Downtown Doug Brown

If you’re new to this series, I’ve been documenting the process I went through upgrading my old PXA166-based Chumby 8’s 2.6.28 Linux kernel to a modern 6.x version. Here are links to parts 1, 2, 3, 4, 5, 6, 7, and 8. At this point in the project, all of the main hardware peripherals were working great. I noticed something odd when running top though. The CPU usage was always really high, and it wasn’t obvious why.

Mem: 47888K used, 55968K free, 168K shrd, 3116K buff, 27480K cached
CPU: 100% usr   0% sys   0% nic   0% idle   0% io   0% irq   0% sirq
Load average: 0.00 0.00 0.00 2/51 269
  PID  PPID USER     STAT   VSZ %VSZ %CPU COMMAND
  267   200 root     R     2936   3% 100% top
  100     1 root     S    12240  12%   0% /sbin/udevd -d
    1     0 root     S     2936   3%   0% init
  200     1 root     S     2936   3%   0% -sh
   65     1 root     S     2936   3%   0% /sbin/syslogd -n
   71     1 root     S     2936   3%   0% /sbin/klogd -n
   34     2 root     SW       0   0%   0% [irq/56-mmc0]
   10     2 root     IW       0   0%   0% [kworker/0:1-eve]
   11     2 root     IW       0   0%   0% [kworker/u2:0-ev]
   41     2 root     IW<      0   0%   0% [kworker/0:2H-kb]
    8     2 root     IW       0   0%   0% [kworker/0:0-lib]
   22     2 root     IW<      0   0%   0% [kworker/0:1H-mm]
   32     2 root     SW       0   0%   0% [irq/55-mmc1]
   14     2 root     IW       0   0%   0% [rcu_preempt]
   17     2 root     IW       0   0%   0% [kworker/u2:1-ev]
   15     2 root     SW       0   0%   0% [kdevtmpfs]
   27     2 root     IW       0   0%   0% [kworker/0:2-pm]
    2     0 root     SW       0   0%   0% [kthreadd]
   13     2 root     SW       0   0%   0% [ksoftirqd/0]
    3     2 root     SW       0   0%   0% [pool_workqueue_]

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After getting many of the PXA16x peripherals working in modern Linux kernels during my Chumby 8 kernel upgrade saga (here are links to parts 1, 2, 3, 4, 5, 6, and 7), I really felt like I was starting to reach the finish line. The display was working well enough to play low-resolution videos, I had basic 2D acceleration up and running, the touchscreen was operational, and Wi-Fi worked flawlessly. Audio was the only major component left to tackle. The Chumby 8 has built-in speakers, a headphone jack, and a microphone.

The Linux sound subsystem is something I had exactly zero experience with prior to this project, so it felt very intimidating. It took me a while to get familiar with it. The relevant project is Advanced Linux Sound Architecture (ALSA) and in particular, the ALSA System on Chip (ASoC) layer is where most of my work would be for this project.

Before digging in too deep, I needed to look at the Chumby 8 schematics and figure out what I would be dealing with. With no prior audio experience to lean on, I wasn’t sure what to expect. Let’s take a look at the relevant section of the schematic:

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