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A Machine-Independent DMA Framework for Netbsd

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A Machine-Independent DMA Framework for Netbsd AMachine-Independent DMA Framework for NetBSD Jason R. Thorpe1 Numerical Aerospace Simulation Facility NASA Ames Research Center Abstract 1.1. Host platform details One of the challenges in implementing a portable In the example platforms listed above,there are at kernel is finding good abstractions for semantically- least three different mechanisms used to perform DMA. similar operations which often have very machine- The first is used by the i386 platform. This mechanism dependent implementations. This is especially impor- can be described as "what you see is what you get": the tant on modern machines which share common archi- address that the device uses to perform the DMA trans- tectural features, e.g. the PCI bus. fer is the same address that the host CPU uses to access This paper describes whyamachine-independent the memory location in question. DMA mapping abstraction is needed, the design consid- DMA address Host address erations for such an abstraction, and the implementation of this abstraction in the NetBSD/alpha and NetBSD/i386 kernels. 1. Introduction NetBSD is a portable, modern UNIX-likeoperat- ing system which currently runs on eighteen platforms covering nine processor architectures. Some of these platforms, including the Alpha and i3862,share the PCI busasacommon architectural feature. In order to share device drivers for PCI devices between different platforms, abstractions that hide the details of bus access must be invented. The details that must be hid- den can be broken down into twoclasses: CPU access Figure 1 - WYSIWYG DMA to devices on the bus (bus_space)and device access to host memory (bus_dma). Here we will discuss the lat- The second mechanism, employed by the Alpha, ter; bus_space is a complicated topic in and of itself, is very similar to the first; the address the host CPU and is beyond the scope of this paper. uses to access the memory location in question is offset Within the scope of DMA, there are twobroad from some base address at which host memory is classes of details that must be hidden from the core direct-mapped on the device bus for the purpose of device driver. The first class, host details, deals with DMA. issues such as the physical mapping of system memory (and the DMA mechanisms employed as a result of such mapping) and cache semantics. The second class, busdetails, deals with issues related to features or limi- tations specific to the bus to which a device is attached, such as DMA bursting and address line limitations. 1Jason R. Thorpe is an employee of MRJ Technology Solu- tions, Inc. This work is funded by NASA contract NAS2-14303. 2The term "i386" is used here to refer to all of the 386-class and higher processors, including the i486, Pentium, Pentium Pro, and Pentium II. DMA address Host address issues exist with the TurboChannel bus used on DEC- stations and early Alpha systems, and with the Q-bus used on some DEC MIPS and VAX-based servers. The semantics of the host system’scache are also important to devices which wish to perform DMA. Some systems are capable of cache-coherent DMA. On such systems, the cache is often write-through (i.e. stores are written both to the cache and to host mem- ory), or the cache has special snooping logic that can detect access to a memory location for which there is a dirty cache line (which causes the cache to be flushed automatically). Other systems are not capable of cache- coherent DMA. On these systems, software must explicitly flush anydata caches before memory-to- Figure 2 - direct-mapped DMA device DMA transfers, as well as invalidate soon-to-be- stale cache lines before device-to-memory DMA. The third mechanism, scatter-gather-mapped DMA, employs an MMU which performs translation of 1.2. Bus details DMA addresses to host memory physical addresses. In addition to hiding the platform-specific DMA This mechanism is also used by the Alpha, because details for a single bus, it is desirable to share as much Alpha platforms implement a physical address space device drivercode as possible for a device which may sometimes significantly larger than the 32-bit address attach to multiple busses. A good example is the Bus- space supported by most currently-available PCI Logic family of SCSI adapters. This family of devices devices. comes in ISA, EISA, VESA local bus, and PCI flavors. DMA address Host address While there are some bus-specific details, such as prob- ing and interrupt initialization, the vast majority of the code that drivesthis family of devices is identical for each flavor. The BusLogic family of SCSI adapters are exam- ples of what are termed busmasters.That is to say,the device itself performs all bus handshaking and host memory access during a DMA transfer.Nothird