A) Problem: When connecting an I2S audio DAC to the Onion Omega 2S it is not possible to playback audio due to "software" issues: no sound card is found.

The Omega is reporting, that it does not have any sound cards:

root@Omega-C8DC:/# cat /proc/asound/cards 
--- no soundcards ---
root@Omega-C8DC:/# 

The kernel modules are loaded:

root@Omega-C8DC:/# lsmod | grep "snd"
regmap_i2c              2298  1 snd_soc_wm8960
snd                    41582 10 snd_soc_wm8960,snd_soc_core,snd_compress,snd_pcm_oss,snd_mixer_oss,snd_pcm,snd_timer,snd_rawmidi,snd_seq_device,snd_hwdep
snd_compress           10436  0 
snd_hwdep               4286  0 
snd_mixer_oss          12729  1 snd_pcm_oss
snd_pcm                61710  5 snd_soc_ralink_i2s,snd_soc_wm8960,snd_soc_core,snd_pcm_dmaengine,snd_pcm_oss
snd_pcm_dmaengine       2971  1 snd_soc_core
snd_pcm_oss            36177  0 
snd_rawmidi            16298  0 
snd_seq_device          2191  1 snd_rawmidi
snd_soc_core          114758  4 snd_soc_simple_card,snd_soc_simple_card_utils,snd_soc_ralink_i2s,snd_soc_wm8960
snd_soc_ralink_i2s      7088  0 
snd_soc_simple_card     4304  0 
snd_soc_simple_card_utils    5095  1 snd_soc_simple_card
snd_soc_wm8960         24224  0 
snd_timer              15806  1 snd_pcm
soundcore               3836  1 snd
root@Omega-C8DC:/# 

The utilities are also compiled correctly but also recognize no sound card:

root@Omega-C8DC:/# aplay -lL
null
    Discard all samples (playback) or generate zero samples (capture)
aplay: device_list:272: no soundcards found...
root@Omega-C8DC:/#

dmesg does not show any information on "sound" at boot:

root@Omega-C8DC:/# dmesg | grep "ralink-i2s"
root@Omega-C8DC:/# dmesg | grep "i2s"
root@Omega-C8DC:/# dmesg | grep "sound"
root@Omega-C8DC:/# dmesg | grep "snd"
root@Omega-C8DC:/# dmesg | grep "wlan"
[   56.522549] IPv6: ADDRCONF(NETDEV_UP): br-wlan: link is not ready
[   70.344185] br-wlan: port 1(ra0) entered blocking state
[   70.349499] br-wlan: port 1(ra0) entered disabled state
[   70.360126] br-wlan: port 1(ra0) entered blocking state
.... just for control, that it works ;-)

The device overlay files target/linux/ramips/dts/OMEGA2.dtsi and target/linux/ramips/dts/OMEGA2.dts are unchanged.

B) My setup: I've checked out https://github.com/OnionIoT/source (used the Docker image for building and did git pull to be on latest on branch openwrt-18.06) and selected the configuration .config.O2-minimum (cp .config.O2-minimum .config). Then I ran make menuconfig and selected (Main Menu --->) Kernel modules ---> Sound Support and selected the options kmod-sound-core, kmod-ac97, kmod-sound-mt7620, kmod-sound-soc-core.

Under (Main Menu --->) Sound I selected alsa-utils, alsa-utils-tests and mpg123 to have software to use to test out the playback.

I wired up the DAC to the Onion Omega 2S as e.g. described by Omega in a blog article herre and also wired the DAC up to a Raspberry Pi and used the hifiberry-dac driver, which is an I2S driver, and tested the hardware which works fine and should cancel out mostly any hardware issues.

C) Other resources: I did a lot of tinkering around with the DTS/DTSI files, poked around inside the system and also search the internet and found the following related resources:

• "official" Omega How-To for I2S -- unfortunately outdated, I could also not find firmware build b177 which is mentioned to work
• post inside this community about the topic ("Trying to get the I2S audio working") -- unfortunately outdated, because "mostly in 2018"
• post inside this community ([Resolved] Attaching PCM5102 to Omega2 (I2S)) -- slightly newer, but no new aspects contained
• a very long post inside this community ("[Resolved] Attaching PCM5102 to Omega2 (I2S)") -- tried it out, but no help, it is also quite old (2018)
• information on how to change DTS/DTSI files -- works, just edit them and run make
• a project which uses a DAC "Hackaday: Onion Omega Hi-Fi Dock" -- unfortunately no information on software etc.
• a patch to enable support of max98357a DAC -- tried it out, nothing "changed"

D) Conclusion: so I had a long tinker day and found many resources but hadn't any luck to find a solution. I've the feeling that the software tooling (also kernel modules) is in place and a configuration is missing which glues everything together.

Does anybody have any ideas?

I finally got it working

TL;DR: you can find under [1] a patch or diff file which will make it work. It basically performs three important steps:

• add the sound device to the device tree along with a codec, which is basically the HW device to play it back
• enable I2S (see pinctrl), gdma and configure I2S
• add the driver for the MAX98357A "codec" or hardware playback device to the kernel

FYI: the last point was the step I was missing and in this case it does not work or it seems that nothing is happening

The "MAX98357A" coded or I2S DAC IC is a generic one which accepts standard I2S audio and therefore others should work like PCM5102A etc. with this driver.

Please note: currently the driver outputs on GPIO18 (<&gpio0 18 GPIO_ACTIVE_HIGH>) the SDMODE signal (active low) which indicates if sound is currently playing. So you cannot use this pin for another peripheral (it is PWM0). However I changed it to 19 (PWM1), because I'm using PWM0 already. I tested both 18/19 and they worked fine for me.

