[alsa-devel] ASoC: fsl_sai: Has anybody attempted to run the i.MX6UL in TDM mode with channels > 2?

[Caleb Crome]

Oh boy,
   I have never used the SAI.

The best test that I've ever used for getting this all straight is to
route the DOUT -> DIN in hardware, set the SSI for symmetric mode, and
send a known signal out and watch the input.  We call this a 'ramp'
test because it's basically sending a sequence 1, 2, 3, 4, 5, ...
(i.e. a ramp) out and verifying that it comes back in in bit-perfect
order. (originally got the idea from the Audio Anecdotes Books)

Arnoud codified that test in his program 'atest', that you can get
here:  https://github.com/amouiche/atest, which works great.

Another comment:  don't use a sine wave that's an exact multiple of
the block size.  i.e. with 1ms blocks, a 441Hz at 44.1kHz sine wave is
the worst possible test signal :-)   If blocks get swapped around,
you'll have no idea.  Good thing that you have a time-changing sine
wave :-)

If you must use a sine wave test, use a non, multiple:  450 Hz would
be fine.   Typical in telephony testing is a test tone of 1004 Hz, so
that 10ms block problems are spotted.


So, that'll help you with your testing, but as far as how DMA works --
I'm pretty clueless.  Arnoud and I (actually mostly Arnoud)  tracked
down several errors with the SSI port implementation.  It wouldn't
surprise me if there were as many problems with the SAI
implementation.

One of the fundamental problems with the freescale SSI/DMA interaction
is that the SSI port doesn't request one FRAME at a time from the DMA,
rather it just requests some number of elements.  This causes a real
implementation headache because if anything ever gets out of sync,
it's forever and irretreivabley out of sync.  IIRC, the TI chips do
DMA requests on a frame by frame basis, so if it goes wrong one frame,
it'll get corrected the next.

Sorry I can't be of more help, but I really don't know how the SAI/DMA
stuff works.

-Caleb


On Wed, Jan 18, 2017 at 10:43 AM, Alexandre Belloni
<alexandre.belloni at free-electrons.com> wrote:
> Hi Caleb,
>
> Fabio pointed to that thread because I'm trying to use the i.mx7d SAI in
> TDM mode with 8 channels (but I'm using only 4).
>
> On 02/05/2016 at 10:52:39 -0700, Caleb Crome wrote :
>> Hi all,
>>    Now that the MX6 fsl_ssi seems to be getting stable for TDM with
>> channels > 2 (thanks all!), the MX6 UL and MX7 have only the SAI port.
>>
>> Does anybody have experience with running the SAI with more than 2 channels?
>>
>
> I have a really small patch to make the SAI driver configure CR4 and CR5
> properly for more than 2 channels.
> Data on the TDM bus seems fine.
>
> However, I have an issue with the sample ordering. Discussing that with
> Lars, it seems the SDMA is getting confused when switching periods. To
> test, I'm playing a 441Hz sine that is fading out. I can definitively
> see block of 1024 samples (it is period_size) that are ok but not
> properly ordered. You can have a look at:
> http://free-electrons.com/~alexandre/playback.png
>
> Did you have the same issue?
>
> --
> Alexandre Belloni, Free Electrons
> Embedded Linux and Kernel engineering
> http://free-electrons.com