Re: [PATCH] iwlwifi: mvm: Use div64_s64 instead of do_div in iwl_mvm_debug_range_resp

From: Luciano Coelho <luciano.coelho@intel.com>
To: Nathan Chancellor <natechancellor@gmail.com>,
	Arnd Bergmann <arnd@arndb.de>
Cc: Johannes Berg <johannes.berg@intel.com>,
	Emmanuel Grumbach <emmanuel.grumbach@intel.com>,
	Intel Linux Wireless <linuxwifi@intel.com>,
	Kalle Valo <kvalo@codeaurora.org>,
	linux-wireless <linux-wireless@vger.kernel.org>,
	Networking <netdev@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>
Subject: Re: [PATCH] iwlwifi: mvm: Use div64_s64 instead of do_div in iwl_mvm_debug_range_resp
Date: Thu, 21 Feb 2019 09:33:32 +0200	[thread overview]
Message-ID: <c1a9091503d780a0405b530d06fd941930b0db4e.camel@intel.com> (raw)
In-Reply-To: <20190220175615.GA1312@archlinux-ryzen>

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@gmail.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.