Create HDMI_AUDIO.md

docs/HDMI_AUDIO.md

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+#### Kernel 5.10.x
+
+KMS also comes with its own arm-side HDMI audio driver. If you want to test this, then remove dtparam=audio=on from config.txt
+Replace /usr/share/alsa/cards/vc4-hdmi.conf with the contents here:
+
+```
+# Configuration for the VC4-HDMI sound card using software IEC958
+# subframe conversion
+
+<confdir:pcm/hdmi.conf>
+vc4-hdmi.pcm.hdmi.0 {
+	@args [ CARD AES0 AES1 AES2 AES3 ]
+	@args.CARD {
+		type string
+	}
+	@args.AES0 {
+		type integer
+	}
+	@args.AES1 {
+		type integer
+	}
+	@args.AES2 {
+		type integer
+	}
+	@args.AES3 {
+		type integer
+	}
+	type iec958
+	slave {
+		format IEC958_SUBFRAME_LE
+		pcm {
+			type hooks
+			slave.pcm {
+				type hw
+				card $CARD
+				device 0
+			}
+			hooks.0 {
+				type ctl_elems
+				hook_args [
+				{
+					name "IEC958 Playback Default"
+					optional true
+					lock true
+					preserve true
+					value [ $AES0 $AES1 $AES2 $AES3 ]
+				}
+				]
+			}
+		}
+	}
+	status [ $AES0 $AES1 $AES2 $AES3 ]
+}
+
+# default with plug
+vc4-hdmi.pcm.default {
+	@args [ CARD ]
+	@args.CARD {
+		type string
+	}
+	type plug
+	slave.pcm {
+		type softvol
+		slave.pcm {
+			@func concat
+			strings [ "hdmi:" $CARD ]
+		}
+		control {
+			name "PCM Playback Volume"
+			card $CARD
+		}
+	}
+}
+```
+
+and you should havea new alsa device. (Only available when running with dtoverlay=vc4-kms-v3d).
+