Linux ALSA声卡驱动之三:PCM设备的创建

1. PCM是什么


        PCM是英文Pulse-code modulation的缩写,中文译名是脉冲编码调制。我们知道在现实生活中,人耳听到的声音是模拟信号,PCM就是要把声音从模拟转换成数字信号的一种技术,他的原理简单地说就是利用一个固定的频率对模拟信号进行采样,采样后的信号在波形上看就像一串连续的幅值不一的脉冲,把这些脉冲的幅值按一定的精度进行量化,这些量化后的数值被连续地输出、传输、处理或记录到存储介质中,所有这些组成了数字音频的产生过程。

           PCM信号的两个重要指标是采样频率和量化精度,目前,CD音频的采样频率通常为44100Hz,量化精度是16bit。通常,播放音乐时,应用程序从存储介质中读取音频数据(MP3、WMA、AAC......),经过解码后,最终送到音频驱动程序中的就是PCM数据,反过来,在录音时,音频驱动不停地把采样所得的PCM数据送回给应用程序,由应用程序完成压缩、存储等任务。所以,音频驱动的两大核心任务就是:

  • playback    如何把用户空间的应用程序发过来的PCM数据,转化为人耳可以辨别的模拟音频
  • capture     把mic拾取到得模拟信号,经过采样、量化,转换为PCM信号送回给用户空间的应用程序

2. alsa-driver中的PCM中间层


      ALSA已经为我们实现了功能强劲的PCM中间层,自己的驱动中只要实现一些底层的需要访问硬件的函数即可。

      要访问PCM的中间层代码,你首先要包含头文件,另外,如果需要访问一些与 hw_param相关的函数,可能也要包含

      每个声卡最多可以包含4个pcm的实例,每个pcm实例对应一个pcm设备文件。pcm实例数量的这种限制源于linux设备号所占用的位大小,如果以后使用64位的设备号,我们将可以创建更多的pcm实例。不过大多数情况下,在嵌入式设备中,一个pcm实例已经足够了。

      一个pcm实例由一个playback stream和一个capture stream组成,这两个stream又分别有一个或多个substreams组成。

                    图2.1  声卡中的pcm结构

       在嵌入式系统中,通常不会像图2.1中这么复杂,大多数情况下是一个声卡,一个pcm实例,pcm下面有一个playback stream和capture stream,playback和capture下面各自有一个substream。

       下面一张图列出了pcm中间层几个重要的结构,他可以让我们从uml的角度看一看这列结构的关系,理清他们之间的关系,对我们理解pcm中间层的实现方式。

 

             图2.2  pcm中间层的几个重要的结构体的关系图 

  • snd_pcm是挂在snd_card下面的一个snd_device,此snd_device保存在snd_card->devices列表中,snd_pcm保存在snd_device->device_data中。
  • snd_pcm中的字段:streams[2],该数组中的两个元素指向两个snd_pcm_str结构,分别代表playback stream和capture stream
  • snd_pcm_str中的substream字段,指向snd_pcm_substream结构
  • snd_pcm_substream是pcm中间层的核心,绝大部分任务都是在substream中处理,尤其是他的ops(snd_pcm_ops)字段,许多user空间的应用程序通过alsa-lib对驱动程序的请求都是由该结构中的函数处理。它的runtime字段则指向snd_pcm_runtime结构,snd_pcm_runtime记录这substream的一些重要的软件和硬件运行环境和参数。
  • 相关数据结构主要定义如下:
struct snd_card {
        ...
	void *private_data;		/* private data for soundcard */
	void (*private_free) (struct snd_card *card); /* callback for freeing of
	...							private data */
	struct list_head devices;	/* devices: snd_device列表*/
        ...
};

struct snd_device {
	struct list_head list;		/* list of registered devices */
	struct snd_card *card;		/* card which holds this device */
	snd_device_state_t state;	/* state of the device */
	snd_device_type_t type;		/* device type */
	void *device_data;		/* device structure: 保存具体snd_device对象指针,如snd_pcm */
	struct snd_device_ops *ops;	/* operations:存有具体snd_device的操作,如snd_pcm*/
};

struct snd_device_ops {
	int (*dev_free)(struct snd_device *dev);
	int (*dev_register)(struct snd_device *dev); 
        /* dev_register: 在snd_card_register时被调用,且创建/dev/snd下的设备文件节点 */
	int (*dev_disconnect)(struct snd_device *dev);
};

