Re: [PATCH] stmmac: avoid ipq806x constant overflow warning

From: David Miller
Date: Thu Nov 12 2015 - 12:25:36 EST

From: Arnd Bergmann <arnd@xxxxxxxx>
Date: Thu, 12 Nov 2015 15:12:48 +0100

> Building dwmac-ipq806x on a 64-bit architecture produces a harmless
> warning from gcc:
> stmmac/dwmac-ipq806x.c: In function 'ipq806x_gmac_probe':
> include/linux/bitops.h:6:19: warning: overflow in implicit constant conversion [-Woverflow]
> stmmac/dwmac-ipq806x.c:333:8: note: in expansion of macro 'QSGMII_PHY_CDR_EN'
> #define QSGMII_PHY_CDR_EN BIT(0)
> #define BIT(nr) (1UL << (nr))
> The compiler warns about the fact that a 64-bit literal is passed
> into a function that takes a 32-bit argument. I could not fully understand
> why it warns despite the fact that this number is always small enough
> to fit, but changing the use of BIT() macros into the equivalent hexadecimal
> representation avoids the warning
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> Fixes: b1c17215d718 ("stmmac: add ipq806x glue layer")

I've seen this warning too on x86_64 and had been meaning to look
into it, thanks for taking the initiative. :)

Moving away from using BIT() is somewhat disappointing, because we
want to encourage people to use these macros.

I think it's also easier from a driver author and auditing
perspective, you can see that something is being defined as bit X and
then check the documentation for the chip to see if bit X is correct
or not.

With the hex values there is more mental work and room for... mistakes.

Also I don't even understand the compiler's behavior, it's warning
about QSGMII_PHY_CDR_EN but if you define only that to "0x1u" it still
warns about QSGMII_PHY_CDR_EN.

The warning goes away only if you change all 5 BIT() uses.

This makes me like the change even less, something foul is going on
here and I'd rather figure out what that is than install this patch.

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at