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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Christophe Leroy <christophe.leroy@c-s.fr>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>, nathanl@linux.ibm.com,
        Arnd Bergmann <arnd@arndb.de>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        Andrew Lutomirski <luto@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        "open list\:MIPS" <linux-mips@vger.kernel.org>,
        X86 ML <x86@kernel.org>
Subject: Re: [RFC PATCH v4 10/11] lib: vdso: Allow arches to override the ns shift operation
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References: <cover.1579196675.git.christophe.leroy@c-s.fr> <c8ce9baaef0dc7273e4bcc31f353b17b655113d1.1579196675.git.christophe.leroy@c-s.fr> <CALCETrWJcB9=MuSw5yx6arcb_np=E=awTyLRSi=r8BJySf_aXw@mail.gmail.com> <877e1rfa40.fsf@nanos.tec.linutronix.de> <CALCETrX5B0SEJN2WG7rzuzbGhWa_dEwVVpMu6deXof3H+K_LdQ@mail.gmail.com>
Date:   Wed, 29 Jan 2020 08:14:27 +0100
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Andy Lutomirski <luto@kernel.org> writes:

> On Thu, Jan 16, 2020 at 11:57 AM Thomas Gleixner <tglx@linutronix.de> wrote:
>>
>> Andy Lutomirski <luto@kernel.org> writes:
>> > On Thu, Jan 16, 2020 at 9:58 AM Christophe Leroy
>> >
>> > Would mul_u64_u64_shr() be a good alternative?  Could we adjust it to
>> > assume the shift is less than 32?  That function exists to benefit
>> > 32-bit arches.
>>
>> We'd want mul_u64_u32_shr() for this. The rules for mult and shift are:
>>
>
> That's what I meant to type...

Just that it does not work. The math is:

     ns = d->nsecs;   // That's the nsec value shifted left by d->shift

     ns += ((cur - d->last) & d->mask) * mult;

     ns >>= d->shift;

So we cannot use mul_u64_u32_shr() because we need the addition there
before shifting. And no, we can't drop the fractional part of
d->nsecs. Been there, done that, got sporadic time going backwards
problems as a reward. Need to look at that again as stuff has changed
over time.

On x86 we enforce that mask is 64bit, so the & operation is not there,
but due to the nasties of TSC we have that conditional

    if (cur > last)
       return (cur - last) * mult;
    return 0;

Christophe, on PPC the decrementer/RTC clocksource masks are 64bit as
well, so you can spare that & operation there too.

Thanks,

        tglx