openwrt/target/linux/brcm2708/patches-4.4/0144-Limit-PCM512x-Digital-gain-to-0dB-by-default-with-Hi.patch

From 8a189f18de820c6facd00ba2db97ba7c4a75156f Mon Sep 17 00:00:00 2001
From: Digital Dreamtime <clive.messer@digitaldreamtime.co.uk>
Date: Thu, 4 Feb 2016 20:04:00 +0000
Subject: [PATCH 144/170] Limit PCM512x "Digital" gain to 0dB by default with
 HiFiBerry 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                       | 17 +++++++++++++++--
 .../arm/boot/dts/overlays/hifiberry-dacplus-overlay.dts |  6 +++++-
 sound/soc/bcm/hifiberry_dacplus.c                       | 14 ++++++++++++++
 3 files changed, 34 insertions(+), 3 deletions(-)

--- a/arch/arm/boot/dts/overlays/README
+++ b/arch/arm/boot/dts/overlays/README
@@ -273,8 +273,21 @@ Params: <None>
 
 Name:   hifiberry-dacplus
 Info:   Configures the HifiBerry DAC+ audio card
-Load:   dtoverlay=hifiberry-dacplus
-Params: <None>
+Load:   dtoverlay=hifiberry-dacplus,<param>=<val>
+Params: 24db_digital_gain       Allow gain to be applied via the PCM512x codec
+                                Digital volume control. Enable with
+                                "dtoverlay=hifiberry-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:   hifiberry-digi
--- a/arch/arm/boot/dts/overlays/hifiberry-dacplus-overlay.dts
+++ b/arch/arm/boot/dts/overlays/hifiberry-dacplus-overlay.dts
@@ -17,7 +17,7 @@
 
 	fragment@1 {
 		target = <&sound>;
-		__overlay__ {
+		frag1: __overlay__ {
 			compatible = "hifiberry,hifiberry-dacplus";
 			i2s-controller = <&i2s>;
 			status = "okay";
@@ -47,4 +47,8 @@
 			};
 		};
 	};
+
+	__overrides__ {
+		24db_digital_gain = <&frag1>,"hifiberry,24db_digital_gain?";
+	};
 };
--- a/sound/soc/bcm/hifiberry_dacplus.c
+++ b/sound/soc/bcm/hifiberry_dacplus.c
@@ -48,6 +48,7 @@ struct pcm512x_priv {
 #define CLK_48EN_RATE 24576000UL
 
 static bool snd_rpi_hifiberry_is_dacpro;
+static bool digital_gain_0db_limit = true;
 
 static void snd_rpi_hifiberry_dacplus_select_clk(struct snd_soc_codec *codec,
 	int clk_id)
@@ -167,6 +168,16 @@ static int snd_rpi_hifiberry_dacplus_ini
 	snd_soc_update_bits(codec, PCM512x_GPIO_OUTPUT_4, 0x0f, 0x02);
 	snd_soc_update_bits(codec, PCM512x_GPIO_CONTROL_1, 0x08, 0x08);
 
+	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;
 }
 
@@ -299,6 +310,9 @@ static int snd_rpi_hifiberry_dacplus_pro
 			dai->platform_name = NULL;
 			dai->platform_of_node = i2s_node;
 		}
+
+		digital_gain_0db_limit = !of_property_read_bool(
+			pdev->dev.of_node, "hifiberry,24db_digital_gain");
 	}
 
 	ret = snd_soc_register_card(&snd_rpi_hifiberry_dacplus);