Re: [PATCH] ASoC: kirkwood: add S/PDIF support

[Russell King - ARM Linux <linux-lFZ/pmaqli7XmaaqVzeoHQ@public.gmane.org>]

On Fri, Oct 18, 2013 at 02:48:29PM +0100, Mark Brown wrote:
> On Fri, Oct 18, 2013 at 01:56:38PM +0100, Russell King - ARM Linux wrote:
> 
> > Let me remind you that I had a definition of what a front end and a back
> > end DAI was from Liam, and your definition is at odds with that.
> 
> Could you please provide some detail on this?  I was aware that you were
> for a time confused about this but had not been aware of the source of
> the confusion.

Let's see:

16:10 < rmk> broonie: can you explain this pcm frontend/backend stuff in
	asoc. what exactly is a frontend and backend pcm, and how do they
	relate to the CPU hardware and codec links?  Which are the ones
	which are given ALSA PCMs?
17:22 < broonie> rmk: A front end is the bit that does DMA from memory, a
	back end is a physical interface.
17:22 < broonie> The bit in the middle is usually a DSP.
17:23 < broonie> To be frank I've not actually *used* this stuff myself
	except in bolting a new CODEC on the edge of it during system
	integrations - going to be rectifying that very soon with the
	Samsung drivers I hope.
17:24 < broonie> But the general idea is that the front end is what the
	application sees, the back end is what comes out of the hardware
	at the other end.
17:25 < broonie> And the bit in the middle does any routing an rewriting
	of formats between the two.
17:26 < broonie> (The Samsung picture should be very similar to Kirkwood
	but the opposite way around - two front ends mixed togther to a
	single back end)
...
16:49 < rmk> oh, another question
16:49 < rmk> .dynamic in the dai link structure, that's for backends only,
	right?
16:50 < broonie> Think so, yes.
16:52 < rmk> hmm
16:52 < rmk>         if (rtd->dai_link->dynamic) {
16:52 < rmk>                 rtd->ops.open           = dpcm_fe_dai_open;
16:52 < rmk> probably needed for frontends
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