From owner-freebsd-hackers@FreeBSD.ORG  Tue May  9 18:44:12 2006
Return-Path: <owner-freebsd-hackers@FreeBSD.ORG>
X-Original-To: freebsd-hackers@freebsd.org
Delivered-To: freebsd-hackers@freebsd.org
Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125])
	by hub.freebsd.org (Postfix) with ESMTP id 10D3216A470
	for <freebsd-hackers@freebsd.org>; Tue,  9 May 2006 18:44:12 +0000 (UTC)
	(envelope-from pm940@yahoo.com)
Received: from web54008.mail.yahoo.com (web54008.mail.yahoo.com
	[206.190.36.232])
	by mx1.FreeBSD.org (Postfix) with SMTP id 78D3743D45
	for <freebsd-hackers@freebsd.org>; Tue,  9 May 2006 18:44:11 +0000 (GMT)
	(envelope-from pm940@yahoo.com)
Received: (qmail 1517 invoked by uid 60001); 9 May 2006 18:44:10 -0000
DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws; s=s1024; d=yahoo.com;
	h=Message-ID:Received:Date:From:Subject:To:MIME-Version:Content-Type:Content-Transfer-Encoding;
	b=omJcmItMWfj/2sJT8JPOMauiwH1CMB9QqWZ/MDZC0w9DIcn+xSVwSgKGkyPHV4Fm6BkLEO9X7+vg6dXqO0PVIO628MvDR0cB7GQ0sMvxq3jihECUBc9nfYgPSwlwtNRuo99h7QdeRVCwoMdqbk7zuZveVVf8Mx7Uog1ZUIhJzV8=
	; 
Message-ID: <20060509184410.1515.qmail@web54008.mail.yahoo.com>
Received: from [71.140.186.33] by web54008.mail.yahoo.com via HTTP;
	Tue, 09 May 2006 11:44:10 PDT
Date: Tue, 9 May 2006 11:44:10 -0700 (PDT)
From: Paul Marciano <pm940@yahoo.com>
To: freebsd-hackers@freebsd.org
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
Content-Transfer-Encoding: 8bit
X-Mailman-Approved-At: Tue, 09 May 2006 21:24:37 +0000
Subject: Question regarding bus_dma_tag_create().
X-BeenThere: freebsd-hackers@freebsd.org
X-Mailman-Version: 2.1.5
Precedence: list
List-Id: Technical Discussions relating to FreeBSD
	<freebsd-hackers.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-hackers>, 
	<mailto:freebsd-hackers-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-hackers>
List-Post: <mailto:freebsd-hackers@freebsd.org>
List-Help: <mailto:freebsd-hackers-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-hackers>,
	<mailto:freebsd-hackers-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Tue, 09 May 2006 18:44:17 -0000

I posted this onto freebsd-questions a couple of days
ago but received no reply.  Hopefully this is a more
appropriate audience.


The man page for bus_dma_map_create() says this about
the "nsegments" parameter:

Number of discontinuities (scatter/gather seg-
ments) allowed in a DMA mapped region.  If there
is no restriction, BUS_SPACE_UNRESTRICTED may be
specified.

BUS_SPACE_UNRESTRICTED is #defined as (~0).

Then, in busdma_map_create() and busdma_mem_alloc():

if (dmat->segments == NULL) {
    dmat->segments = (bus_dma_segment_t *)malloc(
        sizeof(bus_dma_segment_t) * dmat->nsegments,
M_DEVBUF,
        M_NOWAIT);
    ...
}


I don't understand how this works when
BUS_SPACE_UNRESTRICTED is specified.  The malloc will
be fed a -8 (or -12 with PAE) on i386.


I'm reviewing a driver at work that specifies
nsegments = 0, which I think is invalid, but in
reading the man page I came across this ~0 option,
which I've also seen in other FreeBSD drivers and I
just don't get it.


I hope someone can clue me in.


Thanks,
Paul.


__________________________________________________
Do You Yahoo!?
Tired of spam?  Yahoo! Mail has the best spam protection around 
http://mail.yahoo.com