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From: Jamie Lokier <jamie@shareable.org>
To: Brian Gerst <bgerst@didntduck.org>
Cc: Kathy Frazier <kfrazier@mdc-dayton.com>, Andi Kleen <ak@muc.de>,
	linux-kernel@vger.kernel.org
Subject: Re: Interrupt doesn't make it to the 8259 on a ASUS P4PE mobo
Date: Tue, 15 Jul 2003 18:43:37 +0100	[thread overview]
Message-ID: <20030715174337.GC1491@mail.jlokier.co.uk> (raw)
In-Reply-To: <3F14348B.4050606@didntduck.org>

Brian Gerst wrote:
> >>>dmatimer.expires = jiffies + 0.5*HZ;
> >>
> >>That's a serious bug. You cannot use floating point in the kernel.
> >>It will corrupt the FP state of the user process.
> >
> >HZ on the INTEL platform is 100, so this should simply add 50 to the 
> >current
> >value of jiffies.  Besides, assigning the value to the unsigned int field
> >(expires) will truncate it to an integer anyway.
> 
> Use HZ/2 instead.  GCC doesn't optimize floating point constants to the 
> same degree it does integers, because it doesn't know what mode 
> (rounding, precision) the FPU is in.

Optimising this to an integer add of 50 would be incorrect anyway.
Think about jiffies == 0xfffffffe.

The first statement is equivalent to:

	dmatimer.expires = (unsigned long) ((double) + 50)

and would set dmatimer.expires to 0xffffffff.

Whereas the HZ/2 form is correct!

Have a nice day,
-- Jamie

  reply	other threads:[~2003-07-15 17:29 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <m3el0stw23.fsf@averell.firstfloor.org>
2003-07-15 16:14 ` Kathy Frazier
2003-07-15 15:31   ` Richard B. Johnson
2003-07-15 15:58     ` SCSI ATA driver in 2.4.22 ? Jeff Mock
2003-07-15 16:14       ` Jeff Garzik
2003-07-15 17:06   ` Interrupt doesn't make it to the 8259 on a ASUS P4PE mobo Brian Gerst
2003-07-15 17:43     ` Jamie Lokier [this message]
2003-07-15 17:59     ` Luciano Miguel Ferreira Rocha
2003-07-15 18:24       ` Brian Gerst
2003-07-15 18:44         ` Alan Cox
2003-07-15 19:44           ` Jamie Lokier
2003-07-20 21:04           ` Riley Williams
2003-07-20 22:25             ` Jeremy Fitzhardinge
     [not found] <Pine.LNX.4.53.0307141833190.4354@chaos>
2003-07-15 16:52 ` Kathy Frazier
2003-07-14 21:35 Kathy Frazier

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