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:

quoted

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.

quoted

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.

quoted

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.