[alsa-devel] fsl_ssi.c: Roberto's problem: ssi hangs after some number of samples

[Roberto Fichera]

On 11/05/2015 11:25 PM, Caleb Crome wrote:
> On Thu, Nov 5, 2015 at 2:08 PM, Roberto Fichera <kernel at tekno-soft.it> wrote:
>> On 11/05/2015 10:34 PM, Caleb Crome wrote:
>>> On Thu, Nov 5, 2015 at 3:48 AM, Roberto Fichera <kernel at tekno-soft.it> wrote:
>>>> On 11/05/2015 12:30 PM, Fabio Estevam wrote:
>>>>> On Thu, Nov 5, 2015 at 8:03 AM, Roberto Fichera <kernel at tekno-soft.it> wrote:
>>>>>
>>>>>> Following your suggestion, I've increased the buffer size to 2K and set the period to fifo_length - 2 (13),
>>>>>> with that I'm now running substantially smooth except 3 EVTERR on RX DMA over 4 million of interrupts.
>>>>>>
>>>>>> Thanks Nicolin! I'm quite happy now!
>>>>> That's good progress, Roberto.
>>>>>
>>>>> It would be nice if you and Caleb could post the patches to the mailing list.
>>>>>
>>> Yes, when I get something quite solid, I'd like to submit it all to
>>> the list, and hope to get it into the kernel so nobody else has to go
>>> through this pain again.
>>>
>>>> Indeed! Now the TDM is stable, I've also found the reason of the EVTERRs, which was related to some stale
>>>> code I've used to enable and disable both RDMAE and TDMAE bits to try to reset the transfers.
>>>> Once removed that code everything is looks ok now.
>>>>
>>>> Regarding patches, well, from my side there isn't nothing special compared to the original fsl_ssi.c code.
>>>> I'm basically running against a very skinny fsl_ssi.c version, I've just setup a bit larger DMA buffer, from
>>>> 16bytes to 2K, and now reduced the DMA period to 8 because I'm mostly comfortable with that size to simplify
>>>> sampling exchange against DAHDI subsystem within my DMA callbacks.
>>>>
>>>> In a few words, my problem was related due to a DMA buffer too small.
>>>>
>>>> What eventually might be interesting to have is the INTRMASK and EVTERR DMA setting to trigger DMA
>>>> related errors, but I guess this need to be discussed elsewhere.
>>> I have implemented roberto's patch on the 4.2 kernel, and I get a huge
>>> number of EVTERR interrupts.  Something like 7200/second at 16kHz
>>> sample rate.  But strangely, the audio seems to be correct.
>> I've notice that clearing the EVTERR bit seems restarting the given SDMA.
>>
>>> My patch is slightly different in that it just enables EVTERR for all
>>> channels, not just for the SSI.  Might as well see if there are any
>>> other problems.
>> Oh yes! This will overload the SDMA isr.
> It didn't seem to.  There didn't seem to be any other DMA happening in
> my system, definitely none that made the EVTRR trigger.  However, I
> changed it back to the way you had it.  No differences, still got a
> TON of EVTERRs.

This might be related to SDMA request when another is pending.

>
>> How bigger is your audio buffer?
>> In your case I guess you will need something like 16KHz * 16 channels *
>> 2 bytes (16bits) = 512K minimum.
>> I would try to start from 1MB or maybe more.
> That's 2 seconds of audio!  We definitely need less buffering than
> that.    We pretty much need a latency of 100ms, worst case, or 1600
> frames, or 51,200 bytes.

I haven't checked in detail how the DAI buffering is working, but likely
the samples are passed not in buffer size chunks but instead with less
granularity. Having a large buffer gives more chance to the SSI to not
overlap DMA requests, hence no more EVTERRs.

I would give it a try.
>
> I did change the max buffer size to 1MB though, but I'm not sure how
> much is actually being used.

I guess it's 64K, look for IMX_SSI_DMABUF_SIZE.