Messages in this thread | | | Date | Mon, 14 May 2018 13:46:07 +0200 | From | Boris Brezillon <> | Subject | Re: [PATCH] mtd: nand: Fix return type of __DIVIDE() when called with 32-bit |
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On Mon, 14 May 2018 13:32:30 +0200 Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> Hi Boris, > > On Mon, May 14, 2018 at 1:23 PM, Boris Brezillon > <boris.brezillon@bootlin.com> wrote: > > On Mon, 14 May 2018 12:49:37 +0200 > > Geert Uytterhoeven <geert@linux-m68k.org> 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@linux-m68k.org> > >> --- > >> 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.
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