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

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Date:   Wed, 20 Feb 2019 10:56:15 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Johannes Berg <johannes.berg@...el.com>,
        Emmanuel Grumbach <emmanuel.grumbach@...el.com>,
        Luca Coelho <luciano.coelho@...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, 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?

Nathan