From: Ruxandra Ioana Radulescu <ruxandra.radulescu@nxp.com>
To: "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>
CC: Dan Carpenter <dan.carpenter@oracle.com>,
        "devel@driverdev.osuosl.org" <devel@driverdev.osuosl.org>,
        "arnd@arndb.de" <arnd@arndb.de>,
        "stuyoder@gmail.com" <stuyoder@gmail.com>,
        Roy Pledge <roy.pledge@nxp.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "agraf@suse.de" <agraf@suse.de>,
        Bogdan Purcareata <bogdan.purcareata@nxp.com>,
        Laurentiu Tudor <laurentiu.tudor@nxp.com>
Subject: RE: [PATCH] staging: fsl-mc/dpio: Fix incorrect comparison
Thread-Topic: [PATCH] staging: fsl-mc/dpio: Fix incorrect comparison
Thread-Index: AQHTN7oYgMCC9Nvla0+749/uaJzKTqLKQL4AgAAAVyCAAAfdgIAAAOrQ
Date: Thu, 28 Sep 2017 13:23:56 +0000
Message-ID: <VI1PR0402MB284770627319D1B68171C69294790@VI1PR0402MB2847.eurprd04.prod.outlook.com>
References: <20170927175728.3307-1-ruxandra.radulescu@nxp.com>
 <20170928124836.kdqjowqibab53hf5@mwanda>
 <VI1PR0402MB28470F19FE7A585E7F1B184394790@VI1PR0402MB2847.eurprd04.prod.outlook.com>
 <20170928131757.GA23657@kroah.com>
In-Reply-To: <20170928131757.GA23657@kroah.com>
Accept-Language: en-US
Content-Language: en-US
spamdiagnosticoutput: 1:99
spamdiagnosticmetadata: NSPM
Content-Type: text/plain; charset="us-ascii"
MIME-Version: 1.0
X-MS-Exchange-CrossTenant-originalarrivaltime: 28 Sep 2017 13:23:56.2130
 (UTC)
X-MS-Exchange-CrossTenant-fromentityheader: Hosted
X-MS-Exchange-CrossTenant-id: 686ea1d3-bc2b-4c6f-a92c-d99c5c301635
X-MS-Exchange-Transport-CrossTenantHeadersStamped: DB6PR04MB3000
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Content-Transfer-Encoding: 8bit
X-MIME-Autoconverted: from quoted-printable to 8bit by nfs id v8SDOAGP022007
Content-Length: 2547
Lines: 59

> -----Original Message-----
> From: gregkh@linuxfoundation.org [mailto:gregkh@linuxfoundation.org]
> Sent: Thursday, September 28, 2017 4:18 PM
> To: Ruxandra Ioana Radulescu <ruxandra.radulescu@nxp.com>
> Cc: Dan Carpenter <dan.carpenter@oracle.com>;
> devel@driverdev.osuosl.org; arnd@arndb.de; stuyoder@gmail.com; Roy
> Pledge <roy.pledge@nxp.com>; linux-kernel@vger.kernel.org;
> agraf@suse.de; Bogdan Purcareata <bogdan.purcareata@nxp.com>;
> Laurentiu Tudor <laurentiu.tudor@nxp.com>
> Subject: Re: [PATCH] staging: fsl-mc/dpio: Fix incorrect comparison
> 
> On Thu, Sep 28, 2017 at 01:07:48PM +0000, Ruxandra Ioana Radulescu wrote:
> > > -----Original Message-----
> > > From: Dan Carpenter [mailto:dan.carpenter@oracle.com]
> > > Sent: Thursday, September 28, 2017 3:49 PM
> > > To: Ruxandra Ioana Radulescu <ruxandra.radulescu@nxp.com>
> > > Cc: gregkh@linuxfoundation.org; devel@driverdev.osuosl.org;
> > > arnd@arndb.de; stuyoder@gmail.com; Roy Pledge
> <roy.pledge@nxp.com>;
> > > linux-kernel@vger.kernel.org; agraf@suse.de; Bogdan Purcareata
> > > <bogdan.purcareata@nxp.com>; Laurentiu Tudor
> > > <laurentiu.tudor@nxp.com>
> > > Subject: Re: [PATCH] staging: fsl-mc/dpio: Fix incorrect comparison
> > >
> > > On Wed, Sep 27, 2017 at 12:57:28PM -0500, Ioana Radulescu wrote:
> > > > diff --git a/drivers/staging/fsl-mc/bus/dpio/dpio-service.c
> > > b/drivers/staging/fsl-mc/bus/dpio/dpio-service.c
> > > > index f809682..26922fc 100644
> > > > --- a/drivers/staging/fsl-mc/bus/dpio/dpio-service.c
> > > > +++ b/drivers/staging/fsl-mc/bus/dpio/dpio-service.c
> > > > @@ -76,7 +76,7 @@ static inline struct dpaa2_io
> > > *service_select_by_cpu(struct dpaa2_io *d,
> > > >  	if (d)
> > > >  		return d;
> > > >
> > > > -	if (unlikely(cpu >= num_possible_cpus()))
> > > > +	if (unlikely(cpu >= (int)num_possible_cpus()))
> > >
> > >
> > > Drivers shouldn't use likely/unlikley.
> >
> > I was under the impression it's ok to use them on hotpath
> > (and while not entirely obvious, this function is called on
> > other drivers' hotpath).
> 
> Only use it if you can measure the difference.  If you can not, then do
> not use it as the compiler and the CPU will guess it better than you
> will.
> 
> This has been proven many times, something like 80% of our
> likely/unlikely usage in the kernel is wrong because of this, see the
> work from Andi Kleen many years ago in this area.
> 
> So please remove it.  Unless you can prove it matters, and if so,
> document that.
 
Greg, thanks for the explanation. Will remove it in v2.

Ioana