[MPlayer-dev-eng] DMA or not DMA

[Daniel Serpell]

Hi!

El Wed, Nov 24, 2004 at 06:59:33PM +0100, Arpi escribio:
> > 
> > Oh.. sounds fun. 5 DMA channels, all correctly set up and coordinated.
> 
> yeah :)
> 
> btw, is it possible at all to run multiple dma transfers (to
> different cards) in the same time? or bus-mastering is about it?
> (ie only one dma transfer over pci bus)

There should be only one bus-master on the bus at any given time,
so there is bus-arbitration going on. Normally, each bus-master
transfer takes at least about 16 words (ie, 64 bytes in PCI-32)
and then the bus is released. The "latency-timers" on each PCI
card give the bus information about when a master should release
the bus.

So, if you adjust all the latency-timers to the same value, you
will end-up having a round-robbing of bus-master's, giving each
card an equal share of the bus. It's is better to raise the timers,
so the impact of bus adquisition and release is lowered.

Hope it helps,

        Daniel.