[alsa-devel] [PATCH v8] ASoC: bcm2835: Add 8 channel (multitrack) capability

[Emmanuel Fusté]

Le 25/02/2017 à 00:02, Matthias Reichl a écrit :
> On Sat, Feb 25, 2017 at 08:30:03AM +1100, Matt Flax wrote:
>>
>> On 25/02/17 07:25, Matthias Reichl wrote:
>>
>>> I'd put it in another way: it seems to me that the bcm2835 I2S
>>> won't reliably sync in DSP mode A.
>>>
>>>
>> What you are explaining here is the nature of the BCM2835 responding to a
>> codec master. The hardware setup you are describing is elegantly simple, but
>> TOO simple.
> Your patch only changes the CPU DAI driver. At this point it doesn't
> matter if the codec or an external source is driving the clocks.
>
> If the clock timing adheres to DSP mode A timing and you add code
> to the the CPU DAI driver so it can operate in DSP mode A then
> that should also work. If not, it's broken.
>
> The (rather brief) publically available bcm2835 datasheet only
> mentions I2S mode and my tests lead me to the conclusion that the
> bcm2835 is actually expecting I2S timing, not DSP A - at least with
> the current code.
After reading this long thread and gluing all info together, my guess is 
that the "real" thing he's trying to do is to introduce a new DPS mode 
which is nothing more than a multichannel capable I2S mode or a "DSP I" 
mode : DSP A with data_delay 1.
As it is basically an I2S (2 channel) encapsulation of multichannel 
stream, the real question is alsa plugin or new bastardized DSP mode ?

>
>> Measurements on a system which uses a codec master don't represent a robust
>> implementation of multichannel with the BCM2835. My experiments (and now
>> yours) have shown that the system has a bit shift and an uncontrollable
>> channel drift.
>>
>> Multichannel simply can't be done (on the BCM2835) without control at the
>> hardware level - this requires an intermediate master control chip.
>>
>> The Audio Injector Octo machine driver, sets both the codec and the BCM2835
>> as slaves - the FPGA is master because it has to control both the BCM2835
>> and the codec's systems to get reliability. You can see the machine driver
>> on github :
>> https://github.com/flatmax/linux/blob/rpi-4.4.y/sound/soc/bcm/audioinjector-octo-soundcard.c
>>
>> It has taken month to implement a robust solution to this problem using an
>> FPGA and the result is a fantastic (Audio Injector Octo) sound card. I am
>> almost certain this hardware technique can be used on all stereo I2S SoC
>> chips.
> Can you please provide details about the timing / protocol you are
> actually using?
I suspect that the FPGA act as an I2S <=> DSP A translator, taking care 
of data_delay difference translation relative to the falling edge of LRCLK.
It could translate the duration of the one BCLK duration of LRCLK DSP A 
pulse to standard I2S LRCLK even if most codecs works with I2S fs LRCLK 
timing as the sync point is the falling edge.
As the FPGA has to deals with the clocks of the two sides, it's more 
simple if it's the master.
If I'm right, the real used mode from the soc point of view is I2S and 
all of this is valid for all I2S IP, there is nothing BMC2835 specific 
here. It is not the right place to do the mod.

Emmanuel.