[Letux-kernel] [PATCH] MIPS: Fix a longstanding error in div64.h

[Maciej W. Rozycki]

On Wed, 7 Apr 2021, Huacai Chen wrote:

> >  This code is rather broken in an obvious way, starting from:
> >
> >         unsigned long long __n;                                         \
> >                                                                         \
> >         __high = *__n >> 32;                                            \
> >         __low = __n;                                                    \
> >
> > where `__n' is used uninitialised.  Since this is my code originally I'll
> > look into it; we may want to reinstate `do_div' too, which didn't have to
> > be removed in the first place.
> I think we can reuse the generic do_div().

 We can, but it's not clear to me if this is optimal.  We have a DIVMOD 
instruction which original code took advantage of (although I can see 
potential in reusing bits from include/asm-generic/div64.h).  The two 
implementations would have to be benchmarked against each other across a 
couple of different CPUs.

> >  Huacai, thanks for your investigation!  Please be more careful in
> > verifying your future submissions however.
> Sorry, I thought there is only one bug in div64.h, but in fact there
> are three...

 This just shows the verification you made was not good enough, hence my 
observation.

  Maciej