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

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Date:	Fri, 13 Nov 2015 09:00:26 +0100
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Joe Perches <joe@...ches.com>
Cc:	Arnd Bergmann <arnd@...db.de>,
	"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

Hi Joe,

On Fri, Nov 13, 2015 at 8:52 AM, Joe Perches <joe@...ches.com> wrote:
> 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
>
> I still think specific length BIT macros
> can be useful.
>
> https://lkml.org/lkml/2015/10/16/852

Yeah!

I only recently started liking the BIT() macro (before I preferred hex, too).
Perhaps because Renesas datasheets use bit numbers all over the place ;-)

Gr{oetje,eeting}s,

                        Geert

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