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.
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