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

Nathan Chancellor
2019-02-20T17:56:22Z

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?

Nathan