linux-kernel - Re: [PATCH] iwlwifi: mvm: Use div64_s64 instead of do_div in iwl_mvm_debug_range

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Message-ID: <c1a9091503d780a0405b530d06fd941930b0db4e.camel@intel.com>
Date:   Thu, 21 Feb 2019 09:33:32 +0200
From:   Luciano Coelho <luciano.coelho@...el.com>
To:     Nathan Chancellor <natechancellor@...il.com>,
        Arnd Bergmann <arnd@...db.de>
Cc:     Johannes Berg <johannes.berg@...el.com>,
        Emmanuel Grumbach <emmanuel.grumbach@...el.com>,
        Intel Linux Wireless <linuxwifi@...el.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        linux-wireless <linux-wireless@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH] iwlwifi: mvm: Use div64_s64 instead of do_div in
 iwl_mvm_debug_range_resp

On Wed, 2019-02-20 at 10:56 -0700, Nathan Chancellor wrote:
> On Wed, Feb 20, 2019 at 11:51:34AM +0100, Arnd Bergmann wrote:
> > On Tue, Feb 19, 2019 at 7:22 PM Nathan Chancellor
> > <natechancellor@...il.com> wrote:
> > > diff --git a/drivers/net/wireless/intel/iwlwifi/mvm/ftm-
> > > initiator.c b/drivers/net/wireless/intel/iwlwifi/mvm/ftm-
> > > initiator.c
> > > index e9822a3ec373..92b22250eb7d 100644
> > > --- a/drivers/net/wireless/intel/iwlwifi/mvm/ftm-initiator.c
> > > +++ b/drivers/net/wireless/intel/iwlwifi/mvm/ftm-initiator.c
> > > @@ -462,7 +462,7 @@ static void iwl_mvm_debug_range_resp(struct
> > > iwl_mvm *mvm, u8 index,
> > >  {
> > >         s64 rtt_avg = res->ftm.rtt_avg * 100;
> > > 
> > > -       do_div(rtt_avg, 6666);
> > > +       div64_s64(rtt_avg, 6666);
> > 
> > This is wrong: div64_s64 does not modify its argument like
> > do_div(), but
> > it returns the result instead. You also don't want to divide by a
> > 64-bit
> > value when the second argument is a small constant.
> > 
> > I think the correct way should be
> > 
> >        s64 rtt_avg = div_s64(res->ftm.rtt_avg * 100, 6666);
> > 
> > If you know that the value is positive, using unsigned types
> > and div_u64() would be slightly faster.
> > 
> >       Arnd
> 
> Thanks for the review and explanation, Arnd.
> 
> Luca, could you drop this version so I can resend it?

Sure, please do! I already applied this internally, but I can just fix
it with your new patch and that will be squashed before being sent
upstream, so it will look like your second patch.

--
Cheers,
Luca.