linux-kernel - Re: [RFC 01/32] fs: introduce new 'struct inode

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Message-ID: <m2tx86l4ux.fsf@linux-m68k.org>
Date:	Sat, 31 May 2014 10:39:02 +0200
From:	Andreas Schwab <schwab@...ux-m68k.org>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:	Arnd Bergmann <arnd@...db.de>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux-Arch <linux-arch@...r.kernel.org>,
	"Joseph S. Myers" <joseph@...esourcery.com>,
	John Stultz <john.stultz@...aro.org>,
	Christoph Hellwig <hch@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ley Foon Tan <lftan@...era.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>
Subject: Re: [RFC 01/32] fs: introduce new 'struct inode_time'

Geert Uytterhoeven <geert@...ux-m68k.org> writes:

> Hi Arnd,
>
> On Fri, May 30, 2014 at 10:01 PM, Arnd Bergmann <arnd@...db.de> wrote:
>> + * The variant using bit fields is less efficient to access, but
>> + * small and has a wider range as the 32-bit one, plus it keeps
>> + * the signedness of the original timespec.
>> + */
>> +struct inode_time {
>> +       long long       tv_sec  : 34;
>> +       int             tv_nsec : 30;
>> +};
>
> Don't you need 31 bits for tv_nsec, to accommodate for the sign bit?
> I know you won't really store negative numbers there, but storing a large
> positive number will become negative on read out, won't it?

Only if the int bitfield is signed.  Bitfields are weird, aren't they? :-)

Andreas.

-- 
Andreas Schwab, schwab@...ux-m68k.org
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