[$] A security fix briefly breaks DMA

In theory, direct memory access (DMA) operations are simple to understand;a device transfers data directly to or from a memory buffer managed by theCPU. Almost all contemporary devices perform DMA, since it would not bepossible to obtain the needed performance without it. Like so many things,DMA turns out to be a bit more complicated in practice. That complexityled to an erroneous patch, intended to improve security, breaking DMA forsome devices in 5.17 and some stable kernels.