In fact, regular IDE DMA allows arbitrary scatter-gather at least in
theory. Linux has never used it, so I don't know how well it works in
practice - I would not be surprised if it ends up causing no end of nasty
corner-cases that have bugs. It's not as if IDE controllers always follow
the documentation ;)
The _total_ length of the buffers have to be a multiple of the sector
size, and there are some alignment issues (each scatter-gather area has to
be at least 16-bit aligned both in physical memory and in length, and
apparently many controllers need 32-bit alignment). And I'd almost be
surprised if there wouldn't be hardware that wanted cache alignment
because they always expect to burst.
But despite a lot of likely practical reasons why it won't work for
arbitrary sg lists on plain IDE DMA, there is no _theoretical_ reason it
wouldn't. And there are bound to be better controllers that could handle
it.
Linus
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