linux-kernel - Re: [PATCH v2 5/5] lib/cpumask: add FORCE_NR

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Message-ID: <CAMuHMdVeHPR_JH0dJ+WSddjd8_hax70JRzd4pqHLNvgbh+-znw@mail.gmail.com>
Date:   Tue, 18 Oct 2022 15:50:31 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Andreas Schwab <schwab@...e.de>
Cc:     Yury Norov <yury.norov@...il.com>, linux-kernel@...r.kernel.org,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Sander Vanheule <sander@...nheule.net>,
        Alexey Klimov <klimov.linux@...il.com>,
        Eric Biggers <ebiggers@...gle.com>,
        linux-riscv <linux-riscv@...ts.infradead.org>
Subject: Re: [PATCH v2 5/5] lib/cpumask: add FORCE_NR_CPUS config option

Hi Andreas,

On Tue, Oct 18, 2022 at 3:41 PM Andreas Schwab <schwab@...e.de> wrote:
> On Okt 18 2022, Geert Uytterhoeven wrote:
> > Moreover, this cannot be used on all systems.  E.g. on Icicle Kit with
> > Microchip PolarFire SoC, CONFIG_NR_CPUS needs to be larger than 4,
> > as the system has actually 5 CPU cores (1xE51 and 4xU54), but Linux
> > runs only on 4 of them.  So you cannot use FORCE_NR_CPUS=y.
>
> But does Linux acually see the E51 core?  On the Hifive boards it is
> disabled in the device tree, and the cpu probing just skips it,
> effectively resulting in only four cpus.

The E51 is indeed disabled in DT.
The CPU parts of arch/riscv/boot/dts/sifive/fu540-c000.dtsi and
arch/riscv/boot/dts/microchip/mpfs.dtsi arre very similar.
Do you get 4 CPUs on Hifive with CONFIG_NR_CPUS=4?

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds