linux-kernel - Re: [PATCH v1 0/2] Provide a fraemework for RISC-V ISA extensions

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Message-ID: <CAOnJCULirV30g6ti7QvY40=p_W9+-XG+nAbxiZ4qDZLo28RJ6A@mail.gmail.com>
Date:   Fri, 7 Jan 2022 18:24:12 -0800
From:   Atish Patra <atishp@...shpatra.org>
To:     Palmer Dabbelt <palmer@...belt.com>,
        Tsukasa OI <research_trasio@....a4lg.com>
Cc:     "linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>,
        Atish Patra <atishp@...osinc.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Anup Patel <anup@...infault.org>,
        Damien Le Moal <damien.lemoal@....com>,
        devicetree <devicetree@...r.kernel.org>,
        Jisheng Zhang <jszhang@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Rob Herring <robh+dt@...nel.org>
Subject: Re: [PATCH v1 0/2] Provide a fraemework for RISC-V ISA extensions

On Fri, Jan 7, 2022 at 1:58 PM Palmer Dabbelt <palmer@...belt.com> wrote:
>
> On Fri, 24 Dec 2021 13:16:30 PST (-0800), atishp@...shpatra.org wrote:
> > This series implements a generic framework to parse multi-letter ISA
> > extensions. It introduces a new DT node that can be under /cpus or
> > individual cpu depends on the platforms with homogeneous or heterogeneous
> > harts. This version of the series only allows adds support for homogeneous
> > harts as there are no platforms with heterogeneous harts yet. However,
> > the DT binding allows both.
> >
> > The patch also indicates the user space about the available ISA extensions
> > via /proc/cpuinfo.
> >
> > Here is the example output of /proc/cpuinfo:
> > (with debug patches in Qemu and Linux kernel)
> >
> > / # cat /proc/cpuinfo
> > processor     : 0
> > hart          : 0
> > isa           : rv64imafdcsu
> > isa-ext               : sstc,sscofpmf
> > mmu           : sv48
>
> IMO this is the wrong way to go.  I get that the ISA string is very
> complicated to parse, but we've tried to come up with other
> representations of this we've ended up having that interface break when
> the ISA string rules eventually change.  We should just stick to the ISA
> string for these interfaces, as that's at least something everyone can
> agree on because they're defined by the spec.
>

Fair enough.

> That said, we should add the spec versions into this interface.  At
> least the user spec version is relevant here, but given that we're
> passing through some other priv-level details we might as well pass that
> though too.
>

Tsukasa already has a version that extends the isa string parsing for
multi-letter extensions
and versions parsing. We just need to add the ISA bitmap support on top of it.

I will coordinate with Tsukasa to have a complete framework based on
string parsing.
It would be good to have this series asap as all other series  (perf,
svpbmt) will rely on it.

> > processor     : 1
> > hart          : 1
> > isa           : rv64imafdcsu
> > isa-ext               : sstc,sscofpmf
> > mmu           : sv48
> >
> > processor     : 2
> > hart          : 2
> > isa           : rv64imafdcsu
> > isa-ext               : sstc,sscofpmf
> > mmu           : sv48
> >
> > processor     : 3
> > hart          : 3
> > isa           : rv64imafdcsu
> > isa-ext               : sstc,sscofpmf
> > mmu           : sv48
> >
> > Anybody adding support for any new multi-letter extensions should add an
> > entry to the riscv_isa_ext_id and the isa extension array.
> > E.g. The patch[1] adds the support for sscofpmf extension.
> >
> > [1] https://github.com/atishp04/linux/commit/a23157264118d6fd905fd08d8717c7df03078bb1
> >
> > Atish Patra (2):
> > RISC-V: Provide a framework for parsing multi-letter ISA extensions
> > dt-bindings: riscv: Add DT binding for RISC-V ISA extensions
> >
> > .../devicetree/bindings/riscv/cpus.yaml       |  9 +++
> > arch/riscv/include/asm/hwcap.h                | 31 ++++++++++
> > arch/riscv/kernel/cpu.c                       | 16 +++++
> > arch/riscv/kernel/cpufeature.c                | 58 ++++++++++++++++++-
> > 4 files changed, 113 insertions(+), 1 deletion(-)



--
Regards,
Atish