 cores [2].

There is the RISC-V Cache Management Operation, or CMO, task group [3]
but I am not sure if that can help the SoC's that have already been
fabbed like the the D1 and the JH7100.

thanks,
drew

[1] https://github.com/starfive-tech/beaglev_doc/blob/main/JH7100%20Data%20Sheet%20V01.01.04-EN%20(4-21-2021).pdf
[2] https://github.com/starfive-tech/beaglev_doc/blob/main/JH7100%20Cache%20Coherence%20V1.0.pdf
[3] https://github.com/riscv/riscv-CMOs