linux-kernel - Re: [PATCH] cciss: Fix warnings during compilation under 32bitenvironment

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Date:	Fri, 20 Apr 2007 12:30:41 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	James Bottomley <James.Bottomley@...elEye.com>
Cc:	"Cameron, Steve" <Steve.Cameron@...com>,
	"Miller, Mike (OS Dev)" <Mike.Miller@...com>,
	Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp>,
	jens.axboe@...cle.com, linux-kernel@...r.kernel.org,
	linux-scsi@...r.kernel.org
Subject: Re: [PATCH] cciss: Fix warnings during compilation under
 32bitenvironment

On Fri, 20 Apr 2007 14:50:06 -0400
James Bottomley <James.Bottomley@...elEye.com> 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)
+
+
 #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.

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