DMA Mapping

A common DMA driver bug looks like this: the CPU prepared a buffer, but the device reads old data; the device finished DMA, but the CPU still sees old values; the driver works on one SoC but fails after enabling IOMMU.

The usual root cause is treating a CPU pointer as an address the device can use.

In Linux, user virtual addresses, kernel virtual addresses, physical addresses, and DMA addresses are different layers. A device should receive a DMA address, not an ordinary C pointer.