Re: [PATCH] ASoC: bcm2835: Increase channels_max to 8

[Matt Flax <flatmax@flatmax.org>]

On 06/02/17 03:34, Mark Brown wrote:
> On Sun, Feb 05, 2017 at 12:26:31PM +1100, Matt Flax wrote:
>> On 05/02/17 02:49, Mark Brown wrote:
>>> On Thu, Feb 02, 2017 at 10:37:44AM +1100, Matt Flax wrote:
>>> I can't understand how this works.  We're programming the hardware in an
>>> identical fashion for both channel counts, that means that what we're
>> I am not surprised it is confusing, to my knowledge this is the first time
>> that someone has demonstrated this type of I2S signalling.
> More than stereo I2S?  That is actually a thing, while most people use
> DSP modes for higher channel counts it's fairly common to have the
> support especially in CODECs that also support DSP mode since the
> hardware implementation is fairly straightforward if you abstracted in
> the bus interface.
>
>> In this case BCM2835 is a clock slave and not concerned with generating the
>> I2S clocks, only receiving - think of all clocks and synchrony being
>> generated/managed in hardware on the sound card.
>> In short the BCM2835 silicon is only concerned with shifting bits into two
>> hardware registers which (after DMA) are interpreted by ALSA according to
>> the specified channel count.
> Sure, that's what all audio controllers do.  But there's a few problems.
> One is that nothing in your code that prevents this running in master
> mode.  Another is that if this *is* for I2S mode then when doing
> multichannel I2S you usually still respect the left/right indication in
> LRCLK so you will end up with the data in the in memory format:
>
>    L1 R1 L2 R2 L3 R3 L4 R4
>
> but multi channel I2S would usually be rendered with all the left
> channels and all the right channels grouped together so you get:
>
>    L1 L2 L3 L4 R1 R2 R3 R4
>
> It'll also require exact clocking with no spare bits in order to sync
> the right channels with the LRCLK switch but that's not so unusual.
> Looking at the driver it may be possible for the hardware to do DSP
> modes at which point this gets much easier but right now it only has I2S
> support, I've got a feeling that the other end of the link may actually
> be running in a DSP mode.
You've got it !

This can also work with the BCM2835 in master mode, no reason at this 
point to limit it to slave mode only.

I haven't looked into why, but I am getting the correct channel mapping 
under the hood. To me that is a concern of the higher up levels of the 
ALSA driver - it was a pleasant surprise when it worked nicely, because 
to me it indicated a robustly coded system.

My next patch is to allow DSP modes in this I2S driver. I wanted to make 
sure that this got through first however.

The majority of these other concerns you mention are controlled at the 
machine driver level.

Matt