linux-kernel - Re: [PATCH 2/3] Input: alps: drop unlikely before IS_ERR_OR

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Message-ID: <20151002182914.GR8437@dtor-ws>
Date:	Fri, 2 Oct 2015 11:29:14 -0700
From:	Dmitry Torokhov <dmitry.torokhov@...il.com>
To:	Pali Rohár <pali.rohar@...il.com>
Cc:	Geliang Tang <geliangtang@....com>,
	Hans de Goede <hdegoede@...hat.com>,
	Masaki Ota <masaki.ota@...alps.com>,
	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] Input: alps: drop unlikely before IS_ERR_OR_NULL

On Thu, Oct 01, 2015 at 09:30:19AM +0200, Pali Rohár wrote:
> On Thursday 01 October 2015 10:55:30 Geliang Tang wrote:
> > IS_ERR_OR_NULL already contain an unlikely compiler flag. Drop it.
> > 
> > Signed-off-by: Geliang Tang <geliangtang@....com>
> > ---
> >  drivers/input/mouse/alps.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/input/mouse/alps.c b/drivers/input/mouse/alps.c
> > index 4d24686..b4f146a 100644
> > --- a/drivers/input/mouse/alps.c
> > +++ b/drivers/input/mouse/alps.c
> > @@ -1367,7 +1367,7 @@ static void alps_report_bare_ps2_packet(struct psmouse *psmouse,
> >  		/* On V2 devices the DualPoint Stick reports bare packets */
> >  		dev = priv->dev2;
> >  		dev2 = psmouse->dev;
> > -	} else if (unlikely(IS_ERR_OR_NULL(priv->dev3))) {
> > +	} else if (IS_ERR_OR_NULL(priv->dev3)) {
> >  		/* Register dev3 mouse if we received PS/2 packet first time */
> >  		if (!IS_ERR(priv->dev3))
> >  			psmouse_queue_work(psmouse, &priv->dev3_register_work,
> 
> Hm... I do not like this change. If I read code
> 
>  if (unlikely(IS_ERR_OR_NULL(priv->dev3)))
> 
> then I know that it is really unlikely that condition will be truth and
> so this is some case of error/exception or something that normally does
> not happen too much.
> 
> But if I read code
> 
>  if (IS_ERR_OR_NULL(priv->dev3))
> 
> I know nothing about chance that this condition will be truth. Explicit
> unlikely in previous example give me more information.

Yes, given that this is in packet processing path I prefer having
explicit unlikely there.

Thanks.

-- 
Dmitry
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