如snd_pcm的snd_device_ops为:
	static struct snd_device_ops ops = {
		.dev_free = snd_pcm_dev_free,
		.dev_register =	snd_pcm_dev_register,
		.dev_disconnect = snd_pcm_dev_disconnect,
	};

// pcm设备相关数据结构:
struct snd_pcm {
	struct snd_card *card;
        ...         
	struct snd_pcm_str streams[2];
        ...
};

struct snd_pcm_str {
	int stream;	 /* stream (direction) */
	struct snd_pcm *pcm;
	/* -- substreams -- */
	unsigned int substream_count;
	unsigned int substream_opened;

	struct snd_pcm_substream *substream;  /* substream 列表 */
        ...
};

struct snd_pcm_substream {
        ...
	/* -- hardware operations -- */
	struct snd_pcm_ops *ops;    /* 驱动对数据的操作 */

	/* -- runtime information -- */
	struct snd_pcm_runtime *runtime; /* 如通道数、采样率等信息 */
      
	/* -- next substream -- */
	struct snd_pcm_substream *next;  /* 通过它构成了substream链表 */
	/* -- linked substreams -- */
	struct list_head link_list;	/* linked list member */
};

struct snd_pcm_ops {
	int (*open)(struct snd_pcm_substream *substream);
	int (*close)(struct snd_pcm_substream *substream);
	int (*ioctl)(struct snd_pcm_substream * substream,
		     unsigned int cmd, void *arg);
	int (*hw_params)(struct snd_pcm_substream *substream,
			 struct snd_pcm_hw_params *params);
	int (*hw_free)(struct snd_pcm_substream *substream);
	int (*prepare)(struct snd_pcm_substream *substream);
	int (*trigger)(struct snd_pcm_substream *substream, int cmd);
	snd_pcm_uframes_t (*pointer)(struct snd_pcm_substream *substream);
	int (*copy)(struct snd_pcm_substream *substream, int channel,
		    snd_pcm_uframes_t pos,
		    void __user *buf, snd_pcm_uframes_t count);
	int (*silence)(struct snd_pcm_substream *substream, int channel, 
		       snd_pcm_uframes_t pos, snd_pcm_uframes_t count);
	struct page *(*page)(struct snd_pcm_substream *substream,
			     unsigned long offset);
	int (*mmap)(struct snd_pcm_substream *substream, struct vm_area_struct *vma);
	int (*ack)(struct snd_pcm_substream *substream);
}

 

 3. 新建一个pcm  


  •  alsa-driver的中间层已经为我们提供了新建pcm的api:

       int snd_pcm_new(struct snd_card *card, const char *id, int device, int playback_count, int capture_count,
                                     struct snd_pcm ** rpcm);

参数device: 表示目前创建的是该声卡下的第几个pcm,第一个pcm设备从0开始。

参数playback_count: 表示该pcm将会有几个playback substream。

参数capture_count: 表示该pcm将会有几个capture substream。

  • 另一个用于设置pcm操作函数接口的api:

         void snd_pcm_set_ops(struct snd_pcm *pcm, int direction, struct snd_pcm_ops *ops);

         --设定指定方向的snd_pcm_str中的每个snd_pcm_substream的操作为此snd_pcm_ops,snd_pcm_ops定义如下: 

struct snd_pcm_ops {
	int (*open)(struct snd_pcm_substream *substream);
	int (*close)(struct snd_pcm_substream *substream);
	int (*ioctl)(struct snd_pcm_substream * substream,
		     unsigned int cmd, void *arg);
	int (*hw_params)(struct snd_pcm_substream *substream,
			 struct snd_pcm_hw_params *params);
	int (*hw_free)(struct snd_pcm_substream *substream);
	int (*prepare)(struct snd_pcm_substream *substream);
	int (*trigger)(struct snd_pcm_substream *substream, int cmd);
	snd_pcm_uframes_t (*pointer)(struct snd_pcm_substream *substream);
	int (*copy)(struct snd_pcm_substream *substream, int channel,
		    snd_pcm_uframes_t pos,
		    void __user *buf, snd_pcm_uframes_t count);
	int (*silence)(struct snd_pcm_substream *substream, int channel, 
		       snd_pcm_uframes_t pos, snd_pcm_uframes_t count);
	struct page *(*page)(struct snd_pcm_substream *substream,
			     unsigned long offset);
	int (*mmap)(struct snd_pcm_substream *substream, struct vm_area_struct *vma);
	int (*ack)(struct snd_pcm_substream *substream);
}

