Hello Christoph. On Tue, Dec 21, 2021 at 09:56:23AM +0100, Christoph Hellwig wrote: > 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. Oh I misunderstood this. Underlying physical address of vmalloc()-allocated memory can be mapped using DMA API, and it needs to be setup as scatterlist because the allocated memory is not physically continuous. Right? BTW, looking at the API I think the scsi case can be converted to use dma_alloc_pages(). but driver requires 512 bytes of buffer and the API supports allocating by at least page size. It's not a big problem as it allocates a single buffer but in other cases maybe not. Can't we use dma pool for non-coherent pages? Thanks, Hyeonggon. > > 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.
