Re: x86 swiotlb questions

["Jan Beulich" <jbeulich@novell.com>]

One more thing: Is it really necessary to restrict dma_alloc_coherent() to dma_bits?
I.e., couldn't we, once the bit-level page allocator is merged, use the real bit width
needed for the requesting device here? If not, this would then permit using the
original implementation of swiotlb_dma_supported() (as dma_alloc_coherent() then
no longer depends on dma_bits), and perhaps even auto-setting dma_bits based
on what memory we can get out of Xen in swiotlb_init(), making the mismatching of
command line options (between Xen and kernel) impossible (the kernel simply
wouldn't have one anymore).

As a nice side effect, using the original implementation of swiotlb_dma_supported()
would require slightly less tweaking of lib/swiotlb.c, hence slightly raising the
chances of the changes getting accepted into mainline. And clearly, if the kernel
manages to allocate the swiotlb at an address with less than dma_bits bits, there
seems to be no reason to refuse use of I/O devices that the actual buffer fits, but
dma_bits doesn't.

Jan