[$] DMA-BUF cache handling: Off the DMA API map (part 2)

Part 1 of this series, covered somebackground on ION, DMA-BUF heaps, the DMA API, and the concept of"ownership" when it comes to handling CPU-cache maintenance, finally endingon a conventional DMA API view of how DMA-BUF cache handling should bedone. The article concluded with a discussion of why the traditional DMAAPIs can perform poorly on contemporary systems. This article completesthe series with an exploration ofsome of the approaches that DMA-BUF exporters can use to avoidunnecessary cache operations along with some rough proposals for how wemight improve things.