The next thing is now to setup software volume control since we don't want to playback everything at full volume For this you create a file /etc/asound.conf with the following contents:

pcm.!default {
      type plug
      slave.pcm "softvol"
}

pcm.softvol {
     type softvol
      slave {
          pcm "hw:0"
      }
      control {
          name "Master"
          card 0
      }
     min_dB -57.2
     max_dB -6.2
}

ctl.!default {
        type hw
        card 0
}

ctl.softvol {
        type hw
        card 0
}

This will then enable software volume control which can be utilized by the alsa tools. To "activate" this config you need to play a "file"; I'm just doing cat /dev/urandom | aplay and pressing CTRL+C after 1 sec or so. Then, when running amixer controls you get:

root@Omega-C8DC:/mnt/sda1# amixer controls
numid=1,iface=MIXER,name='Master'
root@Omega-C8DC:/mnt/sda1# 

and now you can control the volume e.g. with: amixer sset 'Master' 50%

I didn't yet figure out why you also need to "activate" the config after reboot and how to configure it "properly" so that it works automatically.

References: (further links which helped me):

• the commit e7d598f from the OnionIoT source repo for the working b177 firmware containing essentially the source for my patch [1]
• a community post which also references two patches with a very close similarity
• yet another side-stream where chaps are trying to get it working
• a discussion to get sound working on OpenWRT (not Omega specific)
• a topic to make SDMODE GPIOs optional

I hope that this explanation/question/how-to is helpful for somebody else, saving time in debugging

Have fun!

[1] Patch file to apply on branch openwrt-18.06 of https://github.com/OnionIoT/source

diff --git a/target/linux/ramips/dts/OMEGA2.dtsi b/target/linux/ramips/dts/OMEGA2.dtsi
index 67910b4ce0..0fdffc9067 100644
--- a/target/linux/ramips/dts/OMEGA2.dtsi
+++ b/target/linux/ramips/dts/OMEGA2.dtsi
@@ -30,6 +30,33 @@
 			linux,code = <KEY_RESTART>;
 		};
 	};
+
+
+	sound {
+		compatible = "simple-audio-card";
+		simple-audio-card,name = "Audio-I2S";
+		simple-audio-card,format = "i2s";
+		simple-audio-card,bitclock-master = <&cpu_dai_master>;
+		simple-audio-card,frame-master = <&cpu_dai_master>;
+		simple-audio-card,widgets =
+			"Speaker", "External Speaker";
+
+		cpu_dai_master: simple-audio-card,cpu {
+			sound-dai = <&i2s>;
+		};
+
+		dailink0_master: simple-audio-card,codec {
+			sound-dai = <&codec>;
+		};
+	};
+
+  codec: max98357a {
+		#sound-dai-cells = <0>;
+    compatible = "maxim,max98357a";
+    sdmode-gpios = <&gpio0 18 GPIO_ACTIVE_HIGH>;
+  };
+
+
 };
 
 &pinctrl {
@@ -51,7 +78,7 @@
 
 		i2s {
 			ralink,group = "i2s";
-			ralink,function = "gpio";
+			ralink,function = "i2s";
 		};
 
 		spis {
@@ -164,6 +191,16 @@
 	status = "okay";
 };
 
+&gdma {
+	status = "okay";
+};
+
+&i2s {
+	#sound-dai-cells = <0>;
+	status = "okay";
+};
+
+
 &pwm {
 	status = "okay";
 };
diff --git a/target/linux/ramips/modules.mk b/target/linux/ramips/modules.mk
index 88e83a487f..d9206c1c23 100644
--- a/target/linux/ramips/modules.mk
+++ b/target/linux/ramips/modules.mk
@@ -121,13 +121,15 @@ define KernelPackage/sound-mt7620
 	CONFIG_SND_RALINK_SOC_I2S \
 	CONFIG_SND_SIMPLE_CARD \
 	CONFIG_SND_SIMPLE_CARD_UTILS \
-	CONFIG_SND_SOC_WM8960
+	CONFIG_SND_SOC_WM8960 \
+	CONFIG_SND_SOC_MAX98357A
   FILES:= \
 	$(LINUX_DIR)/sound/soc/ralink/snd-soc-ralink-i2s.ko \
 	$(LINUX_DIR)/sound/soc/generic/snd-soc-simple-card.ko \
 	$(LINUX_DIR)/sound/soc/generic/snd-soc-simple-card-utils.ko \
-	$(LINUX_DIR)/sound/soc/codecs/snd-soc-wm8960.ko
-  AUTOLOAD:=$(call AutoLoad,90,snd-soc-wm8960 snd-soc-ralink-i2s snd-soc-simple-card)
+	$(LINUX_DIR)/sound/soc/codecs/snd-soc-wm8960.ko \
+	$(LINUX_DIR)/sound/soc/codecs/snd-soc-max98357a.ko
+  AUTOLOAD:=$(call AutoLoad,90,snd-soc-wm8960 snd-soc-max98357a snd-soc-ralink-i2s snd-soc-simple-card)
   $(call AddDepends/sound)
 endef

You should be able to run git apply the-patch.diff on the current branch if you put the patch into the file the-patch.diff.

I remember my own long nights back in 2018, where @sza2-sza2 and me finally got i2s working (that forum thread you found and linked above ).

Sorry that I did not see your original question earlier, only the solution now - if I did, I might have been able to help a bit.

Anyway, glad to hear you got it working!

But what I don‘t understand: why did the i2s support disappear again after b177? As I‘m using only custom openwrt images, I lost track of details happening in the official onion releases. Since b177, i2s support was a ticked checkbox in my mind…