delayed dma trigger

[Johannes Berg <johannes@sipsolutions.net>]

Hi,

For AOA, there are some DMA engine programming pitfalls I wasn't aware
of previously (actually, I was aware of the bug it triggered but not
quite why it happened). But now we have two questions about the trigger
callback.

(1)
When correctly using the i2s bus (aoa isn't at the moment), it is not
possible to immediately stop doing DMA, the DMA engine has to finish the
current period first, then go into a stop condition, and only after that
we can stop the engine completely. Hence, this can take quite a while
(up to the period length). Is it ok to set the stop condition and return
from the trigger function without having DMA stopped? It'll be
completely stopped asynchronously on the next interrupt, and we'd have
to make the hw_free callback wait for that, I guess.

(2)
Relating to that, if a start trigger comes in before the DMA engine is
stopped, it would be nice if we could just clear the stop condition and
let it continue to run. Can recording start at any arbitrary period in
the buffer relying only on the pointer callback?
[Actually, come to think of it, isn't this required for proper 'pause'
function during playback or recording? I guess aoa is wrong now then?]

johannes

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