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Subject: Re: [PATCH] riscv: fix multi-letter extension compare
From:   Jessica Clarke <jrtc27@jrtc27.com>
In-Reply-To: <20221006070257.11632-1-nikita.shubin@maquefel.me>
Date:   Thu, 6 Oct 2022 17:10:37 +0100
Cc:     Atish Patra <atishp@rivosinc.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Yury Norov <yury.norov@gmail.com>,
        Nikita Shubin <n.shubin@yadro.com>,
        Anup Patel <anup@brainfault.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux <linux@yadro.com>,
        Tsukasa OI <research_trasio@irq.a4lg.com>,
        Sunil V L <sunilvl@ventanamicro.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Jisheng Zhang <jszhang@kernel.org>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Guo Ren <guoren@kernel.org>, linux-riscv@lists.infradead.org,
        Heiko Stuebner <heiko@sntech.de>
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Message-Id: <E75B9DE8-8943-4F40-8374-2E74C50D7A00@jrtc27.com>
References: <20221006070257.11632-1-nikita.shubin@maquefel.me>
To:     Nikita Shubin <nikita.shubin@maquefel.me>
Precedence: bulk

On 6 Oct 2022, at 08:02, Nikita Shubin <nikita.shubin@maquefel.me> =
wrote:
>=20
> From: Nikita Shubin <n.shubin@yadro.com>
>=20
> Increment ext pointer to match letters after 'z' character, so it =
points
> to actual extension name.
>=20
> Fixes: 4905ec2fb7e6 ("RISC-V: Add sscofpmf extension support")
> Signed-off-by: Nikita Shubin <n.shubin@yadro.com>
> ---
> Hello Atish,
>=20
> Sorry to blame you, but i double checked this and i don't see a way=20
> how extensions from device tree could be successfully matched with=20
> encoded ones without incrementing the ext pointer or prepending=20
> the encoded extensions with 'z' letter.
>=20
> I have no idea how this could slip through review - am i missing =
something ?
> ---
> arch/riscv/kernel/cpufeature.c | 1 +
> 1 file changed, 1 insertion(+)
>=20
> diff --git a/arch/riscv/kernel/cpufeature.c =
b/arch/riscv/kernel/cpufeature.c
> index 3b5583db9d80..031546052dc0 100644
> --- a/arch/riscv/kernel/cpufeature.c
> +++ b/arch/riscv/kernel/cpufeature.c
> @@ -199,6 +199,7 @@ void __init riscv_fill_hwcap(void)
> 				this_hwcap |=3D isa2hwcap[(unsigned =
char)(*ext)];
> 				set_bit(*ext - 'a', this_isa);
> 			} else {
> +				ext++;
> 				SET_ISA_EXT_MAP("sscofpmf", =
RISCV_ISA_EXT_SSCOFPMF);
> 				SET_ISA_EXT_MAP("svpbmt", =
RISCV_ISA_EXT_SVPBMT);
> 				SET_ISA_EXT_MAP("zicbom", =
RISCV_ISA_EXT_ZICBOM);

This makes no sense. The single-letter prefix that=E2=80=99s part of the =
actual
extension name is already in the string there, whether s or z. Your
code means (a) you=E2=80=99d have to double up the prefix in the device =
tree
(which is obviously wrong) (b) you could use s or z interchangeably as
the first character (e.g. you=E2=80=99d make zzicbom and szicbom parsed =
as
zicbom).

Are you actually seeing a bug? Did you test this at all? This looks
like you misunderstood code and wrote an untested patch for a bug you
thought you saw.

Jess

> --=20
> 2.35.1
>=20
>=20
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