Re: [PATCH 5/8] staging: rtl8188eu: Remove function rtw_modular64()

[Ivan Safonov]

On 4/14/20 2:56 PM, Dan Carpenter wrote:
> On Sun, Apr 12, 2020 at 04:34:08PM +0300, Ivan Safonov wrote:
> > > Remove function rtw_modular64 as all it does is call do_div.
> >
> > This is wrong. Macro do_div(x, y) change first argument x, but
> > rtw_modular64(x, y) preserve it.
> >
> > > +			tsf = pmlmeext->TSFValue - do_div(pmlmeext->TSFValue, (pmlmeinfo->bcn_interval*1024)) - 1024; /* us */
> >
> > rounddown(pmlmeext->TSFValue, pmlmeinfo->bcn_interval * 1024) - 1024
> > is a better replacement for
>
> You're absolutely correct that the patch is buggy, but I'm not sure that
> rounddown() is what we want.
>
> rtw_modular64() took the MOD of x.  So it should be something like:
>
> 	tsf = pmlmeext->TSFValue - (pmlmeext->TSFValue % (pmlmeinfo->bcn_interval * 1024)) - 1024; /* us */
>
> But what the heck is that even???  If pmlmeinfo->bcn_interval is zero
> or one then the subtraction ends up giving us a negative.
>
> regards,
> dan carpenter

1. pmlmeext->TSFValue can not be negative, because it is uint64_t;
2. pmlmeext->TSFValue is cyclic value:
    https://en.wikipedia.org/wiki/Timing_synchronization_function ;
3. (rounddown(a, b)) is equal to (a - a % b) by definition.

Ivan Safonov.