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SubjectRE: [PATCH] staging: fsl-mc/dpio: Fix incorrect comparison
Date
> -----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

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