linux-kernel - RE: [PATCH] staging: fsl-mc/dpio: Fix incorrect comparison

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Message-ID: <VI1PR0402MB284770627319D1B68171C69294790@VI1PR0402MB2847.eurprd04.prod.outlook.com>
Date:   Thu, 28 Sep 2017 13:23:56 +0000
From:   Ruxandra Ioana Radulescu <ruxandra.radulescu@....com>
To:     "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
CC:     Dan Carpenter <dan.carpenter@...cle.com>,
        "devel@...verdev.osuosl.org" <devel@...verdev.osuosl.org>,
        "arnd@...db.de" <arnd@...db.de>,
        "stuyoder@...il.com" <stuyoder@...il.com>,
        Roy Pledge <roy.pledge@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "agraf@...e.de" <agraf@...e.de>,
        Bogdan Purcareata <bogdan.purcareata@....com>,
        Laurentiu Tudor <laurentiu.tudor@....com>
Subject: RE: [PATCH] staging: fsl-mc/dpio: Fix incorrect comparison

> -----Original Message-----
> From: gregkh@...uxfoundation.org [mailto:gregkh@...uxfoundation.org]
> Sent: Thursday, September 28, 2017 4:18 PM
> To: Ruxandra Ioana Radulescu <ruxandra.radulescu@....com>
> Cc: Dan Carpenter <dan.carpenter@...cle.com>;
> devel@...verdev.osuosl.org; arnd@...db.de; stuyoder@...il.com; Roy
> Pledge <roy.pledge@....com>; linux-kernel@...r.kernel.org;
> agraf@...e.de; Bogdan Purcareata <bogdan.purcareata@....com>;
> Laurentiu Tudor <laurentiu.tudor@....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@...cle.com]
> > > Sent: Thursday, September 28, 2017 3:49 PM
> > > To: Ruxandra Ioana Radulescu <ruxandra.radulescu@....com>
> > > Cc: gregkh@...uxfoundation.org; devel@...verdev.osuosl.org;
> > > arnd@...db.de; stuyoder@...il.com; Roy Pledge
> <roy.pledge@....com>;
> > > linux-kernel@...r.kernel.org; agraf@...e.de; Bogdan Purcareata
> > > <bogdan.purcareata@....com>; Laurentiu Tudor
> > > <laurentiu.tudor@....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