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

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Message-ID: <53349D92.1050907@linaro.org>
Date:	Thu, 27 Mar 2014 14:52:18 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Andy Lutomirski <luto@...capital.net>
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>,
	Dave Hansen <dave@...1.net>
Subject: Re: [PATCH] x86: hpet: Don't default CONFIG_HPET_TIMER to be y for
 X86_64

On 03/27/2014 02:33 PM, Andy Lutomirski wrote:
> 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.

Actually, an Intel DX79TO. I've harangued some folks I know, but no
fixes for the BIOS have been released.

>> 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.
I know on older hardware it was a bigger win. I'd have to look at what
my current system does.

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

thanks
-john
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