        新建一个pcm可以用下面一张新建pcm的调用的序列图进行描述:

                                                       图3.1 新建pcm的序列图上

       上图中snd_device_new中的ops定义如下:

	static struct snd_device_ops ops = {
		.dev_free = snd_pcm_dev_free,
		.dev_register =	snd_pcm_dev_register,
		.dev_disconnect = snd_pcm_dev_disconnect,
	};
  • snd_card_create    pcm是声卡下的一个设备(部件),所以第一步是要创建一个声卡
  • snd_pcm_new    调用该api创建一个pcm,在该api中会做以下事情
    • 如果有,建立playback stream,相应的substream也同时建立
    • 如果有,建立capture stream,相应的substream也同时建立
    • 调用snd_device_new()把该pcm挂到声卡的snd_card->devices链表中,参数ops中的dev_register字段指向了函数snd_pcm_dev_register,这个回调函数会在声卡的注册阶段被调用。
  • snd_pcm_set_ops    设置操作该pcm的控制/操作接口函数,参数中的snd_pcm_ops结构中的函数通常就是我们驱动要实现的函数
  • snd_card_register    注册声卡,在这个阶段会遍历声卡下的所有逻辑设备,并且调用各设备的注册回调函数,对于pcm,就是第二步提到的snd_pcm_dev_register函数,该回调函数建立了和用户空间应用程序(alsa-lib)通信所用的设备文件节点:/dev/snd/pcmCxxDxxp和/dev/snd/pcmCxxDxxc

4. 设备文件节点的建立(dev/snd/pcmCxxDxxp、pcmCxxDxxc)


 

4.1 struct snd_minor

        每个snd_minor结构体保存了声卡下某个逻辑设备的上下文信息,它在逻辑设备建立阶段被填充,在逻辑设备被使用时就可以从该结构体中得到相应的信息。pcm设备也不例外,也需要使用该结构体。该结构体在include/sound/core.h中定义。

struct snd_minor {
	int type;			/* SNDRV_DEVICE_TYPE_XXX */
	int card;			/* card number */
	int device;			/* device number */
	const struct file_operations *f_ops;	/* file operations */
	void *private_data;		/* private data for f_ops->open, 如snd_pcm对象 */
	struct device *dev;		/* device for sysfs */
};

      在sound/sound.c中定义了一个snd_minor指针的全局数组:

static struct snd_minor *snd_minors[256];

      前面说过,在声卡的注册阶段(snd_card_register),会调用pcm的回调函数snd_pcm_dev_register(),这个函数里会调用函数snd_register_device_for_dev():

static int snd_pcm_dev_register(struct snd_device *device)
{
    ......

	/* register pcm */
	err = snd_register_device_for_dev(devtype, pcm->card,
				         pcm->device,
					&snd_pcm_f_ops[cidx],
					pcm, str, dev);
    ......
}

     我们再进入snd_register_device_for_dev():

int snd_register_device_for_dev(int type, struct snd_card *card, int dev,
				const struct file_operations *f_ops,
				void *private_data,
				const char *name, struct device *device)
{
	int minor;
	struct snd_minor *preg;

	if (snd_BUG_ON(!name))
		return -EINVAL;
	preg = kmalloc(sizeof *preg, GFP_KERNEL);
	if (preg == NULL)
		return -ENOMEM;
	preg->type = type;
	preg->card = card ? card->number : -1;
	preg->device = dev;
	preg->f_ops = f_ops;
	preg->private_data = private_data;
	mutex_lock(&sound_mutex);
#ifdef CONFIG_SND_DYNAMIC_MINORS
	minor = snd_find_free_minor();
#else
	minor = snd_kernel_minor(type, card, dev);
	if (minor >= 0 && snd_minors[minor])
		minor = -EBUSY;
#endif
	if (minor < 0) {
		mutex_unlock(&sound_mutex);
		kfree(preg);
		return minor;
	}
	snd_minors[minor] = preg;
	preg->dev = device_create(sound_class, device, MKDEV(major, minor),
				  private_data, "%s", name);
	if (IS_ERR(preg->dev)) {
		snd_minors[minor] = NULL;
		mutex_unlock(&sound_mutex);
		minor = PTR_ERR(preg->dev);
		kfree(preg);
		return minor;
	}

