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

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Message-Id: <20080229030844.9a7594ba.akpm@linux-foundation.org>
Date:	Fri, 29 Feb 2008 03:08:44 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Julia Lawall <julia@...u.dk>
Cc:	Mark Pearson <devnull.port@...glemail.com>,
	Karol Kozimor <sziwan@...l.org.pl>, corentincj@...aif.net,
	sziwan@...rs.sourceforge.net, acpi4asus-user@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org,
	Len Brown <lenb@...nel.org>
Subject: Re: [PATCH 2/9] drivers/acpi/asus_acpi.c: Correct use of ! and &

On Fri, 29 Feb 2008 12:01:26 +0100 (CET) Julia Lawall <julia@...u.dk> wrote:

> On Thu, 28 Feb 2008, Andrew Morton wrote:
> 
> > On Wed, 27 Feb 2008 19:29:15 +0100 Mark Pearson <devnull.port@...glemail.com> wrote:
> > 
> > > 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.
> > > > Best regards,
> > > > 
> > > 
> > > Seems an odd way of doing:
> > > 
> > > 	led_out ^= 0x01;
> > 
> > It does.
> > 
> > > It this due to some optimisation?
> > 
> > Surely not ;)
> > 
> > That code has been there for many years.
> > 
> > I changed the patch to this:
> > 
> > --- a/drivers/acpi/asus_acpi.c~drivers-acpi-asus_acpic-correct-use-of-and
> > +++ a/drivers/acpi/asus_acpi.c
> > @@ -610,7 +610,7 @@ write_led(const char __user * buffer, un
> >  	    (led_out) ? (hotk->status | ledmask) : (hotk->status & ~ledmask);
> >  
> >  	if (invert)		/* invert target value */
> > -		led_out = !led_out & 0x1;
> > +		led_out = !led_out;
> 
> I don't think this is the same:
> 
> !(0110 & 0x01) = !0 = 1
> !0110 = 0
> 
> led_out ^= 0x01;
> 
> is also not the same:
> 
> 0110 ^ 0x01 = 0111
> 
> Is it desired to keep the value and flip the last bit or just obtain the 
> opposite of the last bit?
> 

led_out can only take the values 0 or 1 here.
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