Re: [PATCH v2] stmmac: avoid ipq806x constant overflow warning

From: Geert Uytterhoeven
Date: Fri Nov 13 2015 - 02:38:46 EST

On Thu, Nov 12, 2015 at 10:03 PM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> 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))
> This is a result of the type conversion rules in C, when we take the
> logical OR of multiple different types. In particular, we have
> and unsigned long
> QSGMII_PHY_CDR_EN == BIT(0) == (1ul << 0) == 0x0000000000000001ul
> and a signed int
> 0xC << QSGMII_PHY_TX_DRV_AMP_OFFSET == 0xc0000000
> which together gives a signed long value
> 0xffffffffc0000001l
> and when this is passed into a function that takes an unsigned int type,
> gcc warns about the signed overflow and the loss of the upper 32-bits that
> are all ones.
> This patch adds 'ul' type modifiers to the literal numbers passed in
> here, so now the expression remains an 'unsigned long' with the upper
> bits all zero, and that avoids the signed overflow and the warning.

FWIW, the 64-bitness of BIT() on 64-bit platforms is also causing subtle
warnings in other places, e.g. when inverting them to create bit mask, cfr.



Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

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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