Re: [PATCH 22/22] mtd: rawnand: Use dma_alloc_noncoherent() for dma buffer

Christoph Hellwig
2022-02-22T08:46:58Z

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