Linux ALSA驱动之四：Control设备创建流程源码分析（5.18）

        Control接口主要让用户空间的应用程序（alsa-lib）可以访问和控制音频codec芯片中的多路开关，滑动控件等。对于 Mixer （混音）来说，Control接口显得尤为重要，从ALSA 0.9.x版本开始，所有的mixer工作都是通过control接口的API来实现的。

        ALSA已经为AC97定义了完整的控制接口模型，如果你的Codec芯片只支持AC97接口，你可以不用关心本节的内容。

   <sound/control.h> 定义了所有的Control API。如果你要为你的codec实现自己的controls，请在代码中包含该头文件。

1、snd_kcontrol_new

struct snd_kcontrol_new {
	snd_ctl_elem_iface_t iface;	/* interface identifier */
	unsigned int device;		/* device/client number */
	unsigned int subdevice;		/* subdevice (substream) number */
	const char *name;		    /* ASCII name of item */
	unsigned int index;		    /* index of item */
	unsigned int access;		/* access rights */
	unsigned int count;		    /* count of same elements */
	snd_kcontrol_info_t *info;
	snd_kcontrol_get_t *get;
	snd_kcontrol_put_t *put;
	union {
		snd_kcontrol_tlv_rw_t *c;
		const unsigned int *p;
	} tlv;
	unsigned long private_value;
};

        iface：表示control的类型，用SNDRV_CTL_ELEM_IFACE_XXX来定义。通常使用MIXER，也可以定于属于全局的CARD类型，如果定义为属于莫雷设备的类型，例如HWDEP、PCMRAWMIDI、TIMER等，此时必须在device和subdevice字段中支出卡的设备逻辑编号。

        name：表示control的名字，用户层可以通过这个名字访问这个control，后续会细聊

        index：存放这个 control 的索引号。如果声卡下不止一个codec。每个codec有相同的名字的control。此时就需要通过index来区分这些controls，当index为0，则可以忽略这种区分策略。

        access：访问权限的控制，READ，WRITE，READWRITE等。每一个bit代表一种访问类型，这些访问类型可以多个或运算组合在一起使用。

        private_value：包含了一个人员的长整数类型的值，该值可以通过info、get、put这几个回调函数访问。

        tlv：该字段为control提供元数据。

2、control的名字

        control的名字需要遵循一些标准，通常可以分成3部分来定义control的名字：源--方向--功能。

        源：可以理解为该control的输入端，alsa已经预定义了一些常用的源，例如：Master，PCM，CD，Line等等。

        方向：代表该control的数据流向，例如：Playback，Capture，Bypass，Bypass Capture等等，也可以不定义方向，这时表示该Control是双向的（playback和capture）。

        功能：根据control的功能，可以是以下字符串：Switch，Volume，Route等等。

也有一些命名上的特例：

                1、全局的capture和playback："Capture Source"，“Capture Volume”，“Capture Switch”，他们用于全局的capture source、switch和volume。同样的“Playback Volume”，“Playeback Switch”，它们用于全局的输出switch和volume。

                2、Tone-controles：音调控制的开关和音量命名为：Tomw Control-XXX，例如，“Tone-Control-Switch”，“Tone Control-Bass”，“Tone Control-Center”。

                3、3D controls：3D控件的命名规则：“3D Control-Switch”,“3D Control-Center”,“3D Control-Space”。

                4、MIC boost：麦克风音量加强空间命名为：“MIC Boost”或“MIC Bosst（6dB）”。

3、访问标志（ACCESS Flags）

        Access字段是一个bitmask，它保存了改control的访问类型。默认的访问类型是：SNDDRV_CTL_ELEM_ACCESS_READWRITE，表明该control支持读和写操作。如果access字段没有定义（.access==0），此时也认为是READWRITE类型。

        如果是一个只读control，access应该设置为：SNDDRV_CTL_ELEM_ACCESS_READ，这时，我们不必定义put回调函数。类似地，如果是只写control，access应该设置为：SNDDRV_CTL_ELEM_ACCESS_WRITE，这时，我们不必定义get回调函数。

