From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2993327AbXDTVMb (ORCPT ); Fri, 20 Apr 2007 17:12:31 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754508AbXDTVMb (ORCPT ); Fri, 20 Apr 2007 17:12:31 -0400 Received: from smtp1.linux-foundation.org ([65.172.181.25]:58509 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754496AbXDTVM3 (ORCPT ); Fri, 20 Apr 2007 17:12:29 -0400 Date: Fri, 20 Apr 2007 14:12:18 -0700 From: Andrew Morton To: James Bottomley Cc: "Cameron, Steve" , "Miller, Mike (OS Dev)" , Hisashi Hifumi , jens.axboe@oracle.com, linux-kernel@vger.kernel.org, linux-scsi@vger.kernel.org Subject: Re: [PATCH] cciss: Fix warnings during compilation under 32bitenvironment Message-Id: <20070420141218.a0222472.akpm@linux-foundation.org> In-Reply-To: <1177100459.3718.45.camel@mulgrave.il.steeleye.com> References: <6.0.0.20.2.20070418160231.043f23c8@172.19.0.2> <226E1C65E4F6164E8EA5FD3CC913AE8CF461DE@G3W0639.americas.hpqcorp.net> <1176995934.31764.7.camel@mulgrave.il.steeleye.com> <226E1C65E4F6164E8EA5FD3CC913AE8CF462EE@G3W0639.americas.hpqcorp.net> <1176999730.31764.15.camel@mulgrave.il.steeleye.com> <558F4D473FD7FE419B019232BF2D37B401167D74@G3W0634.americas.hpqcorp.net> <20070419222028.d33c7f86.akpm@linux-foundation.org> <1177074641.3718.5.camel@mulgrave.il.steeleye.com> <20070420114337.12a67141.akpm@linux-foundation.org> <1177095007.3718.43.camel@mulgrave.il.steeleye.com> <20070420123041.94aac1e1.akpm@linux-foundation.org> <1177100459.3718.45.camel@mulgrave.il.steeleye.com> X-Mailer: Sylpheed version 2.2.7 (GTK+ 2.8.6; i686-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 20 Apr 2007 16:20:59 -0400 James Bottomley wrote: > On Fri, 2007-04-20 at 12:30 -0700, Andrew Morton wrote: > > On Fri, 20 Apr 2007 14:50:06 -0400 > > James Bottomley wrote: > > > > > > CONFIG_LBD=y gives us an additional 3kb of instructions on i386 > > > > allnoconfig. Other architectures might do less well. It's not a huge > > > > difference, but that's the way in which creeping bloatiness happens. > > > > > > OK, sure, but if we really care about this saving, then unconditionally > > > casting to u64 is therefore wrong as well ... this is starting to open > > > quite a large can of worms ... > > > > > > For the record, if we have to do this, I fancy sector_upper_32() ... we > > > should already have some similar accessor for dma_addr_t as well. > > > > hm. How about this? > > > > --- a/include/linux/kernel.h~upper-32-bits > > +++ a/include/linux/kernel.h > > @@ -40,6 +40,17 @@ extern const char linux_proc_banner[]; > > #define DIV_ROUND_UP(n,d) (((n) + (d) - 1) / (d)) > > #define roundup(x, y) ((((x) + ((y) - 1)) / (y)) * (y)) > > > > +/** > > + * upper_32_bits - return bits 32-63 of a number > > + * @n: the number we're accessing > > + * > > + * A basic shift-right of a 64- or 32-bit quantity. Use this to suppress > > + * the "right shift count >= width of type" warning when that quantity is > > + * 32-bits. > > + */ > > +#define upper_32_bits(n) (((u64)(n)) >> 32) > > Won't this have the unwanted side effect of promoting everything in a > calculation to long long on 32 bit platforms, even if n was only 32 > bits? bummer. > > + > > + > > #define KERN_EMERG "<0>" /* system is unusable */ > > #define KERN_ALERT "<1>" /* action must be taken immediately */ > > #define KERN_CRIT "<2>" /* critical conditions */ > > _ > > > > It seems to generate the desired code. I avoided Alan's ((n >> 31) >> 1) > > trick because it'll generate peculiar results with signed 64-bit > > quantities. > > I've seen the trick done similarly with ((n >> 16) >> 16) which > shouldn't have the issue. That works if we know the caller is treating the return value as 32 bits, but we don't know that. If we have #define upper_32_bits(x) ((x >> 16) >> 16) then upper_32_bits(0x8888777766665555) will return 0x88887777 if it's treated as 32-bits, but it'll return 0xffffffff88887777 if the caller is using 64-bits. I spose #define upper_32_bits(x) ((u32)((x >> 16) >> 16)) will do the trick.