On Thu, Nov 29, 2018 at 8:23 AM Christoph Hellwig <hch@xxxxxx> wrote:
>
> We can. At least in theory. The problem is that depending on the
> crazy mapping from physical and kernel virtual address to dma addresses
> these might be pages at pretty random places. Look at fun like
> arch/x86/pci/sta2x11-fixup.c for how ugly these mappings could look.
>
> It also means that we might have setup swiotlb on just about every
> 32-bit architecture, even if it has no real addressing limit except for
> the one we imposed.
No. Really. If there's no iotlb, then you just mark that one page
reserved. It simply doesn't get used. It doesn't mean you suddenly
need a swiotlb.
If there *is* a iotlb, none of this should matter, because you'd just
never map anything into that page.
But whatever. It's independent from the patch series under discussion.
Make dma_mapping_error() at least return a real error (eg -EINVAL, or
whatever is the common error), and we can maybe do this later.
Or, better yet, plan on removing the single-page dma mappign entirely
at a later date, and make the issue moot.
Linus
