linux-kernel - Re: [PATCH 2/9] x86 idle: remove NOP cpuinfo_x86.hlt_works

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Date:	Thu, 31 Mar 2011 15:41:03 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Len Brown <lenb@...nel.org>,
	x86@...nel.org, linux-kernel@...r.kernel.org,
	linux-pm@...ts.linux-foundation.org,
	Len Brown <len.brown@...el.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 2/9] x86 idle: remove NOP cpuinfo_x86.hlt_works_ok flag

On Thu, 31 Mar 2011 07:19:50 -0700
"H. Peter Anvin" <hpa@...or.com> wrote:

> On 03/31/2011 02:35 AM, Ingo Molnar wrote:
> > 
> > * Alan Cox <alan@...rguk.ukuu.org.uk> wrote:
> > 
> >>> Btw., we used to auto-detect broken HLT systems IIRC - but that got lost 
> >>> already. We should at least honor the boot parameter.
> >>
> >> I don't believe we ever auto detected them or found a way to do so. That
> >> was why the HLT message was printed before hlt was executed.
> > 
> > Yeah - the CPU hang was unrecoverably deep so no auto-detection was possible.
> > 
> > That's seriously ancient stuff - still, keeping the boot option around (<10 
> > lines of code) does not hurt anyone.
> > 
> 
> What it was was bad power supplies or low-capacitance, high-inductance
> power distribution that happened to work with MS-DOS which always burned
> the CPU at 100% and therefore left the power draw relatively consistent
> current.  A proper OS putting the CPU in HLT produced a lot more high
> frequency noise on the power busses, with disastrous results without
> proper bypass.

And also chipset errata in some cases - eg some revisions of the CS5510
hung the box solid if a CPU hlt occurred during an IDE transfer. I don't
think any CS5510s are still around although I've had mail from someone
with a CS5520 in use not that long ago so who knows!

The joy of ancient history.
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