        如果control的值会频繁地改变（例如：电平表），我们可以使用VOLATILE类型，这意味着该control会在没有通知的情况下改变，应用程序应该定时地查询该control的值。

4、元数据（METADATA）

        很多mixer control需要提供以dB为单位的信息，我们可以使用DECLARE_TLV_xxx宏来定义一些包含这种信息的变量，然后把control的tlv.p字段指向这些变量，最后，在access字段中加上SNDRV_CTL_ELEM_ACCESS_TLV_READ标志，例如：

static const DECLARE_TLV_DB_SCALE(snd_cx88_db_scale, -6300, 100, 0);

static const struct snd_kcontrol_new snd_cx88_volume = {
	.iface = SNDRV_CTL_ELEM_IFACE_MIXER,
	.access = SNDRV_CTL_ELEM_ACCESS_READWRITE |
		  SNDRV_CTL_ELEM_ACCESS_TLV_READ,
	.name = "Analog-TV Volume",
	.info = snd_cx88_volume_info,
	.get = snd_cx88_volume_get,
	.put = snd_cx88_volume_put,
	.tlv.p = snd_cx88_db_scale,
};

5、函数详解

5.1、snd_ctl_new1函数

/**
 * snd_ctl_new1 - create a control instance from the template
 * @ncontrol: the initialization record
 * @private_data: the private data to set
 *
 * Allocates a new struct snd_kcontrol instance and initialize from the given
 * template.  When the access field of ncontrol is 0, it's assumed as
 * READWRITE access. When the count field is 0, it's assumes as one.
 *
 * Return: The pointer of the newly generated instance, or %NULL on failure.
 */
struct snd_kcontrol *snd_ctl_new1(const struct snd_kcontrol_new *ncontrol,
				  void *private_data)
{
	struct snd_kcontrol *kctl;
	unsigned int count;
	unsigned int access;
	int err;

	if (snd_BUG_ON(!ncontrol || !ncontrol->info))
		return NULL;

	count = ncontrol->count;
	if (count == 0)
		count = 1;

	access = ncontrol->access;
	if (access == 0)
		access = SNDRV_CTL_ELEM_ACCESS_READWRITE;
	access &= (SNDRV_CTL_ELEM_ACCESS_READWRITE |
		   SNDRV_CTL_ELEM_ACCESS_VOLATILE |
		   SNDRV_CTL_ELEM_ACCESS_INACTIVE |
		   SNDRV_CTL_ELEM_ACCESS_TLV_READWRITE |
		   SNDRV_CTL_ELEM_ACCESS_TLV_COMMAND |
		   SNDRV_CTL_ELEM_ACCESS_TLV_CALLBACK |
		   SNDRV_CTL_ELEM_ACCESS_LED_MASK |
		   SNDRV_CTL_ELEM_ACCESS_SKIP_CHECK);

    /* 创建snd_kcontrol */
	err = snd_ctl_new(&kctl, count, access, NULL);
	if (err < 0)
		return NULL;
    
    /* 根据snd_kcontrol_new初始化snd_kcontrol */
	/* The 'numid' member is decided when calling snd_ctl_add(). */
	kctl->id.iface = ncontrol->iface;
	kctl->id.device = ncontrol->device;
	kctl->id.subdevice = ncontrol->subdevice;
	if (ncontrol->name) {
		strscpy(kctl->id.name, ncontrol->name, sizeof(kctl->id.name));
		if (strcmp(ncontrol->name, kctl->id.name) != 0)
			pr_warn("ALSA: Control name '%s' truncated to '%s'\n",
				ncontrol->name, kctl->id.name);
	}
	kctl->id.index = ncontrol->index;

	kctl->info = ncontrol->info;
	kctl->get = ncontrol->get;
	kctl->put = ncontrol->put;
	kctl->tlv.p = ncontrol->tlv.p;

	kctl->private_value = ncontrol->private_value;
	kctl->private_data = private_data;

