As we can now acquire the presence of the full DMA coherency (snooping capability) from ttm_device, we can now map the CPU side memory as write-combined when cached is requested and snooping is not avilable. Signed-off-by: Icenowy Zheng <uwu@icenowy.me> Link: https://lore.kernel.org/r/20240629052247.2653363-3-uwu@icenowy.me [ Han Gao: add conditional compilation for dma coherent operations ] Signed-off-by: Han Gao <rabenda.cn@gmail.com>