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

[Boris Brezillon]

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.