linux-kernel - Re: [PATCH] riscv: Add RISC-V svpbmt extension

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Date:   Thu, 23 Sep 2021 15:27:34 +0530
From:   Anup Patel <anup@...infault.org>
To:     Nick Kossifidis <mick@....forth.gr>
Cc:     Guo Ren <guoren@...nel.org>, Anup Patel <anup.patel@....com>,
        Atish Patra <atish.patra@....com>,
        Palmer Dabbelt <palmerdabbelt@...gle.com>,
        Christoph Müllner <christoph.muellner@...ll.eu>,
        Philipp Tomsich <philipp.tomsich@...ll.eu>,
        Christoph Hellwig <hch@....de>,
        liush <liush@...winnertech.com>, wefu@...hat.com,
        Wei Wu (吴伟) <lazyparser@...il.com>,
        Drew Fustini <drew@...gleboard.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>,
        taiten.peng@...onical.com, aniket.ponkshe@...onical.com,
        Heinrich Schuchardt <heinrich.schuchardt@...onical.com>,
        gordan.markus@...onical.com, Guo Ren <guoren@...ux.alibaba.com>,
        Arnd Bergmann <arnd@...db.de>, Chen-Yu Tsai <wens@...e.org>,
        Maxime Ripard <maxime@...no.tech>,
        Daniel Lustig <dlustig@...dia.com>,
        Greg Favor <gfavor@...tanamicro.com>,
        Andrea Mondelli <andrea.mondelli@...wei.com>,
        Jonathan Behrens <behrensj@....edu>,
        Xinhaoqu <xinhaoqu@...wei.com>,
        Bill Huffman <huffman@...ence.com>,
        Allen Baum <allen.baum@...erantotech.com>,
        Josh Scheid <jscheid@...tanamicro.com>,
        Richard Trauben <rtrauben@...il.com>
Subject: Re: [PATCH] riscv: Add RISC-V svpbmt extension

On Thu, Sep 23, 2021 at 3:18 PM Nick Kossifidis <mick@....forth.gr> wrote:
>
> Στις 2021-09-23 12:42, Nick Kossifidis έγραψε:
> > Στις 2021-09-23 12:37, Anup Patel έγραψε:
> >> On Thu, Sep 23, 2021 at 2:55 PM Nick Kossifidis <mick@....forth.gr>
> >> wrote:
> >>>
> >>> Hello Guo,
> >>>
> >>> Στις 2021-09-23 10:27, guoren@...nel.org έγραψε:
> >>> diff --git a/Documentation/devicetree/bindings/riscv/cpus.yaml
> >>> b/Documentation/devicetree/bindings/riscv/cpus.yaml
> >>> index e534f6a7cfa1..1825cd8db0de 100644
> >>> --- a/Documentation/devicetree/bindings/riscv/cpus.yaml
> >>> +++ b/Documentation/devicetree/bindings/riscv/cpus.yaml
> >>> @@ -56,7 +56,9 @@ properties:
> >>>       enum:
> >>>         - riscv,sv32
> >>>         - riscv,sv39
> >>> +      - riscv,sv39,svpbmt
> >>>         - riscv,sv48
> >>> +      - riscv,sv48,svpbmt
> >>>         - riscv,none
> >>>
> >>> Isn't svpbmt orthogonal to the mmu type ? It's a functionality that
> >>> can
> >>> be present on either sv39/48/57 so why not have another "svpbmt"
> >>> property directly on the cpu node ?
> >>
> >> Actually, "mmu-type" would be a good place because it's page based
> >> memory attribute and paging can't exist without mmu translation mode.
> >>
> >> Also, "svpmbt" is indeed a CPU property so has to be feature
> >> individual
> >> CPU node. Hypothetically, a heterogeneous system is possible where
> >> some CPUs have "svpmbt" and some CPUs don't have "svpmbt". For
> >> example, a future FUxxx SoC might have a E-core and few S-cores
> >> where S-cores have Svpmbt whereas E-core does not have Svpmbt
> >> because it's an embedded core.
> >>
> >
> > I should say cpuX node, not the root /cpu node. We can have an svpbmt
> > property in the same way we have an mmu-type property.
> >
>
> I'm also thinking of future mmu-related extensions, e.g. what about
> svnapot ? Should we have mmu-type be riscv,sv39,svnapot and e.g.
> riscv.sv39,svpbmt,svnapot ? It'll become messy.

I agree, "mmu-type" can become longer in future but I was thinking
if all MMU related features can simply be comma-separated values
of one DT property.

Alternately, we can have "riscv,svpmbt" bool DT property in each
CPU node which will keep things simpler as compared to parsing
comma-separate string in "mmu-type" DT property.

Regards,
Anup

>
> Regards,
> Nick