Date: Fri, 29 Feb 2008 12:01:26 +0100 (CET)
From: Julia Lawall <julia@diku.dk>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Mark Pearson <devnull.port@googlemail.com>,
       Karol Kozimor <sziwan@hell.org.pl>, corentincj@iksaif.net,
       sziwan@users.sourceforge.net, acpi4asus-user@lists.sourceforge.net,
       linux-kernel@vger.kernel.org, kernel-janitors@vger.kernel.org,
       Len Brown <lenb@kernel.org>
Subject: Re: [PATCH 2/9] drivers/acpi/asus_acpi.c: Correct use of ! and &
In-Reply-To: <20080228215503.b942bb3c.akpm@linux-foundation.org>
Message-ID: <Pine.LNX.4.64.0802291154100.1081@ask.diku.dk>
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 <7C626792-02A6-4BE6-B5F2-D75899F3990C@hell.org.pl> <47C5ABFB.3050607@googlemail.com>
 <20080228215503.b942bb3c.akpm@linux-foundation.org>
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On Thu, 28 Feb 2008, Andrew Morton wrote:

> On Wed, 27 Feb 2008 19:29:15 +0100 Mark Pearson <devnull.port@googlemail.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?

julia

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