On Sat, Feb 19, 2022 at 11:18:24AM +0000, Hyeonggon Yoo wrote:

quoted

I think it would be better to still allocate the buffer at allocation
time and then just transfer ownership using dma_sync_single* in the I/O
path to avoid the GFP_ATOMIC allocation.

This driver allocates the buffer at initialization step and maps the buffer
for DMA_TO_DEVICE and DMA_FROM_DEVICE when processing IO.

But after making this driver to use dma_alloc_noncoherent(), remapping
dma_alloc_noncoherent()-ed buffer is strange So I just made it to allocate
the buffer in IO path.

You should not remap it.  Just use dma_sync_single* to transfer ownership.

Hmm.. for this specific case, What about allocating two buffers
for DMA_TO_DEVICE and DMA_FROM_DEVICE at initialization time?

That will work, but I don't see the benefit as you'd still need to call
dma_sync_single* before and after each data transfer.