linux-kernel - Re: [PATCH] x86: hpet: Don't default CONFIG_HPET_TIMER to be y for X86

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Date:	Thu, 27 Mar 2014 14:33:56 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	John Stultz <john.stultz@...aro.org>
Cc:	Clemens Ladisch <clemens@...isch.de>,
	Feng Tang <feng.tang@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: hpet: Don't default CONFIG_HPET_TIMER to be y for X86_64

On Thu, Mar 27, 2014 at 2:27 PM, John Stultz <john.stultz@...aro.org> wrote:
> On Thu, Mar 27, 2014 at 12:52 PM, Andy Lutomirski <luto@...capital.net> wrote:
>> On 03/27/2014 04:02 AM, Clemens Ladisch wrote:
>>> Feng Tang wrote:
>>> The help text still says:
>>> | You can safely choose Y here.  [...]
>>> | Choose N to continue using the legacy 8254 timer.
>>>
>>> Are these statements still true for those platforms?
>>
>> They aren't true for modern desktop and server platforms -- the TSC is
>> used regardless of hpet availability.
>
> While I suspect the comment above is in relation to the non-apic
> timer. But with respect to timekeeping, our point is true assuming the
> TSC isn't mucked up by the BIOS.  My 1yr old i7-3930k single socket
> system still has some wonky BIOS bug that offsets the boot core's TSC.
> And that's intel's bios, so I can only imagine other vendors have
> found other ways to cause trouble.

Is this, perhaps, an MSI X79A-GD65 (8D) (MS-7760)?  If so, there's a fixed BIOS.

>
> So yea, the hpet availability for timekeeping is still important, as
> the TSC can still be problematic.

Is HPET really that much better than acpi_pm?  I can read my HPET in
~584ns (vdso) or ~649ns (syscall) and my acpi_pm in 753ns.  So it's
better, but not by a whole lot.

But yes, I see no good reason to disable it, except specifically on
systems where there are known bugs.

>
> thanks
> -john



-- 
Andy Lutomirski
AMA Capital Management, LLC
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