From: Roland Dreier <rdreier@cisco.com>
To: lkml@MoreThan.org
Cc: linux-kernel@vger.kernel.org
Subject: Re: [Compile Warning] 2.6.30-rc8 build
References: <200906051150.17889.lkml@morethan.org>
Date: Fri, 05 Jun 2009 10:28:00 -0700
In-Reply-To: <200906051150.17889.lkml@morethan.org> (Michael S. Zick's message
	of "Fri, 5 Jun 2009 11:50:15 -0500")
Message-ID: <ada1vpywpy7.fsf@cisco.com>
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 > I can't argue with gcc on this one either:
 > 
 > drivers/scsi/sd.c: In function 'sd_read_capacity':
 > drivers/scsi/sd.c:1451: warning: comparison is always false due to limited range of data type
 > 
 > It reads to my eyes as if the function can never
 > select read_capacity_16 for very large devices.

The code is:

		if ((sizeof(sdkp->capacity) > 4) &&
		    (sdkp->capacity > 0xffffffffULL)) {

sdkp->capacity is a sector_t, and <linux/types.h> has:

	#ifdef CONFIG_LBD
	typedef u64 sector_t;
	typedef u64 blkcnt_t;
	#else
	typedef unsigned long sector_t;
	typedef unsigned long blkcnt_t;
	#endif

so if you don't set CONFIG_LBD on a 32-bit architecture, then you are
correct that sd.c won't ever hit the READ_CAPACITY(16) case, and the
kernel won't be able to handle large SCSI disks.
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