linux-kernel - Re: [RFC PATCH 1/2] riscv: vendors: andes: Add support to configure the PMA regions

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Date:   Thu, 8 Sep 2022 15:39:49 +0100
From:   "Lad, Prabhakar" <prabhakar.csengg@...il.com>
To:     Conor Dooley <Conor.Dooley@...rochip.com>,
        Palmer Dabbelt <palmer@...belt.com>
Cc:     Biju Das <biju.das.jz@...renesas.com>, atishp@...shpatra.org,
        guoheyi@...ux.alibaba.com, guoren@...ux.alibaba.com,
        "Lad, Prabhakar" <prabhakar.mahadev-lad.rj@...renesas.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>, atishp@...osinc.com,
        apatel@...tanamicro.com,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 1/2] riscv: vendors: andes: Add support to configure
 the PMA regions

On Thu, Sep 8, 2022 at 3:04 PM <Conor.Dooley@...rochip.com> wrote:
>
> On 08/09/2022 14:01, Biju Das wrote:
> > EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
> >
> > Hi Conor,
> >
> > Thanks for the feedback.
> >
> >> Subject: Re: [RFC PATCH 1/2] riscv: vendors: andes: Add support to
> >> configure the PMA regions
> >>
> >> On 08/09/2022 09:39, Biju Das wrote:
> >>> EXTERNAL EMAIL: Do not click links or open attachments unless you know
> >>> the content is safe
> >>>
> >>> Hi Conor, Atish,
> >>>
> >>> What RISC-V devices you have?
> >>
> >> A bunch ;)
> >>
> >> A __couple__ PolarFire SoC boards, HiFive Unleashed, D1 Nezha, Canaan
> >> k210 MAIX something & the VisionFive.
> >
> > If standard DMA api works without any issue means, on these platforms
> > IO Coherence port is enabled in the hardware. So all peripherals
> > involving DMA work as expected.
> >
> >>> Ours is RISC-V uniprocessor without IO Coherence Port.
> >>
> >> What does "IO Coherence Port" mean? Zicbo*?
> >
> > The HW will provide coherency between CPU and peripheral.
> >
> > If Zibco* is uniprocessor, then highly it may not have IO coherence
> > Port enabled in their design.
>
> Zicbo* are cache management extensions as Geert pointed out.
>
> >
> > Guo, Please confirm.
> >
> >>
> >>> Currently USB, ethernet, SDHI/eMMC doesn't work with standard DMA
> >>> api's.
> >>
> >> Sounds pretty similar to the D1 so.
> >>
> >>> On RISC-V world, how do we handle DMA api for uniprocessor without IO
> >>> Coherence Port?
> >>
> >> If you do mean Zicbo* you're into errata territory there & I don't know
> >> if that'll be acceptable upstream - not for me to make that call...
> >
> > It is not errata for sure. It is a HW design where we don't have
> > IO cache coherency port enabled in the HW. So looks like it is not
> > an extension or errata but it is core stuff.
>
> If you do non-coherent stuff that is not Zicbo*, the precedence set by
> the D1 is errata. As I said to Prabhakar earlier, do a
> `git grep "ERRATA_THEAD*`. I am not a maintainer so I don't know the
> "rules" about doing cache management without the dedicated extensions
> are.
>
Maybe we could have a discussion about this topic at LPC too ;)

Cheers,
Prabhakar