linux-kernel - Re: [PATCH 5/7] RISC-V: arch/riscv/lib

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Date:   Tue, 23 May 2017 13:19:42 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Palmer Dabbelt <palmer@...belt.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Olof Johansson <olof@...om.net>, albert@...ive.com
Subject: Re: [PATCH 5/7] RISC-V: arch/riscv/lib

On Tue, May 23, 2017 at 2:41 AM, Palmer Dabbelt <palmer@...belt.com> wrote:
> diff --git a/arch/riscv/lib/Makefile b/arch/riscv/lib/Makefile
> new file mode 100644
> index 000000000000..f644e582f4b8
> --- /dev/null
> +++ b/arch/riscv/lib/Makefile

> +
> +void __delay(unsigned long cycles)
> +{
> +       u64 t0 = get_cycles();
> +
> +       while ((unsigned long)(get_cycles() - t0) < cycles)
> +               cpu_relax();
> +}
> +
> +void udelay(unsigned long usecs)
> +{
> +       u64 ucycles = (u64)usecs * timebase;
> +       do_div(ucycles, 1000000U);
> +       __delay((unsigned long)ucycles);
> +}
> +EXPORT_SYMBOL(udelay);
> +
> +void ndelay(unsigned long nsecs)
> +{
> +       u64 ncycles = (u64)nsecs * timebase;
> +       do_div(ncycles, 1000000000U);
> +       __delay((unsigned long)ncycles);
> +}

I'd be slightly worried about a global 'timebase' identifier that
might conflict with a variable in some random driver.

Also, it would be good to replace the multiply+div64
with a single multiplication here, see how x86 and arm do it
(for the tsc/__timer_delay case).

      Arnd