linux-kernel - Re: [PATCH v2] x86: hpet: Don't default CONFIG_HPET_TIMER to be y for X86

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Date:	Wed, 16 Apr 2014 08:51:48 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Feng Tang <feng.tang@...el.com>
Cc:	Clemens Ladisch <clemens@...isch.de>, tglx@...utronix.de,
	mingo@...e.hu, hpa@...or.com, linux-kernel@...r.kernel.org,
	John Stultz <john.stultz@...aro.org>,
	Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH v2] x86: hpet: Don't default CONFIG_HPET_TIMER to be y
 for X86_64


* Feng Tang <feng.tang@...el.com> wrote:

> On Tue, Apr 15, 2014 at 12:42:06PM +0200, Ingo Molnar wrote:
> > 
> > * Feng Tang <feng.tang@...el.com> wrote:
> > 
> > > On Tue, Apr 15, 2014 at 11:00:25AM +0200, Ingo Molnar wrote:
> > > > 
> > > > * Feng Tang <feng.tang@...el.com> wrote:
> > > > 
> > > > > Hi Ingo,
> > > > > 
> > > > > On Fri, Mar 28, 2014 at 09:11:17AM +0100, Ingo Molnar wrote:
> > > > > > 
> > > > > > * Clemens Ladisch <clemens@...isch.de> wrote:
> > > > > > 
> > > > > > > Feng Tang wrote:
> > > > > > > > On Fri, Mar 28, 2014 at 08:17:16AM +0100, Ingo Molnar wrote:
> > > > > > > >> * Feng Tang <feng.tang@...el.com> wrote:
> > > > > > > >>  - or the kernel should have a quirk to reliably disable it. Why
> > > > > > > >>    should we crash or misbehave if a driver is built into the
> > > > > > > >>    kernel?
> > > > > > > >
> > > > > > > > I thought about this before, HPET doesn't have PCI ID like stuff,
> > > > > > > 
> > > > > > > HPET does have the PCI vendor ID in the first register.
> > > > > > > 
> > > > > > > > only thing I can think of to identify them may be the CPU family/ID.
> > > > > > > 
> > > > > > > The HPET is implemented by some actual chip, and that chip also has lots
> > > > > > > of PCI devices.  (In the case of a SoC, the CPU ID would work, too).
> > > > > > 
> > > > > > Correct. See arch/x86/kernel/hpet.c, which has a large number of HPET 
> > > > > > quirks keyed off chipset PCI IDs:
> > > > > > 
> > > > > >   DECLARE_PCI_FIXUP_HEADER(PCI_VENDOR_ID_INTEL, PCI_DEVICE_ID_INTEL_ESB2_0,
> > > > > >                            ich_force_enable_hpet);
> > > > > >   DECLARE_PCI_FIXUP_HEADER(PCI_VENDOR_ID_INTEL, PCI_DEVICE_ID_INTEL_ICH6_0,
> > > > > >                            ich_force_enable_hpet);
> > > > > >   DECLARE_PCI_FIXUP_HEADER(PCI_VENDOR_ID_INTEL, PCI_DEVICE_ID_INTEL_ICH6_1,
> > > > > >                            ich_force_enable_hpet);
> > > > > >   [...]
> > > > > 
> > > > > I just gave it another thought, that the HPET on our platform 
> > > > > currently do have some problem to be used as clocksource/clockevent, 
> > > > > but it may get fixed in future version (by Silicon or BIOS).
> > > > > 
> > > > > If I add quirk to block it now, I may revert this code in future 
> > > > > when it get fixed, same problem applis for the future generation of 
> > > > > platform.
> > > > 
> > > > If the hardware or BIOS gets fixed then that will be visible in the 
> > > > revision level of the hardware, right?
> > > 
> > > AFAIK, if it is fixed in a new silicon version, we should be able to 
> > > detect it by the "stepping", but the PCI DEV ID is not likely to 
> > > change.
> > > 
> > > If it is fixed by BIOS or PUNIT FW, then we may go through the 
> > > DMI info to check the BIOS version, One problem is there is some 
> > > Baytrail tablet platforms that doesn't provide DMI info as its 
> > > FW is not UEFI/Legacy BIOS compatible version.
> > 
> > So just turn it off for the current hardware, via PCI ID. When you 
> > get a fix, you can turn it back on again by refining the quirk 
> > check, for that specific hardware.
> 
> got it, thanks!

Btw., note that such narrow quirk refinings are pretty uncontroversial 
and have a pretty fast path upstream, once you have the initial quirk 
in there.

Thanks,

	Ingo
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