Re: [PATCH] mtd: nand: Fix return type of __DIVIDE() when called with 32-bit

[Boris Brezillon]

On Mon, 14 May 2018 13:46:07 +0200
Boris Brezillon <boris.brezillon@xxxxxxxxxxx> wrote:

> On Mon, 14 May 2018 13:32:30 +0200
> Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote:
> 
> > Hi Boris,
> > 
> > On Mon, May 14, 2018 at 1:23 PM, Boris Brezillon
> > <boris.brezillon@xxxxxxxxxxx> wrote:  
> > > On Mon, 14 May 2018 12:49:37 +0200
> > > Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote:    
> > >> The __DIVIDE() macro checks whether it is called with a 32-bit or 64-bit
> > >> dividend, to select the appropriate divide-and-round-up routine.
> > >> As the check uses the ternary operator, the result will always be
> > >> promoted to a type that can hold both results, i.e. unsigned long long.
> > >>
> > >> When using this result in a division on a 32-bit system, this may lead
> > >> to link errors like:
> > >>
> > >>     ERROR: "__udivdi3" [drivers/mtd/nand/raw/nand.ko] undefined!
> > >>
> > >> Fix this by casting the result of the 64-bit division to the type of the
> > >> dividend.
> > >>
> > >> Fixes: 8878b126df769831 ("mtd: nand: add ->exec_op() implementation")
> > >> Signed-off-by: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
> > >> ---
> > >> This fixes the root cause of the link failure seen with
> > >> m68k/allmodconfig since commit 3057fcef385348fe ("mtd: rawnand: Make
> > >> sure we wait tWB before polling the STATUS reg").
> > >>
> > >> An alternative mitigation was posted as "[PATCH] m68k: Implement
> > >> ndelay() as an inline function to force type checking/casting"
> > >> (https://lkml.org/lkml/2018/5/13/102).
> > >> ---
> > >>  include/linux/mtd/rawnand.h | 2 +-
> > >>  1 file changed, 1 insertion(+), 1 deletion(-)
> > >>
> > >> diff --git a/include/linux/mtd/rawnand.h b/include/linux/mtd/rawnand.h
> > >> index 5dad59b312440a9c..d06dc428ea0102ae 100644
> > >> --- a/include/linux/mtd/rawnand.h
> > >> +++ b/include/linux/mtd/rawnand.h
> > >> @@ -871,7 +871,7 @@ struct nand_op_instr {
> > >>  #define __DIVIDE(dividend, divisor) ({                                       \
> > >>       sizeof(dividend) == sizeof(u32) ?                               \
> > >>               DIV_ROUND_UP(dividend, divisor) :                       \
> > >> -             DIV_ROUND_UP_ULL(dividend, divisor);                    \
> > >> +             (__typeof__(dividend))DIV_ROUND_UP_ULL(dividend, divisor); \    
> > >
> > > Hm, it's a bit hard to follow when you place the cast here. One could
> > > wonder why a cast to (__typeof__(dividend)) is needed since
> > > DIV_ROUND_UP_ULL() already returns a (__typeof__(dividend)) type.    
> > 
> > DIV_ROUND_UP_ULL() does not return __typeof__(dividend), but
> > unsigned long long.  
> 
> Except if you entered this branch, that means you passed an unsigned
> long long dividend (AKA u64), otherwise you would go in DIV_ROUND_UP().
> Am I missing something?
> 
> >   
> > > How about:
> > >
> > >         /*
> > >          * Cast to type of dividend is needed here to guarantee that the
> > >          * result won't be an unsigned long long when the dividend is an
> > >          * unsigned long, which is what the compiler does when it sees a    
> > 
> > s/an unsigned long/32-bit/
> >   
> > >          * ternary operator with 2 different return types.
> > >          */
> > >         (__typeof__(dividend))(sizeof(dividend) == sizeof(u32) ?        \  
> 
> To be completely safe and handle cases where dividend is an unsigned
> short or an unsigned, we should probably have:
> 
> 	(__typeof__(dividend))(sizeof(dividend) == sizeof(unsigned long long) ?	\
> 			       DIV_ROUND_UP_ULL(dividend, divisor) :
> 			       DIV_ROUND_UP(dividend, divisor));
> 
> > >                                DIV_ROUND_UP(dividend, divisor) :        \
> > >                                DIV_ROUND_UP_ULL(dividend, divisor));    
> > 
> > Looks fine to me, too.
> >   
> > > Actually, I'm not even sure we care about the truncation that could
> > > happen on an unsigned long long -> unsigned long cast because the
> > > delays we express here will anyway be hundreds of nanosecs/millisecs,
> > > so nothing close to the billions of nanosecs/millisecs you can express
> > > with an unsigned long.
> > >
> > > So, maybe we should just do:
> > >
> > >         (unsigned long)(sizeof(dividend) == sizeof(u32) ?               \
> > >                         DIV_ROUND_UP(dividend, divisor) :               \
> > >                         DIV_ROUND_UP_ULL(dividend, divisor));
> > >
> > > to make things more readable.    
> > 
> > That would break callers who pass a 64-bit dividend, and expect to receive
> > a 64-bit quotient back (on 32-bit systems).
> > Calling e.g. PSEC_TO_NSEC(1000000000000ULL) is valid, passing the
> > result to ndelay() isn't ;-)  
> 
> Well, theoretically, yes it's possible, in practice, we only ever pass
> u32 types to PSEC_TO_NSEC() and u64 types to PSEC_TO_MSEC(), so why
> bother.

Anyway, will apply your patch with the comment (and the fix you
suggested). I was just continuing the discussion to point out that we
don't care that much about the return type here, an u32 would be just
fine.

Thanks,

Boris