Direct Memory Access DMA

All PCs include an auxiliary processor called the Direct Memory Access Controller (DMAC), which can be instructed to transfer data between the RAM and an I/O device. Once activated by the CPU, the DMAC is able to continue the data transfer on its own; when the data transfer is completed, the DMAC issues an interrupt request. The conflicts that occur when both CPU and DMAC need to access the same memory location at the same time are resolved by a hardware circuit called a memory arbiter (see Section 5.3.1).

The DMAC is mostly used by disk drivers and other slow devices that transfer a large number of bytes at once. Because setup time for the DMAC is relatively high, it is more efficient to directly use the CPU for the data transfer when the number of bytes is small.

The first DMACs for the old ISA buses were complex, hard to program, and limited to the lower 16 MB of physical memory. More recent DMACs for the PCI and SCSI buses rely on dedicated hardware circuits in the buses and make life easier for device driver developers.

Until now we have distinguished three kinds of memory addresses: logical and linear addresses, which are used internally by the CPU, and physical addresses, which are the memory addresses used by the CPU to physically drive the data bus. However, there is a fourth kind of memory address: the so-called bus address. It corresponds to the memory addresses used by all hardware devices except the CPU to drive the data bus. In the PC architecture, bus addresses coincide with physical addresses; however, in other architectures (like Sun's SPARC and Hewlett-Packard's Alpha), these two kinds of addresses differ.

Why should the kernel be concerned at all about bus addresses? Well, in a DMA operation, the data transfer takes place without CPU intervention; the data bus is driven directly by the I/O device and the DMAC. Therefore, when the kernel sets up a DMA operation, it must write the bus address of the memory buffer involved in the proper I/O ports of the DMAC or I/O device.

13.1.4.1 Putting DMA to work

Several I/O drivers use the Direct Memory Access Controller (DMAC) to speed up operations. The DMAC interacts with the device's I/O controller to perform a data transfer and the kernel includes an easy-to-use set of routines to program the DMAC. The I/O controller signals to the CPU, via an IRQ, when the data transfer has finished.

When a device driver sets up a DMA operation for some I/O device, it must specify the memory buffer involved by using bus addresses. The kernel provides the virt_to_bus and bus_to_virt macros, respectively, to translate a linear address into a bus address and vice

As with IRQ lines, the DMAC is a resource that must be assigned dynamically to the drivers that need it. The way the driver starts and ends DMA operations depends on the type of bus.

For recent buses, such as PCI or SCSI, there are two main steps to perform: allocating an IRQ line and triggering the DMA transfer. The IRQ line used for signaling the termination of the DMA operation is allocated when opening the device file (see the later section Section 13.3.4). To start a DMA operation, the device driver simply writes the bus address of the DMA buffer, the transfer direction, and the size of the data in an I/O port of the hardware device; the driver then suspends the current process. When the DMA transfer ends, the hardware device raises an interrupt that wakes the device driver. The release method of the device file releases the IRQ line when the file object is closed by the last process.

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