From: Jarek Poplawski <redacted>
Date: Wed, 20 Jan 2010 21:58:42 +0100

Yes, and it's mainly for lib/dma-debug (until it's not verified dmar
errors reported by Michael Breuer could be connected). So, I'm not
sure for now how serious this warning could be. On the other hand,
Documentation/(PCI-?)DMA-mapping, mentioned by Stephen, doesn't seem
to allow or use in examples the "size" different than mapped.

Someone find me even one actual implemenetation of the DMA APIs, not
the debugging code, which can't handle a partial sync correctly, then
we can talk.

As the person who originally created the PCI DMA APIs I can tell
you the intention was definitely to allow arbitrary SYNC lengths
and there was no requirement that the length needed to be the
same as that used in the mapping call.

Nothing else makes sense.  People sync when they have a partially used
buffer and want to copy the data out then recycle the big original
buffer back to the chip.  I knew this when creating the PCI DMA APIs
because I first tried to use them in network drivers and that's what
so many of them do.

Currently all we have is an overly anal DMA debugging layer check, and
some speculation that it might or might not fix some real bug.