[$] DMA-BUF cache handling: Off the DMA API map (part 1)

Recently, the DMA-BUF heapsinterface was added to the 5.6 kernel. Thisinterface is similar to ION,which has been used for years by Android vendors. However, in trying to move vendors touse DMA-BUF heaps, we have begun to see how the DMA API modeldoesn't fit well for modern mobile devices. Additionally, the lack of clearguidance in how to handle cache operations efficiently, results in vendorsusing custom device-specific optimizations that aren't generic enough foran upstream solution. This article will describe the nature of theproblem; the upcoming second installment will look at the path toward asolution.