	return kctl;
}

        分配一个新的snd_kcontrol实例,并把my_control中相应的值复制到该实例中,所以,在定义my_control时,通常我们可以加上__devinitdata前缀.snd_ctl_add则把该control绑定到声卡对象card当中。

struct snd_kcontrol {
	struct list_head list;		                /* list of controls */
	struct snd_ctl_elem_id id;
	unsigned int count;		                    /* count of same elements */
	snd_kcontrol_info_t *info;
	snd_kcontrol_get_t *get;
	snd_kcontrol_put_t *put;
	union {
		snd_kcontrol_tlv_rw_t *c;
		const unsigned int *p;
	} tlv;
	unsigned long private_value;
	void *private_data;
	void (*private_free)(struct snd_kcontrol *kcontrol);
	struct snd_kcontrol_volatile vd[];	        /* volatile data */
};

#define snd_kcontrol(n) list_entry(n, struct snd_kcontrol, list)

5.2、 snd_ctl_add函数

/* add/replace a new kcontrol object; call with card->controls_rwsem locked */
static int __snd_ctl_add_replace(struct snd_card *card,
				 struct snd_kcontrol *kcontrol,
				 enum snd_ctl_add_mode mode)
{
	struct snd_ctl_elem_id id;
	unsigned int idx;
	struct snd_kcontrol *old;
	int err;

	id = kcontrol->id;
	if (id.index > UINT_MAX - kcontrol->count)
		return -EINVAL;

	old = snd_ctl_find_id(card, &id);
	if (!old) {
		if (mode == CTL_REPLACE)
			return -EINVAL;
	} else {
		if (mode == CTL_ADD_EXCLUSIVE) {
			dev_err(card->dev,
				"control %i:%i:%i:%s:%i is already present\n",
				id.iface, id.device, id.subdevice, id.name,
				id.index);
			return -EBUSY;
		}

		err = snd_ctl_remove(card, old);
		if (err < 0)
			return err;
	}

	if (snd_ctl_find_hole(card, kcontrol->count) < 0)
		return -ENOMEM;
    
    /* 把snd_kcontrol挂入snd_card的controls链表 */
	list_add_tail(&kcontrol->list, &card->controls);
	card->controls_count += kcontrol->count;
    /* 设置元素ID */
	kcontrol->id.numid = card->last_numid + 1;
	card->last_numid += kcontrol->count;

	for (idx = 0; idx < kcontrol->count; idx++)
		snd_ctl_notify_one(card, SNDRV_CTL_EVENT_MASK_ADD, kcontrol, idx);

	return 0;
}

static int snd_ctl_add_replace(struct snd_card *card,
			       struct snd_kcontrol *kcontrol,
			       enum snd_ctl_add_mode mode)
{
	int err = -EINVAL;

	if (! kcontrol)
		return err;
	if (snd_BUG_ON(!card || !kcontrol->info))
		goto error;

	down_write(&card->controls_rwsem);
	err = __snd_ctl_add_replace(card, kcontrol, mode);
	up_write(&card->controls_rwsem);
	if (err < 0)
		goto error;
	return 0;

 error:
	snd_ctl_free_one(kcontrol);
	return err;
}

/**
 * snd_ctl_add - add the control instance to the card
 * @card: the card instance
 * @kcontrol: the control instance to add
 *
 * Adds the control instance created via snd_ctl_new() or
 * snd_ctl_new1() to the given card. Assigns also an unique
 * numid used for fast search.
 *
 * It frees automatically the control which cannot be added.
 *
 * Return: Zero if successful, or a negative error code on failure.
 *
 */
int snd_ctl_add(struct snd_card *card, struct snd_kcontrol *kcontrol)
{
	return snd_ctl_add_replace(card, kcontrol, CTL_ADD_EXCLUSIVE);
}