party MMU is involved in the transfer.Such devices, when per- forming a DMA transfer,present the DMA address on the bus address lines, execute the bus’sfetch or store operation, increment the address, and so forth until the transfer is complete. Because the device is using the busaddress lines, the range of host physical addresses the device can access is limited by the number of such Figure 3 - scatter-gather-mapped DMA lines. On the PCI bus, which has at least 32 address The second and third DMA mechanisms above lines, the device may be able to access the entire physi- are combined on the Alpha through the use of DMA cal address space of a 32-bit architecture, such as the windows.The ASIC which implements the PCI bus on i386. ISA, however, only has 24 address lines. This aparticular platform has at least twoofthese DMA means that the device can directly access only 16MB of windows. Each windowmay be configured for direct- physical address space. mapped or scatter-gather-mapped DMA. Windows are Acommon solution to the limited-address-lines chosen based on the type of DMA transfer being per- problem is a technique known as DMA bouncing.This formed, the bus type, and the physical address range of technique involves a second memory area, located in the host memory being accessed. the physical address range accessible by the device, These concepts apply to platforms other than known as a bounce buffer.Inamemory-to-device those listed above and busses other than PCI. Similar transfer,the data is copied by the CPU to the bounce buffer,and the DMA operation is started. Conversely, in a device-to-memory transfer,the DMA operation is space of a specific process. It is most commonly used started, and the CPU then copies the data from the by the read(2) and write(2) system calls. While it bounce buffer once the DMA operation has completed. would be possible for the device drivertotreat the two While simple to implement, DMA bouncing is more complexbuffer structures as sets of multiple sim- not the most elegant way to solvethe limited-address- ple linear buffers, this is undesirable in terms of source line problem. On the Alpha, for example, scatter- code maintenance; the code to handle these data buffer gather-mapped DMA may be used to translate the out- structures can be complex, especially in terms of error of-range memory physical addresses to in-range DMA handling. addresses that the device may use. This solution tends In addition to the obvious need to DMA to and to offer better performance due to eliminated data from memory mapped into kernel address space, it is copies, and is less expensive interms of memory usage. common in modern operating systems to implement an Returning to the BusLogic SCSI example, it is optimized I/O interface for user processes which pro- undesirable to place intimate knowledge of direct-map- vides a method for devices to DMA directly to or from ping, scatter-gather-mapping, and DMA bouncing in memory regions mapped into a process’saddress space. the core device driver. Clearly,anabstraction that hides While this facility is partially provided for character these details and presents a consistent interface, regard- device I/O by double-mapping the user buffer into ker- less of the DMA mechanism being used, is needed. nel address space, the interface is not sufficiently gen- eral, and consumes kernel resources. This is somewhat 2. Design considerations related to the uio structure, in that the uio is capable of addressing buffers in a process’saddress space. How- Hiding host and bus details is actually very ev eritmay be desirable to use an alternate data format, straightforward. Handling WYSIWYG and direct- such as a linear buffer,insome applications. In order to mapped DMA mechanisms is trivial. Handling scatter- implement this, the DMA mapping framework must gather-mapped DMA is also very easy,with the help of have access to processes’ virtual memory structures. state kept in machine-dependent code layers. The pres- ence and semantics of caches are also easy to handle It may also be desirable to DMA to or from with a set of four "synchronization" operations, and buffers not mapped into anyaddress space. The obvi- once caches are handled, DMA bouncing is conceptu- ous example is frame grabbers. These devices, which ally trivial if viewed as a non-DMA-coherent cache. capture video images, often require large, physically Unfortunately,while these operations are quite easy to contiguous memory regions to store the captured image do individually,traditional kernels do not provide a suf- data. On some architectures, mapping of virtual ficiently abstract interface to the operations. This address space is expensive.Anapplication may wish to means that device drivers in these traditional kernels give a large buffer to the device, allowthe device to must handle each case explicitly.
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