On Thu, Nov 12, 2015 at 10:03 PM, Arnd Bergmann <a...@arndb.de> 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] > val = QSGMII_PHY_CDR_EN | > 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. https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=a9efeca613a8fe5281d7c91f5c8c9ea46f2312f6 Gr{oetje,eeting}s, Geert -- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- ge...@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 -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
