Re: Access to DMA memory while DMA in progress?

[Mouse <mouse%Rodents-Montreal.ORG@localhost>]

>> I would like to read the DMA buffer while DMA is still going on.
>> [...]
> This is all very hardware dependent.

That doesn't really surprise me; as MI as bus_dma/bus_space tries to
make device drivers, there are still some things it can't really
compensate for (or can compensate for in theory, but only at a ruinous
performance penalty).

> Make sure you map that area with the BUS_DMA_COHERENT flag.

Thank you!  The manpage called it a hint, so I left it off.  I've added
it; I'll see if that helps anything.

> Many NICs do something like this.

Hmm, true; a NIC descriptor ring is not a bad example of the sort of
thing I want to do: a block of memory I want to read while DMA is
mutating it.

>> But I'm not sure what sort of sync calls I need to make.  [...]
> You want to do a bus_dmamap_sync(BUS_DMASYNC_POSTREAD) [...]
> In the NIC example above, you map the ring buffer with
> BUS_DMA_COHERENT, fill it up and do a
> bus_dmamap_sync(BUS_DMASYNC_PREREAD).  When you want to read it
> (usually after getting an interrupt) you do
> bus_dmamap_sync(BUS_DMASYNC_POSTREAD) before doing the read.

Don't you need to PREWRITE after filling it?  Based on the mental
models I've formed, that feels necessary.

>> [...] bus_dmamem_mmap [...]
> Trying to do this in userland on a machine with an I/O cache won't
> work too good.

That's about what I'd expect.

> AFAIK amd64 disables all caches on BUS_DMAMAP_COHERENT, so the sync
> operations aren't really necessary.  But jumping through all these
> hoops is important on other hardware.

Okay.  If I can make it work on amd64 I'll be content for the moment,
even if I have to be horribly MD about it.  It'd be nice for this to be
MI, but not necessary.  (The hardware is a data acquisition card which
DMAs received data into host memory, but as far as I have been able to
tell has no way for the host to tell how far a DMA transfer in progress
has progressed.  In my case, the data is strongly enough patterned I
can fill the buffer with impossible values and tell how far it's gone
by looking at the buffer...but that means reading the buffer.)

While I don't always mind being MD, I _do_ rather like to know when and
how I am. :-)

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