Sound stoppage: TRIAL code to re-start DEAD dma

[Iain Sandoe]

> Paul experienced some problems with the DEC ethernet controller of his old
> PowerBook 3400 for example. It may be useful to flush the dbdma command buffer
> after modifying and invalidate it before it gets modified by the controller...

OK, that's not too difficult - we get a chance to do this before the cmd
block is queued on TX... and after we read the data on RX.

(although we don't *seem* to be seeing a coherency problem here - the
xfer_status is {AFAICT} getting updated properly for dmasound).

I'll figure it into the next lot of changes.

Iain.

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