	mutex_unlock(&sound_mutex);
	return 0;
}
  • 首先,分配并初始化一个snd_minor结构中的各字段
    • type:SNDRV_DEVICE_TYPE_PCM_PLAYBACK/SNDRV_DEVICE_TYPE_PCM_CAPTURE
    • card: card的编号
    • device:pcm实例的编号,大多数情况为0
    • f_ops:snd_pcm_f_ops
    • private_data:指向该snd_pcm的实例对象
  • 根据type,card和pcm的编号,确定数组的索引值minor,minor也作为pcm设备的此设备号
  • 把该snd_minor结构的地址放入全局数组snd_minors[minor]中
  • 最后,调用device_create创建设备节点

4.2 设备文件的建立


        在4.1节的最后,设备文件已经建立,不过4.1节的重点在于snd_minors数组的赋值过程,在本节中,我们把重点放在设备文件中。

        回到pcm的回调函数snd_pcm_dev_register()中:

static int snd_pcm_dev_register(struct snd_device *device)
{
	int cidx, err;
	char str[16];
	struct snd_pcm *pcm;
	struct device *dev;

	pcm = device->device_data;
         ......
	for (cidx = 0; cidx < 2; cidx++) {
                  ......
		switch (cidx) {
		case SNDRV_PCM_STREAM_PLAYBACK:
			sprintf(str, "pcmC%iD%ip", pcm->card->number, pcm->device);
			devtype = SNDRV_DEVICE_TYPE_PCM_PLAYBACK;
			break;
		case SNDRV_PCM_STREAM_CAPTURE:
			sprintf(str, "pcmC%iD%ic", pcm->card->number, pcm->device);
			devtype = SNDRV_DEVICE_TYPE_PCM_CAPTURE;
			break;
		}
		/* device pointer to use, pcm->dev takes precedence if
		 * it is assigned, otherwise fall back to card's device
		 * if possible */
		dev = pcm->dev;
		if (!dev)
			dev = snd_card_get_device_link(pcm->card);
		/* register pcm */
		err = snd_register_device_for_dev(devtype, pcm->card,
						  pcm->device,
						  &snd_pcm_f_ops[cidx],
						  pcm, str, dev);
                  ......
	}
         ......
}

     以上代码我们可以看出,对于一个pcm设备,可以生成两个设备文件,一个用于playback,一个用于capture,代码中也确定了他们的命名规则:

  • playback  --  pcmCxDxp,通常系统中只有一个声卡和一个pcm,它就是pcmC0D0p
  • capture  --  pcmCxDxc,通常系统中只有一个声卡和一个pcm,它就是pcmC0D0c

snd_pcm_f_ops

     snd_pcm_f_ops是一个标准的文件系统file_operations结构数组,它的定义在sound/core/pcm_native.c中:

const struct file_operations snd_pcm_f_ops[2] = {
	{
		.owner =		THIS_MODULE,
		.write =		snd_pcm_write,
		.aio_write =		snd_pcm_aio_write,
		.open =			snd_pcm_playback_open,
		.release =		snd_pcm_release,
		.llseek =		no_llseek,
		.poll =			snd_pcm_playback_poll,
		.unlocked_ioctl =	snd_pcm_playback_ioctl,
		.compat_ioctl = 	snd_pcm_ioctl_compat,
		.mmap =			snd_pcm_mmap,
		.fasync =		snd_pcm_fasync,
		.get_unmapped_area =	snd_pcm_get_unmapped_area,
	},
	{
		.owner =		THIS_MODULE,
		.read =			snd_pcm_read,
		.aio_read =		snd_pcm_aio_read,
		.open =			snd_pcm_capture_open,
		.release =		snd_pcm_release,
		.llseek =		no_llseek,
		.poll =			snd_pcm_capture_poll,
		.unlocked_ioctl =	snd_pcm_capture_ioctl,
		.compat_ioctl = 	snd_pcm_ioctl_compat,
		.mmap =			snd_pcm_mmap,
		.fasync =		snd_pcm_fasync,
		.get_unmapped_area =	snd_pcm_get_unmapped_area,
	}
};

        snd_pcm_f_ops作为snd_register_device_for_dev的参数被传入,并被记录在snd_minors[minor]中的字段f_ops中。最后,在snd_register_device_for_dev中创建设备节点

	snd_minors[minor] = preg;
	preg->dev = device_create(sound_class, device, MKDEV(major, minor),
				  private_data, "%s", name);

