[alsa-devel] [PATCH 0/3] ASoC: Enable a new IC master mode: bcm2835<=>IC<=>cs42xx8

Mark Brown broonie at kernel.org
Wed Mar 15 12:01:08 PDT 2017

On Tue, Feb 28, 2017 at 09:59:29AM +0000, Charles Keepax wrote:
> On Mon, Feb 27, 2017 at 12:51:08PM +0100, Matthias Reichl wrote:

> > > I have a bcm2835 (Pi 2 and 3) SoC here. It is producing multichannel (8 out,
> > > 6 in) audio. In ALSA we call that DSP mode - right ?!

> > No. DSP modes are protocol/timing specifications as I2S, PDP, S/PDIF, ...
> > You can look these up in datasheets and if a chip implements such a
> > protocol you can be sure that it adheres to that standard - i.e. it
> > will sync the frames to the pulses on LRclk.

> I agree with the thoughts in this thread really if the AP doesn't
> actually support DSP A mode we shouldn't add DSP A mode.

The trouble here is that this isn't 100% clear, the specifications of
the DSP modes are such that only one edge in the LRCLK matters and so
providing you're doing mono or have exact clocking they interoperate
perfectly well.  We already frequently do similar things the other way,
most of the programmable serial ports can't actually do I2S modes
properly and rely on exact clocking to get things right when operating
as I2S since they only sync on one edge (though they can generally
generate the clocks correctly when operating as master, they just don't
pay attention to the left/right switch edge).

That said unless we're doing something with the data layout or similar
configuration there's a fairly strong case for putting the mangling for
this in the core, something like just falling back to I2S mode if we set
DSP A and so on.  Which would be a lot nicer if we actually got round to
putting mode capability information in the drivers.
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