Re: [PATCH 7/8] docs: dma-api: update streaming DMA API physical address constraints

[Christoph Hellwig]

On Thu, Jun 26, 2025 at 05:45:18PM +0100, Robin Murphy wrote:
> Indeed that might actually end up pushing things in the opposite direction,
> at least in some cases. Right now, a driver with, say, a 40-bit DMA mask is
> usually better off not special-casing DMA buffers, and just making plain
> GFP_KERNEL allocations for everything (on the assumption that 64-bit systems
> with masses of memory *should* have SWIOTLB to cover things in the worst
> case), vs. artificially constraining its DMA buffers to GFP_DMA32 and having
> to deal with allocation failure more often. However with a more precise and
> flexible allocator, there's then a much stronger incentive for such drivers
> to explicitly mark *every* allocation that may be used for DMA, in order to
> get the optimal behaviour.

It really should be using dma_alloc_pages to ensure it gets addressable
memory for these cases.  For sub-page allocations it could use dmapool,
but that's a little annoying because it does coherent allocations which
95% of the users don't actually need.