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From: Christophe Leroy <christophe.leroy@c-s.fr>
To: Thomas Gleixner <tglx@linutronix.de>, Andy Lutomirski <luto@kernel.org>
Cc: nathanl@linux.ibm.com, Arnd Bergmann <arnd@arndb.de>,
	X86 ML <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>,
	"open list:MIPS" <linux-mips@vger.kernel.org>,
	Paul Mackerras <paulus@samba.org>,
	Vincenzo Frascino <vincenzo.frascino@arm.com>,
	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
	linux-arm-kernel <linux-arm-kernel@lists.infradead.org>
Subject: Re: [RFC PATCH v4 10/11] lib: vdso: Allow arches to override the ns shift operation
Date: Wed, 29 Jan 2020 08:26:39 +0100
Message-ID: <1329105a-aa6b-bd40-3a62-5c7e130d2a7b@c-s.fr> (raw)
In-Reply-To: <87mua64tv0.fsf@nanos.tec.linutronix.de>



Le 29/01/2020 à 08:14, Thomas Gleixner a écrit :
> 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.
> 

Yes, I did it already. It spares reading d->mast, that the main advantage.

Christophe

  reply index

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-16 17:58 [RFC PATCH v4 00/11] powerpc: switch VDSO to C implementation Christophe Leroy
2020-01-16 17:58 ` [RFC PATCH v4 01/11] powerpc/64: Don't provide time functions in compat VDSO32 Christophe Leroy
2020-01-16 17:58 ` [RFC PATCH v4 02/11] powerpc/vdso: Switch VDSO to generic C implementation Christophe Leroy
2020-01-16 17:58 ` [RFC PATCH v4 03/11] lib: vdso: only read hrtimer_res when needed in __cvdso_clock_getres() Christophe Leroy
2020-01-16 17:58 ` [RFC PATCH v4 04/11] powerpc/vdso: simplify __get_datapage() Christophe Leroy
2020-01-16 17:58 ` [RFC PATCH v4 05/11] lib: vdso: allow arches to provide vdso data pointer Christophe Leroy
2020-01-16 17:58 ` [RFC PATCH v4 06/11] powerpc/vdso: provide inline alternative to __get_datapage() Christophe Leroy
2020-01-16 17:58 ` [RFC PATCH v4 07/11] powerpc/vdso: provide vdso data pointer from the ASM caller Christophe Leroy
2020-01-16 17:58 ` [RFC PATCH v4 08/11] lib: vdso: allow fixed clock mode Christophe Leroy
2020-01-16 20:13   ` Thomas Gleixner
2020-01-16 20:19     ` Andy Lutomirski
2020-01-16 21:07       ` Thomas Gleixner
2020-01-16 17:58 ` [RFC PATCH v4 09/11] powerpc/vdso: override __arch_vdso_capable() Christophe Leroy
2020-01-16 17:58 ` [RFC PATCH v4 10/11] lib: vdso: Allow arches to override the ns shift operation Christophe Leroy
2020-01-16 19:47   ` Andy Lutomirski
2020-01-16 19:57     ` Thomas Gleixner
2020-01-16 20:20       ` Andy Lutomirski
2020-01-29  7:14         ` Thomas Gleixner
2020-01-29  7:26           ` Christophe Leroy [this message]
2020-01-16 17:58 ` [RFC PATCH v4 11/11] powerpc/32: provide vdso_shift_ns() Christophe Leroy
2020-01-17  8:58 ` [RFC PATCH v4 00/11] powerpc: switch VDSO to C implementation Segher Boessenkool
2020-01-17  9:26   ` Christophe Leroy
2020-01-20 14:56   ` Christophe Leroy
2020-01-20 15:19     ` Segher Boessenkool
2020-01-20 17:08       ` Christophe Leroy
2020-01-20 17:27         ` Segher Boessenkool

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