About DMA

https://www.kernel.org/doc/Documentation/DMA-API-HOWTO.txt

https://stackoverflow.com/questions/52658418/gfp-kernel-vs-gfp-dma-and-kmalloc-vs-dma-alloc-coherent

http://linuxkernelhacker.blogspot.com/2014/07/arm-dma-mapping-explained.html