4.3 层层深入,从应用程序到驱动层pcm


4.3.1 字符设备注册

      在sound/core/sound.c中有alsa_sound_init()函数,定义如下:

static int __init alsa_sound_init(void)
{
	snd_major = major;
	snd_ecards_limit = cards_limit;
	if (register_chrdev(major, "alsa", &snd_fops)) {
		snd_printk(KERN_ERR "unable to register native major device number %d/n", major);
		return -EIO;
	}
	if (snd_info_init() < 0) {
		unregister_chrdev(major, "alsa");
		return -ENOMEM;
	}
	snd_info_minor_register();
	return 0;
}

       register_chrdev中的参数major与之前创建pcm设备是device_create时的major是同一个<即116>,这样的结果是,当应用程序open设备文件/dev/snd/pcmCxDxp时,会进入snd_fops的open回调函数,我们将在下一节中讲述open的过程。

4.3.2 打开pcm设备

      从上一节中我们得知,open一个pcm设备时,将会调用snd_fops的open回调函数,我们先看看snd_fops的定义:

static const struct file_operations snd_fops =
{
	.owner =	THIS_MODULE,
	.open =		snd_open
};

     进入snd_open函数,它首先从inode中取出次设备号,然后以次设备号为索引,从snd_minors全局数组中取出当初注册pcm设备时填充的snd_minor结构(参看4.1节的内容),然后从snd_minor结构中取出pcm设备的f_ops,并且把file->f_op替换为pcm设备的f_ops,紧接着直接调用pcm设备的f_ops->open(),然后返回。因为file->f_op已经被替换,以后,应用程序的所有read/write/ioctl调用都会进入pcm设备自己的回调函数中,也就是4.2节中提到的snd_pcm_f_ops结构中定义的回调

static int snd_open(struct inode *inode, struct file *file)
{
	unsigned int minor = iminor(inode);
	struct snd_minor *mptr = NULL;
	const struct file_operations *old_fops;
	int err = 0;

	if (minor >= ARRAY_SIZE(snd_minors))
		return -ENODEV;
	mutex_lock(&sound_mutex);
	mptr = snd_minors[minor];
	if (mptr == NULL) {
		mptr = autoload_device(minor);
		if (!mptr) {
			mutex_unlock(&sound_mutex);
			return -ENODEV;
		}
	}
	old_fops = file->f_op;
	file->f_op = fops_get(mptr->f_ops);
	if (file->f_op == NULL) {
		file->f_op = old_fops;
		err = -ENODEV;
	}
	mutex_unlock(&sound_mutex);
	if (err < 0)
		return err;

	if (file->f_op->open) {
		err = file->f_op->open(inode, file);
		if (err) {
			fops_put(file->f_op);
			file->f_op = fops_get(old_fops);
		}
	}
	fops_put(old_fops);
	return err;
}

       下面的序列图展示了应用程序如何最终调用到snd_pcm_f_ops结构中的回调函数:



                             图4.3.2.1    应用程序操作pcm设备

    在上图中,file->f_op为snd_pcm_f_ops[0]或snd_pcm_f_ops[1],以playback为例,接下来的调用顺序为:

1) snd_pcm_playback_open(struct inode *inode, struct file *file)->
   /* snd_pcm根据文件节点的minor从snd_minors中获取*/

2) snd_pcm_open(struct file *file, struct snd_pcm *pcm, int stream)->

3) static int snd_pcm_open_file(struct file *file, struct snd_pcm *pcm, int stream, struct snd_pcm_file **rpcm_file)->
snd_pcm_file的定义如下:

struct snd_pcm_file {
 struct snd_pcm_substream *substream;
 int no_compat_mmap;
};


  a) 调用snd_pcm_open_substream根据stream获取对应的snd_pcm_substream
  b) 创建一个pcm_file对象,并把a)中获取的snd_pcm_substream赋值给pcm_file->substream
  c) file->private_data = pcm_file, 这样根据file->private_data就可以找到对应的snd_pcm_substream
  d) 然后就可以调用snd_pcm_substream->ops执行具体的操作

