From owner-freebsd-hackers@FreeBSD.ORG  Mon Jun 13 19:59:27 2005
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Date: Mon, 13 Jun 2005 20:59:17 +0100
From: Christoph Hellwig <hch@infradead.org>
To: Julian Elischer <julian@elischer.org>
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Cc: Daniel Eischen <deischen@freebsd.org>, Scott Long <scottl@samsco.org>,
	Apache Xie <apachexm@hotmail.com>, freebsd-hackers@freebsd.org
Subject: Re: contigmalloc() and mmap()
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On Mon, Jun 13, 2005 at 12:54:40PM -0700, Julian Elischer wrote:
> though, some people use it for that purpose (e.g. in the original posting).

driver writers do all kinds of odd things ;-)

> it might not be such a bad idea..
> I don't see why the device entrypoints shouldn't have that argument
> available.. (file descriptor by which we are getting here)
> As long as it can take account of the fact that not all accesses come 
> via an FD
> (e.g mounted disks).

disk drivers use a completely different set of entry points in Linux,
and don't have access to per-fd data even in the case they're opened
from userland.  Character drivers to which this applies OTOH always
get a valid struct file, it's guranteed as part of the driver API.