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