4.3.3 写PCM设备

写PCM流程如下:

1) snd_pcm_write-> (用户态)
2) write-> (系统调用)
以下为Kernel态:
3) snd_pcm_write(struct file *file, const char __user *buf,
        size_t count, loff_t * offset)
   a) 从file中获取pcm_file
   b) 从pcm_file中获取snd_pcm_substream
4) snd_pcm_lib_write(struct snd_pcm_substream *substream,
                 const void __user *buf, snd_pcm_uframes_t size)
5) snd_pcm_lib_write1(struct snd_pcm_substream *substream,
         unsigned long data,
         snd_pcm_uframes_t size,
         int nonblock,
         transfer_f transfer)
   即:snd_pcm_lib_write1(substream, (unsigned long)buf, size, nonblock,
      snd_pcm_lib_write_transfer)
 
6) snd_pcm_lib_write_transfer(struct snd_pcm_substream *substream,
          unsigned int hwoff,
          unsigned long data, unsigned int off,
          snd_pcm_uframes_t frames)

static int snd_pcm_lib_write_transfer(struct snd_pcm_substream *substream,
				      unsigned int hwoff,
				      unsigned long data, unsigned int off,
				      snd_pcm_uframes_t frames)
{
	struct snd_pcm_runtime *runtime = substream->runtime;
	int err;
	char __user *buf = (char __user *) data + frames_to_bytes(runtime, off);
	if (substream->ops->copy) {
		if ((err = substream->ops->copy(substream, -1, hwoff, buf, frames)) < 0)
			return err;
	} else {
		char *hwbuf = runtime->dma_area + frames_to_bytes(runtime, hwoff);
		if (copy_from_user(hwbuf, buf, frames_to_bytes(runtime, frames)))
			return -EFAULT;
	}
	return 0;
}


     在此函数中调用snd_pcm_substream->ops->copy来传递数据给ALSA Driver,再由ALSA Driver把此数据发送给hardware playback。

4.3.4 读PCM设备

读PCM流程如下:

1) snd_pcm_read-> (用户态)
2) read-> (系统调用)
以下为Kernel态:
3) snd_pcm_read(struct file *file, char __user *buf,
                size_t count,loff_t * offset)
4) snd_pcm_lib_read(struct snd_pcm_substream *substream,
                   void __user *buf, snd_pcm_uframes_t size)
5) snd_pcm_lib_read1(struct snd_pcm_substream *substream,
        unsigned long data,
        snd_pcm_uframes_t size,
        int nonblock,
        transfer_f transfer)
   
即:snd_pcm_lib_read1(substream, (unsigned long)buf, size, nonblock,
                                  snd_pcm_lib_read_transfer);

static int snd_pcm_lib_read_transfer(struct snd_pcm_substream *substream, 
				     unsigned int hwoff,
				     unsigned long data, unsigned int off,
				     snd_pcm_uframes_t frames)
{
	struct snd_pcm_runtime *runtime = substream->runtime;
	int err;
	char __user *buf = (char __user *) data + frames_to_bytes(runtime, off);
	if (substream->ops->copy) {
		if ((err = substream->ops->copy(substream, -1, hwoff, buf, frames)) < 0)
			return err;
	} else {
		char *hwbuf = runtime->dma_area + frames_to_bytes(runtime, hwoff);
		if (copy_to_user(buf, hwbuf, frames_to_bytes(runtime, frames)))
			return -EFAULT;
	}
	return 0;
}

5. 总结

    1) 通过device_create创建的设备文件节点中包含major和minor

    2) pcm设备的文件操作(file_operations snd_pcm_f_ops[2])被保存在snd_minors全局数据中,以minor为索引

    3) snd_minors的private_data为snd_pcm实例

    4) open文件节点时,根据其major寻找已经为此major注册的open函数,在此open函数中,则根据其minor在snd_minors中找到对应的f_ops,然后调用此f_ops->open<即snd_pcm_playback_open>


转自:http://blog.csdn.net/droidphone/article/details/6308006

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