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From: Julia Lawall <julia@diku.dk>
To: Karol Kozimor <sziwan@hell.org.pl>
Cc: corentincj@iksaif.net, sziwan@users.sourceforge.net,
	acpi4asus-user@lists.sourceforge.net,
	linux-kernel@vger.kernel.org, kernel-janitors@vger.kernel.org
Subject: Re: [PATCH 2/9] drivers/acpi/asus_acpi.c: Correct use of ! and &
Date: Wed, 27 Feb 2008 18:41:31 +0100 (MET)	[thread overview]
Message-ID: <Pine.LNX.4.58.0802271838190.1187@pc-041.diku.dk> (raw)
In-Reply-To: <7C626792-02A6-4BE6-B5F2-D75899F3990C@hell.org.pl>

On Wed, 27 Feb 2008, Karol Kozimor wrote:

> On 26-02-2008, at 21:42, Julia Lawall wrote:
> >  	if (invert)		/* invert target value */
> > -		led_out = !led_out & 0x1;
> > +		led_out = !(led_out & 0x1);
> >
> >  	if (!write_acpi_int(hotk->handle, ledname, led_out, NULL))
> >  		printk(KERN_WARNING "Asus ACPI: LED (%s) write failed\n",
>
>
> IIRC we're just supposed to flip the last bit here, so the original
> code is correct.

I spent some time thinking about this one.  The original code is ok if
led_out is always either 0x01 or 0x00.  But what if it is eg 0xc0?  Then
the negation amounts to the negation of a nonzero number, so the result is
0.  So the result of the bit and is 0.  So the last bit is not flipped.
But I don't know what is the range of led_out.  If it is always 1 or 0,
then why bother with the bit and?

julia


  reply	other threads:[~2008-02-27 17:41 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-02-26 20:42 Julia Lawall
2008-02-27 17:03 ` Karol Kozimor
2008-02-27 17:41   ` Julia Lawall [this message]
2008-02-27 18:29   ` Mark Pearson
2008-02-29  5:55     ` Andrew Morton
2008-02-29  6:10       ` Matthew Wilcox
2008-02-29  6:14         ` Matthew Wilcox
2008-02-29  6:19         ` Andrew Morton
2008-02-29 11:01       ` Julia Lawall
2008-02-29 11:08         ` Andrew Morton
2008-02-29 18:06       ` Mark Pearson
2008-02-29 21:33         ` Andrew Morton

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