Re: Interrupt doesn't make it to the 8259 on a ASUS P4PE mobo

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