Advantages of User DMA

Advantages of User DMA

The dma_start() function and its related functions operate in user space; they do not make system calls to the kernel. This has two important effects. First, overhead is reduced, since there are no mode switches between user and kernel, as there are for read() and write(). This is important, because the DMA engine is often used for frequent, small inputs and outputs.

Second, dma_start() does not block the calling process, in the sense of suspending it and possibly allowing another process to use the CPU. It waits in a test loop until the operation is complete. As you can infer from Table 4-4, typical transfer times range from 50 to 250 microseconds. You can calculate the approximate duration of a call to dma_start() based on the amount of data and the operational mode.

You can use the udmalib functions to access a VME Bus Master device, if the device can respond in slave mode. However, this would normally be less efficient than using the Master device's own DMA circuitry.

While you can initiate only one DMA engine transfer per bus, it is possible to program a DMA engine transfer from each bus in the system, concurrently.