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.
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