linux-kernel - Re: [PATCH 2/2] x86 platform driver: intelligent power sharing driver

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Date:	Sat, 15 May 2010 01:54:56 -0700
From:	Andy Isaacson <adi@...apodia.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Jesse Barnes <jbarnes@...tuousgeek.org>,
	linux-kernel@...r.kernel.org, intel-gfx@...ts.freedesktop.org,
	Matthew Garrett <mjg59@...f.ucam.org>
Subject: Re: [PATCH 2/2] x86 platform driver: intelligent power sharing
	driver

On Mon, May 10, 2010 at 10:00:46PM -0400, Andrew Morton wrote:
> > +	int i;
> > +	u16 avg;
> > +
> > +	for (i = 0; i < IPS_SAMPLE_COUNT; i++)
> > +		total += (u64)(array[i] * 100);
> 
> Actually, that does work.  Somehow the compiler will promote array[i]
> to u64 _before_ doing the multiplication.  I think.  Still, it looks
> like a deliberate attempt to trick the compiler into doing a
> multiplicative overflow ;)

It seems to promote to int, probably due to the implicit type of
"100".  Aind since array is u16, * 100 can't overflow int.  So yes, it's
safe, but it does catch the eye as potentially unsafe.

> > +		cur_seqno = (thm_readl(THM_ITV) & ITV_ME_SEQNO_MASK) >>
> > +			ITV_ME_SEQNO_SHIFT;
> > +		if (cur_seqno == last_seqno &&
> > +		    time_after(jiffies, seqno_timestamp + HZ)) {
> > +			dev_warn(&ips->dev->dev, "ME failed to update for more than 1s, likely hung\n");
> > +		} else {
> > +			seqno_timestamp = get_jiffies_64();
> > +			last_seqno = cur_seqno;
> > +		}
> > +
> > +		last_msecs = jiffies_to_msecs(jiffies);

Once it triggers, this will print the "likely hung" message every second
until the end of time, won't it?

-andy
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