lets say, hypothetically, that i have an ioctl interface to my device
driver that takes a buffer that looks something like:

struct {
  int length;
  char *buf;
} mystruct;

and lets say, hypothetically, that i wanted to dma directly
to/from 'buf'.  how would i do that?

i've tried doing vtophys on buf, but that doesn't seem to work.

i've tried doing vtophys on a kernel-allocated block and the mmapping it
through /dev/mem in userland, but that doesn't seem to work either (this
may have a bug, though - it worked under linux, but was about 10x slower
than just leaving the kernel/user copy in there).

so what am i missing here?

- j
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