[alsa-devel] fsl_ssi.c: Getting channel slips with fsl_ssi.c in TDM (network) mode.

[Caleb Crome]

On Thu, Oct 29, 2015 at 6:44 AM, Caleb Crome <caleb at crome.org> wrote:
> On Wed, Oct 28, 2015 at 9:53 PM, Nicolin Chen <nicoleotsuka at gmail.com> wrote:
>>
>> I am actually thinking about setting a watermark to a larger number.
>> I forgot how the SDMA script handles this number. But if this burst
>> size means the overall data count per transaction, it might indicate
>> that each FIFO only gets half of the burst size due to dual FIFOs.
>>
>> Therefore, if setting watermark to 8, each FIFO has 7 (15 - 8) space
>> left, the largest safe burst size could be 14 (7 * 2) actually.
>
> Oh, does this depend on the data size?  I'm using 16-bit data, so I
> guess the bursts are measured in 2 byte units?  Does this mean that
> the burst size should be dynamically adjusted depending on word size
> (I guess done in hw_params)?
>
>> Nicolin

Okay, so wm=8 and maxburst=14 definitely does not work at all,.  wm=8,
maxburst=8 works okay, but still not perfect.

I just discovered some new information:

With wm=8 and maxburst=8 (which is my best setting so far), I just
captured a problem at the very start of playing a file, and restarted
enough times to capture it starting wrong:

Instead of the playback starting with

(hex numbers:  my ramp file has first nibble as channel, second nibble as frame)

frame 0:  00, 10, 20, 30, 40, 50, 60, 70, 80, 90, a0, b0, c0, d0, e0, f0
frame 1:  01, 11, 21, 31, 41, 51, 61, 71, 81, 91, a1, b1, c1, d1, e1, f1

It started with:

frame 0:  00, 00, 10, 20, 30, 40, 50, 60, 70, 80, 90, a0, b0, c0, d0, e0
frame 1:  f0, 01, 11, 21, 31, 41, 51, 61, 71, 81, 91, a1, b1, c1, d1, e1

So, the transfer started wrong right out of the gate -- with an extra
sample inserted at the beginning. Again, my setup is:
1) use scope to capture the TDM bus.  Trigger on first data change
2) aplay myramp.wav
3) If okay, ctrl-c and goto 2.
4) The capture below shows everything off by 1 sample.

The capture is here:
https://drive.google.com/open?id=0B-KUa9Yf1o7iOXFtWXk2ZXdoUXc

This test definitely reveals that there is a startup issue.  Now for
the $64,000 question: what to do with this knowledge?  I'm quite
unfamiliar with how the DMA works at all.

I'll start poking around the DMA I guess.

Thanks,
  -Caleb