Allow up to 24dB digital gain to be applied when using IQAudIO DAC+

24db_digital_gain DT param can be used to specify that PCM512x
codec "Digital" volume control should not be limited to 0dB gain,
and if specified will allow the full 24dB gain.

arch/arm/boot/dts/overlays/README

@@ -362,8 +362,21 @@ Params: <None>
 
 Name:   iqaudio-dacplus
 Info:   Configures the IQaudio DAC+ audio card
-Load:   dtoverlay=iqaudio-dacplus
-Params: <None>
+Load:   dtoverlay=iqaudio-dacplus,<param>=<val>
+Params: 24db_digital_gain       Allow gain to be applied via the PCM512x codec
+                                Digital volume control. Enable with
+                                "dtoverlay=iqaudio-dacplus,24db_digital_gain"
+                                (The default behaviour is that the Digital
+                                volume control is limited to a maximum of
+                                0dB. ie. it can attenuate but not provide
+                                gain. For most users, this will be desired
+                                as it will prevent clipping. By appending
+                                the 24db_digital_gain parameter, the Digital
+                                volume control will allow up to 24dB of
+                                gain. If this parameter is enabled, it is the
+                                responsibility of the user to ensure that
+                                the Digital volume control is set to a value
+                                that does not result in clipping/distortion!)
 
 
 Name:   lirc-rpi

arch/arm/boot/dts/overlays/iqaudio-dacplus-overlay.dts

@@ -7,7 +7,7 @@
 
 	fragment@0 {
 		target = <&sound>;
-		__overlay__ {
+		frag0: __overlay__ {
 			compatible = "iqaudio,iqaudio-dac";
 			i2s-controller = <&i2s>;
 			status = "okay";
@@ -36,4 +36,8 @@
 			};
 		};
 	};
+
+	__overrides__ {
+		24db_digital_gain = <&frag0>,"iqaudio,24db_digital_gain?";
+	};
 };

sound/soc/bcm/iqaudio-dac.c

@@ -23,14 +23,19 @@
 #include <sound/soc.h>
 #include <sound/jack.h>
 
+static bool digital_gain_0db_limit = true;
+
 static int snd_rpi_iqaudio_dac_init(struct snd_soc_pcm_runtime *rtd)
 {
-	int ret;
-	struct snd_soc_card *card = rtd->card;
-
-	ret = snd_soc_limit_volume(card, "Digital Playback Volume", 207);
-	if (ret < 0)
-		dev_warn(card->dev, "Failed to set volume limit: %d\n", ret);
+	if (digital_gain_0db_limit)
+	{
+		int ret;
+		struct snd_soc_card *card = rtd->card;
+
+		ret = snd_soc_limit_volume(card, "Digital Playback Volume", 207);
+		if (ret < 0)
+			dev_warn(card->dev, "Failed to set volume limit: %d\n", ret);
+	}
 
 	return 0;
 }
@@ -94,6 +99,9 @@ static int snd_rpi_iqaudio_dac_probe(struct platform_device *pdev)
 		dai->platform_name = NULL;
 		dai->platform_of_node = i2s_node;
 	    }
+
+	    digital_gain_0db_limit = !of_property_read_bool(pdev->dev.of_node,
+					"iqaudio,24db_digital_gain");
 	}
 
 	ret = snd_soc_register_card(&snd_rpi_iqaudio_dac);