[alsa-devel] Slow SPI reads in ALSA causing mic recording error

[noman pouigt]

On Thu, Jun 14, 2018 at 3:42 AM, Mark Brown <broonie at kernel.org> wrote:
> On Thu, Jun 14, 2018 at 02:12:35AM -0700, noman pouigt wrote:
>
>> I have also used high priority workqueues but that didn't
>> help either. Wondering if I can get some pointers to debug this?
>> also if there any alsa-utility which can simulate closely audio flinger?
>
> You probably just need to ensure that you've got more than one period
> so there's always one ready, increase min_periods or possibly make the
> individual periods bigger.  It's normally going to be very hard to get
> everything scheduled reliably if there's no margin for error, ensuring
> there's some grace in case things run over will tend to be more robust.

Thanks, but can you kindly explain how userspace application read which
is happening via pcm_read (mmap) affects kernel SPI workqueue thread doing
SPI read in a while loop i.e. causing SPI reads to get delayed?

In kernel, we have two workqueues:
1. First to do SPI reads after trigger callback trigger and call
elapsed callback
once SoC gets more than period size.
2. Second to handle interrupts from DSP.

First workqueue SPI reads is getting delayed based on the what application is
reading data. In case of tinycap, we don't see much time difference
between SPI reads
but with AudioFlinger we see lot of time difference between consecutive SPI
reads? How can application affect kernel threads and cause this delay?

Please let me know where is this dependency coming from?