5.3、info回调函数

        用于得到对应control的详细信息，需要把信息存入snd_ctl_elem_info 对象中。

struct snd_ctl_elem_info {
	struct snd_ctl_elem_id id;	/* W: element ID */
	snd_ctl_elem_type_t type;	/* R: value type - SNDRV_CTL_ELEM_TYPE_* */
	unsigned int access;		/* R: value access (bitmask) - SNDRV_CTL_ELEM_ACCESS_* */
	unsigned int count;		/* count of values */
	__kernel_pid_t owner;		/* owner's PID of this control */
	union {
		struct {
			long min;		/* R: minimum value */
			long max;		/* R: maximum value */
			long step;		/* R: step (0 variable) */
		} integer;
		struct {
			long long min;		/* R: minimum value */
			long long max;		/* R: maximum value */
			long long step;		/* R: step (0 variable) */
		} integer64;
		struct {
			unsigned int items;	/* R: number of items */
			unsigned int item;	/* W: item number */
			char name[64];		/* R: value name */
			__u64 names_ptr;	/* W: names list (ELEM_ADD only) */
			unsigned int names_length;
		} enumerated;
		unsigned char reserved[128];
	} value;
	unsigned char reserved[64];
};

        其中的value是个一个共用体，需要根据control的类型，确定值的类型，control type包括如下几类：

typedef int __bitwise snd_ctl_elem_type_t;
#define	SNDRV_CTL_ELEM_TYPE_NONE	((__force snd_ctl_elem_type_t) 0)     /* invalid */
#define	SNDRV_CTL_ELEM_TYPE_BOOLEAN	((__force snd_ctl_elem_type_t) 1)     /* boolean type */
#define	SNDRV_CTL_ELEM_TYPE_INTEGER	((__force snd_ctl_elem_type_t) 2)     /* integer type */
#define	SNDRV_CTL_ELEM_TYPE_ENUMERATED	((__force snd_ctl_elem_type_t) 3) /* enumerated type */
#define	SNDRV_CTL_ELEM_TYPE_BYTES	((__force snd_ctl_elem_type_t) 4)     /* byte array */
#define	SNDRV_CTL_ELEM_TYPE_IEC958	((__force snd_ctl_elem_type_t) 5)     /* IEC958 (S/PDIF) setup */
#define	SNDRV_CTL_ELEM_TYPE_INTEGER64	((__force snd_ctl_elem_type_t) 6) /* 64-bit integer type */
#define	SNDRV_CTL_ELEM_TYPE_LAST	SNDRV_CTL_ELEM_TYPE_INTEGER64

        下面是以SNDRV_CTL_ELEM_TYPE_INTEGER和以SNDRV_CTL_ELEM_TYPE_BOOLEAN为例定义的info回调函数：

static int snd_cx88_volume_info(struct snd_kcontrol *kcontrol,
				struct snd_ctl_elem_info *info)
{
	info->type = SNDRV_CTL_ELEM_TYPE_INTEGER;
	info->count = 2;
	info->value.integer.min = 0;
	info->value.integer.max = 0x3f;

	return 0;
}

static int snd_saa7134_capsrc_info(struct snd_kcontrol * kcontrol,
				   struct snd_ctl_elem_info * uinfo)
{
	uinfo->type = SNDRV_CTL_ELEM_TYPE_BOOLEAN;
	uinfo->count = 2;
	uinfo->value.integer.min = 0;
	uinfo->value.integer.max = 1;
	return 0;
}

5.4、get回调函数

        这个函数用来读取当前 control 的值并返回到用户空间，需要把值放在snd_ctl_elem_value结构体中，与info结构体类似，value字段是一个共用体，与类型相关。如果value的cont大于1, 需要把值全部放入到 value[]数组中。

static int snd_cx88_volume_info(struct snd_kcontrol *kcontrol,
				struct snd_ctl_elem_info *info)
{
	info->type = SNDRV_CTL_ELEM_TYPE_INTEGER;
	info->count = 2;
	info->value.integer.min = 0;
	info->value.integer.max = 0x3f;

