Specifications

This is the Title of the Book, eMatter Edition

Direct Memory Access

441

Overview of a DMA Data Transfer

Before introducing the programming details, let’s review how a DMA transfer takes

place, considering only input transfers to simplify the discussion.

Data transfer can be triggered in two ways: either the software asks for data (via a

function such as read) or the hardware asynchronously pushes data to the system.

In the first case, the steps involved can be summarized as follows:

1. When a process calls read, the driver method allocates a DMA buffer and

instructs the hardware to transfer its data into that buffer. The process is put to

sleep.

2. The hardware writes data to the DMA buffer and raises an interrupt when it’s

done.

3. The interrupt handler gets the input data, acknowledges the interrupt, and

awakens the process, which is now able to read data.

The second case comes about when DMA is used asynchronously. This happens, for

example, with data acquisition devices that go on pushing data even if nobody is

reading them. In this case, the driver should maintain a buffer so that a subsequent

read call will return all the accumulated data to user space. The steps involved in this

kind of transfer are slightly different:

1. The hardware raises an interrupt to announce that new data has arrived.

2. The interrupt handler allocates a buffer and tells the hardware where to transfer

its data.

3. The peripheral device writes the data to the buffer and raises another interrupt

when it’s done.

4. The handler dispatches the new data, wakes any relevant process, and takes care

of housekeeping.

A variant of the asynchronous approach is often seen with network cards. These

cards often expect to see a circular buffer (often called a DMA ring buffer) estab-

lished in memory shared with the processor; each incoming packet is placed in the

next available buffer in the ring, and an interrupt is signaled. The driver then passes

the network packets to the rest of the kernel and places a new DMA buffer in the

ring.

The processing steps in all of these cases emphasize that efficient DMA handling

relies on interrupt reporting. While it is possible to implement DMA with a polling

driver, it wouldn’t make sense, because a polling driver would waste the perfor-

mance benefits that DMA offers over the easier processor-driven I/O.

* There are, of course, exceptions to everything; see the section “Receive Interrupt Mitigation” in Chapter 17

for a demonstration of how high-performance network drivers are best implemented using polling.

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