Messages in this thread | | | From | Geert Uytterhoeven <> | Date | Mon, 14 May 2018 13:32:30 +0200 | Subject | Re: [PATCH] mtd: nand: Fix return type of __DIVIDE() when called with 32-bit |
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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.
> 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) ? \ > 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 ;-)
Gr{oetje,eeting}s,
Geert
-- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org
In personal conversations with technical people, I call myself a hacker. But when I'm talking to journalists I just say "programmer" or something like that. -- Linus Torvalds
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