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

Nick Desaulniers
2019-02-19T19:06:03Z

On Tue, Feb 19, 2019 at 10:21 AM Nathan Chancellor
<natechancellor@gmail.com> wrote:
>
> Clang warns:
>
> drivers/net/wireless/intel/iwlwifi/mvm/ftm-initiator.c:465:2: warning:
> comparison of distinct pointer types ('typeof ((rtt_avg)) *' (aka 'long
> long *') and 'uint64_t *' (aka 'unsigned long long *'))
> [-Wcompare-distinct-pointer-types]
> do_div(rtt_avg, 6666);
> ^~~~~~~~~~~~~~~~~~~~~
> include/asm-generic/div64.h:222:28: note: expanded from macro 'do_div'
> (void)(((typeof((n)) *)0) == ((uint64_t *)0)); \
> ~~~~~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~~
> 1 warning generated.
>
> do_div expects an unsigned dividend. Use div64_s64, which expects a
> signed dividend.

Eh, IIRC, signed vs unsigned division has implications for rounding
towards zero or not, but I doubt that the round trip time average (RTT
avg) should ever be negative. General rule of thumb for C is to keep
arithmetic signed (even when working with non zero values), so rather
than make the literal (6666) a unsigned long, I agree with your change
to keep the division signed as well. Thanks for the fix.
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

>
> Fixes: 937b10c0de68 ("iwlwifi: mvm: add debug prints for FTM")
> Link: https://github.com/ClangBuiltLinux/linux/issues/372
> Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
> ---
> drivers/net/wireless/intel/iwlwifi/mvm/ftm-initiator.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> 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);
>
> IWL_DEBUG_INFO(mvm, "entry %d\n", index);
> IWL_DEBUG_INFO(mvm, "\tstatus: %d\n", res->status);
> --
> 2.21.0.rc1
>

--
Thanks,
~Nick Desaulniers