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