Hi @gustavo.tx,
i’ve tested your device tree, unfortunately it failed with “no sound card found”. After changing it to following I’m getting rid of the error:
/ {
sound-wm8978 {
compatible = "simple-audio-card";
status="okay";
simple-audio-card,name = "Wolfson WM8978";
simple-audio-card,widgets =
"Microphone", "Built-in Microphone",
"Line", "Line-in Jack",
"Line", "Aux-in Jack",
"Headphone", "Headphone Jack",
"Speaker", "Built-in Speaker";
simple-audio-card,routing =
"LMICP", "Built-in Microphone",
"RMICP", "Built-in Microphone",
"L2", "Line-in Jack",
"R2", "Line-in Jack",
"LAUX", "Aux-in Jack",
"RAUX", "Aux-in Jack",
"Headphone Jack", "LHP",
"Headphone Jack", "RHP";
simple-audio-card,format = "i2s";
simple-audio-card,cpu {
sound-dai = <&sai2 0>;
system-clock-frequency = <3072000>;
};
simple-audio-card,codec {
sound-dai = <&wm8978 0>;
clocks = <&clks IMX7D_AUDIO_MCLK_ROOT_CLK>;
bitclock-master;
frame-master;
};
};
};
&i2c3 {
clock-frequency = <400000>;
pinctrl-names = "default";
status = "okay";
pinctrl-0 = <&pinctrl_i2c3>;
wm8978: wm8978@1a {
compatible = "wlf,wm8978";
#sound-dai-cells = <0>;
clocks = <&clks IMX7D_AUDIO_MCLK_ROOT_CLK>;
reg = <0x1a>;
};
};
It’s now playing sound. Anyway I have to test the correct output tomorrow or on Monday. It would be great not to close this post until then 
Can you also give me some info, why I need to set this clock?
Best regards