Linux声卡驱动添加虚拟codec

在调试Linux声音相关驱动时，有时只使用i2s等接口对接其他芯片而非codec，此时也需要按照alsa框架配置codec等信息。本文给出了一个虚拟codec，即dummy-codec的配置源码。

1，在dts中配置i2s、dummy-codec和声卡相关节点

i2s0: i2s@ff400000 {
	compatible = "test,rk3399-i2s", "test,test-i2s";
	reg = <0x0 0xff400000 0x0 0x1000>;
	test,grf = <&grf>;
	interrupts = <GIC_SPI 39 IRQ_TYPE_LEVEL_HIGH 0>;
	dmas = <&dmac_bus 0>, <&dmac_bus 1>;
	dma-names = "tx", "rx";
	clock-names = "i2s_clk", "i2s_hclk";
	clocks = <&cru SCLK_I2S0_8CH>, <&cru HCLK_I2S0_8CH>;
	resets = <&cru SRST_I2S0_8CH>, <&cru SRST_H_I2S0_8CH>;
	reset-names = "reset-m", "reset-h";
	pinctrl-names = "default";
	pinctrl-0 = <&i2s0_8ch_bus>;
	power-domains = <&power RK3399_PD_SDIOAUDIO>;
	test,clk-trcm = <1>;
	status = "disabled";
};
dummy_codec: dummy-codec {
    status = "okay";
    compatible = "test,dummy-codec";
    #sound-dai-cells = <0>;
    clocks = <&cru SCLK_I2S_8CH_OUT>;
    clock-names = "mclk";
    pinctrl-names = "default";
    pinctrl-0 = <&i2s_8ch_mclk>;
};

test_i2s_sound: test-i2s-sound {
    status = "okay";
    compatible = "simple-audio-card";
    simple-audio-card,name = "radia,radia-i2s-sound";
    simple-audio-card,format = "i2s";
    simple-audio-card,mclk-fs = <256>;
    simple-audio-card,cpu {
        sound-dai = <&i2s0>;
    };
    simple-audio-card,codec {
        sound-dai = <&dummy_codec>;
    };
};

2，添加源代码linux/sound/soc/codecs/dummy-codec.c

/*
 * dummy_codec.c  --  dummy audio codec 
 *
 *
 * This software is licensed under the terms of the GNU General Public
 * License version 2, as published by the Free Software Foundation, and
 * may be copied, distributed, and modified under those terms.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 */

#include <linux/clk.h>
#include <linux/module.h>
#include <linux/moduleparam.h>
#include <linux/slab.h>
#include <linux/of.h>
#include <linux/of_gpio.h>
#include <sound/soc.h>
#include <sound/pcm.h>
#include <sound/initval.h>

struct dummy_codec_priv {
	struct snd_soc_codec *codec;
	struct clk *mclk;
};

static int dummy_codec_startup(struct snd_pcm_substream *substream,
			       struct snd_soc_dai *dai)
{
	struct snd_soc_codec *codec = dai->codec;
	struct dummy_codec_priv *dummy_codec = snd_soc_codec_get_drvdata(codec);

	if (!IS_ERR(dummy_codec->mclk))
		clk_prepare_enable(dummy_codec->mclk);
	return 0;
}

static void dummy_codec_shutdown(struct snd_pcm_substream *substream,
				 struct snd_soc_dai *dai)
{
	struct snd_soc_codec *codec = dai->codec;
	struct dummy_codec_priv *dummy_codec = snd_soc_codec_get_drvdata(codec);

	if (!IS_ERR(dummy_codec->mclk))
		clk_disable_unprepare(dummy_codec->mclk);
}

static struct snd_soc_dai_ops dummy_codec_dai_ops = {
	.startup	= dummy_codec_startup,
	.shutdown	= dummy_codec_shutdown,
};

struct snd_soc_dai_driver dummy_dai = {
	.name = "dummy_codec",
	.playback = {
		.stream_name = "Dummy Playback",
		.channels_min = 2,
		.channels_max = 384,
		.rates = SNDRV_PCM_RATE_8000_192000,
		.formats = (SNDRV_PCM_FMTBIT_S16_LE |
			    SNDRV_PCM_FMTBIT_S20_3LE |
			    SNDRV_PCM_FMTBIT_S24_LE |
			    SNDRV_PCM_FMTBIT_S32_LE),
	},
	.capture = {
		.stream_name = "Dummy Capture",
		.channels_min = 2,
		.channels_max = 384,
		.rates = SNDRV_PCM_RATE_8000_192000,
		.formats = (SNDRV_PCM_FMTBIT_S16_LE |
			    SNDRV_PCM_FMTBIT_S20_3LE |
			    SNDRV_PCM_FMTBIT_S24_LE |
			    SNDRV_PCM_FMTBIT_S32_LE),
	},
	.ops = &dummy_codec_dai_ops,
};

static struct snd_soc_codec_driver soc_dummy_codec;

static int radia_dummy_codec_probe(struct platform_device *pdev)
{
	struct dummy_codec_priv *codec_priv;

	codec_priv = devm_kzalloc(&pdev->dev, sizeof(*codec_priv),
				  GFP_KERNEL);
	if (!codec_priv)
		return -ENOMEM;
	platform_set_drvdata(pdev, codec_priv);

	codec_priv->mclk = devm_clk_get(&pdev->dev, "mclk");
	if (IS_ERR(codec_priv->mclk)) {
		/* some devices may not need mclk,so warnnig */
		dev_warn(&pdev->dev, "Unable to get mclk\n");
		if (PTR_ERR(codec_priv->mclk) == -EPROBE_DEFER)
			return -EPROBE_DEFER;
		else if (PTR_ERR(codec_priv->mclk) != -ENOENT)
			return -EINVAL;
	} else {
		dev_info(&pdev->dev, "get mclk success\n");
	}

	return snd_soc_register_codec(&pdev->dev, &soc_dummy_codec,
				      &dummy_dai, 1);
}

static int radia_dummy_codec_remove(struct platform_device *pdev)
{
	snd_soc_unregister_codec(&pdev->dev);

	return 0;
}

static const struct of_device_id radia_dummy_codec_of_match[] = {
	{ .compatible = "test,dummy-codec", },
	{},
};
MODULE_DEVICE_TABLE(of, radia_dummy_codec_of_match);

static struct platform_driver radia_dummy_codec_driver = {
	.driver = {
		.name = "dummy_codec",
		.of_match_table = of_match_ptr(radia_dummy_codec_of_match),
	},
	.probe = radia_dummy_codec_probe,
	.remove = radia_dummy_codec_remove,
};

module_platform_driver(radia_dummy_codec_driver);

MODULE_AUTHOR("radia <radia@stackdump.cn>");
MODULE_DESCRIPTION("radia Dummy Codec Driver");
MODULE_LICENSE("GPL v2");