From d95f04489acd8ee514595966eda29526b485427c Mon Sep 17 00:00:00 2001
From: ehb <2213942+ehb@users.noreply.github.com>
Date: Tue, 10 Nov 2026 21:06:20 +0100
Subject: [PATCH] overlays: Add spdif-gpio21

Add an overlay that turns the I2S block into a sound card for an S/PDIF
transmitter on GPIO21 (PCM_DOUT), without a transceiver chip: the generic
linux,spdif-dit codec on a simple-audio-card, the Pi as bit and frame
clock producer, 2 x 32-bit slots per frame, and a pin group for GPIO21
alone, so GPIOs 18-20 stay free. Userspace supplies the biphase-mark
coded line signal as S32_LE at twice the audio rate.

The pin group uses the legacy brcm,pins/brcm,function properties, which
the RP1 pin driver maps as well (GPIO21 ALT0 to i2s0), so the same
overlay should also serve the Pi 5.

Assisted-by: claude-code:claude-opus-5-5
Signed-off-by: ehb <2213942+ehb@users.noreply.github.com>
---
 arch/arm/boot/dts/overlays/Makefile           |  1 +
 arch/arm/boot/dts/overlays/README             | 12 ++++
 .../dts/overlays/spdif-gpio21-overlay.dts     | 62 +++++++++++++++++++
 3 files changed, 75 insertions(+)
 create mode 100644 arch/arm/boot/dts/overlays/spdif-gpio21-overlay.dts

diff --git a/arch/arm/boot/dts/overlays/Makefile b/arch/arm/boot/dts/overlays/Makefile
index 020213c7936d6..c1b8633700b56 100644
--- a/arch/arm/boot/dts/overlays/Makefile
+++ b/arch/arm/boot/dts/overlays/Makefile
@@ -287,6 +287,7 @@ dtbo-$(CONFIG_ARCH_BCM2835) += \
 	smi.dtbo \
 	smi-dev.dtbo \
 	smi-nand.dtbo \
+	spdif-gpio21.dtbo \
 	spi-gpio35-39.dtbo \
 	spi-gpio40-45.dtbo \
 	spi-rtc.dtbo \
diff --git a/arch/arm/boot/dts/overlays/README b/arch/arm/boot/dts/overlays/README
index 5f35ee995124b..e27c4c68bbdd6 100644
--- a/arch/arm/boot/dts/overlays/README
+++ b/arch/arm/boot/dts/overlays/README
@@ -5115,6 +5115,18 @@ Load:   dtoverlay=smi-nand
 Params: <None>
 
 
+Name:   spdif-gpio21
+Info:   Sound card that drives an S/PDIF transmitter (TOSLINK module or coaxial
+        driver) directly from GPIO21 (PCM_DOUT, 40-pin header pin 40), without
+        a transceiver chip. The I2S block is the bit and frame clock producer
+        and sends 2 x 32-bit slots per frame; userspace has to supply the
+        biphase-mark coded S/PDIF line signal as S32_LE at twice the audio
+        sample rate (e.g. through an ALSA plugin) - plain PCM does not produce
+        audio. Only GPIO21 is claimed, GPIOs 18-20 stay free.
+Load:   dtoverlay=spdif-gpio21
+Params: <None>
+
+
 Name:   spi-gpio35-39
 Info:   Move SPI function block to GPIO 35 to 39
 Load:   dtoverlay=spi-gpio35-39
diff --git a/arch/arm/boot/dts/overlays/spdif-gpio21-overlay.dts b/arch/arm/boot/dts/overlays/spdif-gpio21-overlay.dts
new file mode 100644
index 0000000000000..870350fb07b51
--- /dev/null
+++ b/arch/arm/boot/dts/overlays/spdif-gpio21-overlay.dts
@@ -0,0 +1,62 @@
+// SPDX-License-Identifier: GPL-2.0
+// S/PDIF transmitter on GPIO21 (PCM_DOUT) without a transceiver: the I2S block is the clock
+// producer; software supplies the biphase-mark coded bitstream. Only GPIO21 is claimed.
+/dts-v1/;
+/plugin/;
+
+/ {
+	compatible = "brcm,bcm2835";
+
+	fragment@0 {
+		target = <&gpio>;
+		__overlay__ {
+			spdif_gpio21_pins: spdif_gpio21_pins {
+				brcm,pins = <21>;
+				brcm,function = <4>; /* ALT0: PCM_DOUT, on RP1 i2s0 */
+			};
+		};
+	};
+
+	fragment@1 {
+		target = <&i2s_clk_producer>;
+		__overlay__ {
+			pinctrl-names = "default";
+			pinctrl-0 = <&spdif_gpio21_pins>;
+			status = "okay";
+		};
+	};
+
+	fragment@2 {
+		target-path = "/";
+		__overlay__ {
+			spdif_gpio21_codec: spdif-gpio21-codec {
+				compatible = "linux,spdif-dit";
+				#sound-dai-cells = <0>;
+				status = "okay";
+			};
+		};
+	};
+
+	fragment@3 {
+		target = <&sound>;
+		__overlay__ {
+			compatible = "simple-audio-card";
+			status = "okay";
+
+			simple-audio-card,name = "spdif-gpio21";
+			simple-audio-card,format = "i2s";
+			simple-audio-card,bitclock-master = <&dailink0_cpu>;
+			simple-audio-card,frame-master = <&dailink0_cpu>;
+
+			dailink0_cpu: simple-audio-card,cpu {
+				sound-dai = <&i2s_clk_producer>;
+				dai-tdm-slot-num = <2>;
+				dai-tdm-slot-width = <32>;
+			};
+
+			simple-audio-card,codec {
+				sound-dai = <&spdif_gpio21_codec>;
+			};
+		};
+	};
+};