linux-kernel - Re: [PATCH 2/9] drivers/acpi/asus

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Date:	Wed, 27 Feb 2008 18:41:31 +0100 (MET)
From:	Julia Lawall <julia@...u.dk>
To:	Karol Kozimor <sziwan@...l.org.pl>
Cc:	corentincj@...aif.net, sziwan@...rs.sourceforge.net,
	acpi4asus-user@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
	kernel-janitors@...r.kernel.org
Subject: Re: [PATCH 2/9] drivers/acpi/asus_acpi.c: Correct use of ! and &

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

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