linux-kernel - Re: [PATCH 1/1 linux-next] ACPI / SBS: fix sparse warning

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Date:	Mon, 29 Sep 2014 18:30:13 +0200 (CEST)
From:	Fabian Frederick <fabf@...net.be>
To:	Sudip Mukherjee <sudipm.mukherjee@...il.com>
Cc:	Len Brown <lenb@...nel.org>, linux-acpi@...r.kernel.org,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1 linux-next] ACPI / SBS: fix sparse warning



> On 29 September 2014 at 07:40 Sudip Mukherjee <sudipm.mukherjee@...il.com>
> wrote:
>
>
> On Sun, Sep 28, 2014 at 08:44:47PM +0200, Fabian Frederick wrote:
> > Adding parentheses around expression to avoid:
> > drivers/acpi/sbs.c:444:28: warning: dubious: !x & y
> >
> > Signed-off-by: Fabian Frederick <fabf@...net.be>
> > ---
> >  drivers/acpi/sbs.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/acpi/sbs.c b/drivers/acpi/sbs.c
> > index 32aecea..a7a3edd 100644
> > --- a/drivers/acpi/sbs.c
> > +++ b/drivers/acpi/sbs.c
> > @@ -441,7 +441,7 @@ static int acpi_ac_get_present(struct acpi_sbs *sbs)
> >      * The spec requires that bit 4 always be 1. If it's not set, assume
> >      * that the implementation doesn't support an SBS charger
> >      */
> > -   if (!(status >> 4) & 0x1)
> > +   if (!((status >> 4) & 0x1))
>
> i think the logic changed over here.
> it was !x & y , but now it has become !(x & y)
> ! is having higher priority than & so !x & y will mean ((!x) & y)
>
> thanks
> sudip
Hello Sudip,

You're right, logic has changed but IMHO expression was wrong.

Let's take value 100, decimal.
Binary: 0110>0<100

Spec states that bit 4 must be 1 so value is wrong.

We don't go in
if (!(status>>4) & 0x1)

but

if (!((status>>4) & 0x1))
        return -ENODEV;

is triggered.

Regards,
Fabian
> >             return -ENODEV;
> > 
> >     sbs->charger_present = (status >> 15) & 0x1;
> > --
> > 1.9.1
> >
> > --
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