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Date: Mon, 14 May 2018 13:55:43 +0200
From: Boris Brezillon <boris.brezillon@...tlin.com>
To: Geert Uytterhoeven <geert@...ux-m68k.org>
Cc: Linux/m68k <linux-m68k@...r.kernel.org>,
Richard Weinberger <richard@....at>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Marek Vasut <marek.vasut@...il.com>,
MTD Maling List <linux-mtd@...ts.infradead.org>,
Miquel Raynal <miquel.raynal@...tlin.com>,
Brian Norris <computersforpeace@...il.com>,
David Woodhouse <dwmw2@...radead.org>
Subject: Re: [PATCH] mtd: nand: Fix return type of __DIVIDE() when called
with 32-bit
On Mon, 14 May 2018 13:46:07 +0200
Boris Brezillon <boris.brezillon@...tlin.com> wrote:
> On Mon, 14 May 2018 13:32:30 +0200
> Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
>
> > Hi Boris,
> >
> > On Mon, May 14, 2018 at 1:23 PM, Boris Brezillon
> > <boris.brezillon@...tlin.com> wrote:
> > > On Mon, 14 May 2018 12:49:37 +0200
> > > Geert Uytterhoeven <geert@...ux-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@...ux-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.
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
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