On Fri, Dec 17, 2021 at 11:38:27AM +0000, Hyeonggon Yoo wrote: > My understanding is any buffer requested from kmalloc (without > GFP_DMA/DMA32) can be used by device driver because it allocates > continuous physical memory. It doesn't mean that buffer allocated > with kmalloc is free of addressing limitation. Yes. > > the addressing limitation comes from the capability of device, not > allocation size. if you allocate memory using alloc_pages() or kmalloc(), > the device has same limitation. and vmalloc can't be used for > devices because they have no MMU. vmalloc can be used as well, it just needs to be setup as a scatterlist and needs a little lover for DMA challenged platforms with the invalidate_kernel_vmap_range and flush_kernel_vmap_range helpers. > But we can map memory outside DMA zone into bounce buffer (which resides > in DMA zone) using DMA API. Yes, although in a few specific cases the bounce buffer could also come from somewhere else.