	return 0;
}

static int snd_cx88_volume_get(struct snd_kcontrol *kcontrol,
			       struct snd_ctl_elem_value *value)
{
	struct cx88_audio_dev *chip = snd_kcontrol_chip(kcontrol);
	struct cx88_core *core = chip->core;
	int vol = 0x3f - (cx_read(AUD_VOL_CTL) & 0x3f),
	    bal = cx_read(AUD_BAL_CTL);

	value->value.integer.value[(bal & 0x40) ? 0 : 1] = vol;
	vol -= (bal & 0x3f);
	value->value.integer.value[(bal & 0x40) ? 1 : 0] = vol < 0 ? 0 : vol;

	return 0;
}

5.5、put回调函数

         put回调函数用于把应用程序的控制值设置到control中。

static void snd_cx88_wm8775_volume_put(struct snd_kcontrol *kcontrol,
				       struct snd_ctl_elem_value *value)
{
	struct cx88_audio_dev *chip = snd_kcontrol_chip(kcontrol);
	struct cx88_core *core = chip->core;
	u16 left = value->value.integer.value[0];
	u16 right = value->value.integer.value[1];
	int v, b;

	/* Pass volume & balance onto any WM8775 */
	if (left >= right) {
		v = left << 10;
		b = left ? (0x8000 * right) / left : 0x8000;
	} else {
		v = right << 10;
		b = right ? 0xffff - (0x8000 * left) / right : 0x8000;
	}
	wm8775_s_ctrl(core, V4L2_CID_AUDIO_VOLUME, v);
	wm8775_s_ctrl(core, V4L2_CID_AUDIO_BALANCE, b);
}

/* OK - TODO: test it */
static int snd_cx88_volume_put(struct snd_kcontrol *kcontrol,
			       struct snd_ctl_elem_value *value)
{
	struct cx88_audio_dev *chip = snd_kcontrol_chip(kcontrol);
	struct cx88_core *core = chip->core;
	int left, right, v, b;
	int changed = 0;
	u32 old;

	if (core->sd_wm8775)
		snd_cx88_wm8775_volume_put(kcontrol, value);

	left = value->value.integer.value[0] & 0x3f;
	right = value->value.integer.value[1] & 0x3f;
	b = right - left;
	if (b < 0) {
		v = 0x3f - left;
		b = (-b) | 0x40;
	} else {
		v = 0x3f - right;
	}
	/* Do we really know this will always be called with IRQs on? */
	spin_lock_irq(&chip->reg_lock);
	old = cx_read(AUD_VOL_CTL);
	if (v != (old & 0x3f)) {
		cx_swrite(SHADOW_AUD_VOL_CTL, AUD_VOL_CTL, (old & ~0x3f) | v);
		changed = 1;
	}
	if ((cx_read(AUD_BAL_CTL) & 0x7f) != b) {
		cx_write(AUD_BAL_CTL, b);
		changed = 1;
	}
	spin_unlock_irq(&chip->reg_lock);

	return changed;
}

6、Control设备创建流程

        Control设备和PCM设备一样，都属于声卡下的逻辑设备。用户空间的应用程序通过alsa-lib访问该Control设备，读取或设置control的控制状态，从而达到控制音频Codec进行各种Mixer等控制操作。

        Control设备的创建过程大体上和PCM设备的创建过程相同。详细的创建过程可以参考下方时序图。

      我们需要在我们的驱动程序初始化时主动调用snd_pcm_new()函数创建pcm设备，而control设备则在snd_ctl_new1()内被创建，snd_ctl_new1()通过调用snd_ctl_create()函数创建control设备节点。所以我们无需显式地创建control设备，只要建立声卡，control设备被自动地创建。

      和pcm设备一样，control设备的名字遵循一定的规则：controlCxx，这里的xx代表声卡的编号。

        snd_ctl_dev_register()函数会在snd_card_register()中，即声卡的注册阶段被调用。注册完成后，control设备的相关信息被保存在snd_minors[]数组中，用control设备的次设备号作索引，即可在snd_minors[]数组中找出相关的信息。注册完成后的数据结构关系可以用下图进行表述：