On Tue, Nov 01, 2022 at 06:24:16PM +0100, Christoph Hellwig wrote:

On Tue, Nov 01, 2022 at 05:19:46PM +0000, Catalin Marinas wrote:

quoted

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.

There's also the case of low-end phones with all RAM below 4GB and arm64
doesn't allocate the swiotlb. Not sure those vendors would go with a
recent kernel anyway.

So the need for swiotlb now changes from 32-bit DMA to any DMA
(non-coherent but we can't tell upfront when booting, devices may be
initialised pretty late).

quoted

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.

I'll give it a try this week, hopefully post patches soon(ish).

-- 
Catalin

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