linux-kernel - Re: [PATCH][RFC] Fix for LS21 + HPET = boot hang (since 2.6.24-rc1)

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Date:	Fri, 13 Feb 2009 09:12:32 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	john stultz <johnstul@...ibm.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	lkml <linux-kernel@...r.kernel.org>,
	Clark Williams <williams@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH][RFC] Fix for LS21 + HPET = boot hang (since 2.6.24-rc1)


* john stultz <johnstul@...ibm.com> wrote:

> Between 2.6.23 and 2.6.24-rc1 a change was made that broke IBM LS21
> systems that had the HPET enabled in the BIOS, resulting in boot hangs
> for x86_64. 
> 
> Specifically commit b8ce33590687888ebb900d09557b8807c4539022, which
> merges the i386 and x86_64 HPET code.
> 
> Prior to this commit, when we setup the HPET timers in x86_64, we did
> the following:
> 
> 	hpet_writel(HPET_TN_ENABLE | HPET_TN_PERIODIC | HPET_TN_SETVAL |
>                     HPET_TN_32BIT, HPET_T0_CFG);
> 
> However after the i386/x86_64 HPET merge, we do the following:
> 
> 	cfg = hpet_readl(HPET_Tn_CFG(timer));
> 	cfg |= HPET_TN_ENABLE | HPET_TN_PERIODIC |
> 			HPET_TN_SETVAL | HPET_TN_32BIT;
> 	hpet_writel(cfg, HPET_Tn_CFG(timer));
> 
> 
> However on LS21s with HPET enabled in the BIOS, the HPET_T0_CFG register
> boots with Level triggered interrupts (HPET_TN_LEVEL) enabled. This
> causes the periodic interrupt to be not so periodic, and that results in
> the boot time hang I reported earlier in the delay calibration.
> 
> 
> My fix: Always disable HPET_TN_LEVEL when setting up periodic mode.
> 
> Does that seem ok to folks? I've not been able to run this on an i386
> system, so it could use some extra testing. So while it is a regression
> fix, the bug has been around for awhile, so I'd probably queue it for
> 2.6.30.

Makes perfect sense - and i dont think we can actually survive the
bootup with this IRQ being level-triggered, so there's little risk
of introducing additional regressions. I'll give it a good workout
nevertheless.

Applied to tip:x86/urgent, thanks John for tracking this one down!

	Ingo
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