On Tue, Nov 01, 2022 at 05:19:46PM +0000, Catalin Marinas wrote: > > The main downside of bouncing is mobile phones where those vendors are > in the habit of passing noswiotlb on the kernel command line or they > want a very small bounce buffer (hard to pick a universally small size). > I guess we can go ahead with this and, depending on how bad the problem > is, we can look at optimising swiotlb to use a kmem_cache (or aligned > kmalloc) as fall-back for bouncing. Theses phones setups are completely broken already. There is a reason why it needs a debug option to disable swiotlb, and that reason is that Linux guarantees that the 32-bit DMA always works. If they force swiotlb off they can keep the pieces as this is not a supported configuration. > As a less than optimal solution, we can force bouncing for the whole > list if any of the sg elements is below the alignment size. Hopefully we > won't have many such mixed size cases. I suspect we won't see any such cases. The scatterlist is usually used for large data transfers, and many devices won't like unaligned buffers for SG operations to start with. So I think it is a perfectly fine tradeoff.
