78 lines
4.2 KiB
Plaintext
78 lines
4.2 KiB
Plaintext
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.\" $NetBSD: 5.me,v 1.1 1998/07/15 00:34:54 thorpej Exp $
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.\"
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.\" Copyright (c) 1998 Jason R. Thorpe.
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgements:
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.\" This product includes software developed for the NetBSD Project
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.\" by Jason R. Thorpe.
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.\" 4. The name of the author may not be used to endorse or promote products
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.\" derived from this software without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
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.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
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.\" BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
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.\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
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.\" AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
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.\" OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.sh 1 "Conclusions"
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.pp
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The \fIbus_dma\fR interface was introduced into the NetBSD kernel
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at development version 1.2G, just before the release cycle for NetBSD 1.3
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began. When the code was committed to the NetBSD master sources, several
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drivers, mostly for SCSI controllers, were converted to the interface at
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the same time. (All of these drivers had been previously converted to
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use the \fIbus_space\fR interface.) Not only did these drivers provide
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an example of the correct use of \fIbus_dma\fR, but they provided
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functionality that had
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not previously existed in the NetBSD kernel: support for bus mastering ISA
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devices in PCs with more than 16MB of RAM.
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.pp
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The first real test of the interface on the Alpha platform came by
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installing a bus mastering ISA device (an Adaptec 1542 SCSI controller)
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in an AXPpci33 computer. After addressing a small bug in the Alpha
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implementation of \fIbus_dmamap_load\fR(), the device worked flawlessly.
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.pp
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When converting device drivers to use the new interface, developers
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discovered that a fair amount of mostly-similar code could be removed
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from each driver converted. The code in question was the loop that
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built the software scatter-gather list. In some cases, the drivers
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performed noticeably better, due to the fact that the implementation
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of this loop within \fIbus_dmamap_load()\fR is more efficient and
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supports segment concatenation.
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.pp
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Most of the machine-independent drivers that use DMA have been converted
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to the new interface, and more platforms have implemented the
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necessary back-ends. The results have been very encouraging. Nearly
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every device/platform combination that has been tested has worked without
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additional modifications to the device driver. The few exceptions to this
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have generally been to handle differences in host and device byte-order,
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and are not directly related to DMA.
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.pp
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The \fIbus_dma\fR interface has also paved the way for additional
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machine-independent bus autoconfiguration frameworks, such as for VME.
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Eventually, this will help support PCI-to-VME bridges, and allow
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Sun, Motorola, and Intel systems to share common VME device drivers.
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.pp
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We have found the \fIbus_dma\fR interface to be a major architectural
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benefit in the NetBSD kernel, greatly simplifying the process of porting
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the kernel to new platforms, and making portable device driver development
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considerably easier. In short, the abstraction has delivered what it was
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designed to deliver: a means of supporting a wide range of platforms with
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maximum code reuse.
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