API change for bus_dma

Terry Lambert tlambert2 at mindspring.com
Sat Jun 28 00:54:06 PDT 2003

Andrew Gallatin wrote:
> Scott Long writes:
>  > The approach taken with busdma is that you don't assume coherency.  The
> Unfortunately, in our application we must assume coherency in some
> situations.  We have kernel memory mmap'ed into user space for
> zero-copy io of small messages.  Doing a system call to force the dma
> sync would add unacceptable latency. (we're talking sub 10us latencies
> here, without syscalls).


You have to do the same thing for BT848 and fram buffers that eat
host memory instead of having their own to play in.  In general,
this has to be done in the device driver, very early on in the life
of the system to stand any chance of succeeding: because we do not
properly use ELF section attribute tags, it's not possible to defrag
physical memory in FreeBSD to do these allocations later.  The tags
are necessary to identify code in the defragmentation code path so
that it does not attempt to relocate itself while it's running itself.

NB: FreeBSD doesn't support kernel paging and discard of init routines
in device drivers, once the driver is operational, for the same reason:
lack of section tags indicating "paging path" in the first instance,
and lack of section tags indicating "discard after initialization" in
the second.

-- Terry

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