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

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Message-ID: <1447401166.6012.2.camel@perches.com>
Date:	Thu, 12 Nov 2015 23:52:46 -0800
From:	Joe Perches <joe@...ches.com>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Arnd Bergmann <arnd@...db.de>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	David Miller <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	mathieu@...eaurora.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Giuseppe Cavallaro <peppe.cavallaro@...com>
Subject: Re: [PATCH v2] stmmac: avoid ipq806x constant overflow warning

On Fri, 2015-11-13 at 08:37 +0100, Geert Uytterhoeven wrote:
> On Thu, Nov 12, 2015 at 10:03 PM, Arnd Bergmann <arnd@...db.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/commi
> t/?id=a9efeca613a8fe5281d7c91f5c8c9ea46f2312f6
> 
> Gr{oetje,eeting}s,

I still think specific length BIT macros
can be useful.

https://lkml.org/lkml/2015/10/16/852

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