linux-kernel - Re: [PATCH] Input: tca8418_keypad: hide gcc-4.9 -Wmaybe-uninitialized warning

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Message-ID: <20161026225954.GD27930@dtor-ws>
Date:   Wed, 26 Oct 2016 15:59:54 -0700
From:   Dmitry Torokhov <dmitry.torokhov@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     linux-input@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Input: tca8418_keypad: hide gcc-4.9
 -Wmaybe-uninitialized warning

On Tue, Oct 25, 2016 at 11:59:22AM +0200, Arnd Bergmann wrote:
> On Monday, October 24, 2016 4:45:13 PM CEST Dmitry Torokhov wrote:
> > > diff --git a/drivers/input/keyboard/tca8418_keypad.c b/drivers/input/keyboard/tca8418_keypad.c
> > > index 9002298698fc..3048ef3e3e16 100644
> > > --- a/drivers/input/keyboard/tca8418_keypad.c
> > > +++ b/drivers/input/keyboard/tca8418_keypad.c
> > > @@ -164,11 +164,18 @@ static void tca8418_read_keypad(struct tca8418_keypad *keypad_data)
> > >       int error, col, row;
> > >       u8 reg, state, code;
> > >  
> > > -     /* Initial read of the key event FIFO */
> > > -     error = tca8418_read_byte(keypad_data, REG_KEY_EVENT_A, &reg);
> > > +     do {
> > > +             error = tca8418_read_byte(keypad_data, REG_KEY_EVENT_A, &reg);
> > > +             if (error < 0) {
> > > +                     dev_err(&keypad_data->client->dev,
> > > +                             "unable to read REG_KEY_EVENT_A\n");
> > > +                     break;
> > > +             }
> > > +
> > > +             /* Assume that key code 0 signifies empty FIFO */
> > > +             if (reg <= 0)
> > > +                     break;
> > 
> > I am unconvinced that this rearrangement fixes the issue for all older
> > GCCs. Can we simply do:
> > 
> >         u8 uninitialized_var(reg);
> > 
> > and be done with it?
> 
> Yes, that would work. However:
> 
> - avoiding the fake intialization tends to produce better object
>   code, as gcc actually knows what's going on
> - Linus doesn't like uninitialized_var() and would rather see it
>   removed from the kernel
> - llvm produces warnings for uninitialized_var()
> 
> I have checked gcc-4.6/4.7/4.8/4.9/5.x/6.x, and only gcc-4.9
> produces the warning. 4.9 changed the detection for uninitialized
> variables significantly, which generally caused fewer false
> positives but unfortunately introduced a couple of new ones
> like this.

OK, I'll apply it then.

Thanks.

-- 
Dmitry