sound/core/oss/mulaw.c - maze/linux - Git at Google

 /*
  *  Mu-Law conversion Plug-In Interface
  *  Copyright (c) 1999 by Jaroslav Kysela <perex@perex.cz>
  *                        Uros Bizjak <uros@kss-loka.si>
  *
  *  Based on reference implementation by Sun Microsystems, Inc.
  *
  *   This library is free software; you can redistribute it and/or modify
  *   it under the terms of the GNU Library General Public License as
  *   published by the Free Software Foundation; either version 2 of
  *   the License, or (at your option) any later version.
  *
  *   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 Library General Public License for more details.
  *
  *   You should have received a copy of the GNU Library General Public
  *   License along with this library; if not, write to the Free Software
  *   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307 USA
  *
  */

 #include <linux/time.h>
 #include <sound/core.h>
 #include <sound/pcm.h>
 #include "pcm_plugin.h"

 #define	SIGN_BIT	(0x80)		/* Sign bit for a u-law byte. */
 #define	QUANT_MASK	(0xf)		/* Quantization field mask. */
 #define	NSEGS		(8)		/* Number of u-law segments. */
 #define	SEG_SHIFT	(4)		/* Left shift for segment number. */
 #define	SEG_MASK	(0x70)		/* Segment field mask. */

 static inline int val_seg(int val)
 {
 	int r = 0;
 	val >>= 7;
 	if (val & 0xf0) {
 		val >>= 4;
 		r += 4;
 	}
 	if (val & 0x0c) {
 		val >>= 2;
 		r += 2;
 	}
 	if (val & 0x02)
 		r += 1;
 	return r;
 }

 #define	BIAS		(0x84)		/* Bias for linear code. */

 /*
  * linear2ulaw() - Convert a linear PCM value to u-law
  *
  * In order to simplify the encoding process, the original linear magnitude
  * is biased by adding 33 which shifts the encoding range from (0 - 8158) to
  * (33 - 8191). The result can be seen in the following encoding table:
  *
  *	Biased Linear Input Code	Compressed Code
  *	------------------------	---------------
  *	00000001wxyza			000wxyz
  *	0000001wxyzab			001wxyz
  *	000001wxyzabc			010wxyz
  *	00001wxyzabcd			011wxyz
  *	0001wxyzabcde			100wxyz
  *	001wxyzabcdef			101wxyz
  *	01wxyzabcdefg			110wxyz
  *	1wxyzabcdefgh			111wxyz
  *
  * Each biased linear code has a leading 1 which identifies the segment
  * number. The value of the segment number is equal to 7 minus the number
  * of leading 0's. The quantization interval is directly available as the
  * four bits wxyz.  * The trailing bits (a - h) are ignored.
  *
  * Ordinarily the complement of the resulting code word is used for
  * transmission, and so the code word is complemented before it is returned.
  *
  * For further information see John C. Bellamy's Digital Telephony, 1982,
  * John Wiley & Sons, pps 98-111 and 472-476.
  */
 static unsigned char linear2ulaw(int pcm_val)	/* 2's complement (16-bit range) */
 {
 	int mask;
 	int seg;
 	unsigned char uval;

 	/* Get the sign and the magnitude of the value. */
 	if (pcm_val < 0) {
 		pcm_val = BIAS - pcm_val;
 		mask = 0x7F;
 	} else {
 		pcm_val += BIAS;
 		mask = 0xFF;
 	}
 	if (pcm_val > 0x7FFF)
 		pcm_val = 0x7FFF;

 	/* Convert the scaled magnitude to segment number. */
 	seg = val_seg(pcm_val);

 	/*
 	 * Combine the sign, segment, quantization bits;
 	 * and complement the code word.
 	 */
 	uval = (seg << 4) | ((pcm_val >> (seg + 3)) & 0xF);
 	return uval ^ mask;
 }

 /*
  * ulaw2linear() - Convert a u-law value to 16-bit linear PCM
  *
  * First, a biased linear code is derived from the code word. An unbiased
  * output can then be obtained by subtracting 33 from the biased code.
  *
  * Note that this function expects to be passed the complement of the
  * original code word. This is in keeping with ISDN conventions.
  */
 static int ulaw2linear(unsigned char u_val)
 {
 	int t;

 	/* Complement to obtain normal u-law value. */
 	u_val = ~u_val;

 	/*
 	 * Extract and bias the quantization bits. Then
 	 * shift up by the segment number and subtract out the bias.
 	 */
 	t = ((u_val & QUANT_MASK) << 3) + BIAS;
 	t <<= ((unsigned)u_val & SEG_MASK) >> SEG_SHIFT;

 	return ((u_val & SIGN_BIT) ? (BIAS - t) : (t - BIAS));
 }

 /*
  *  Basic Mu-Law plugin
  */

 typedef void (*mulaw_f)(struct snd_pcm_plugin *plugin,
 			const struct snd_pcm_plugin_channel *src_channels,
 			struct snd_pcm_plugin_channel *dst_channels,
 			snd_pcm_uframes_t frames);

 struct mulaw_priv {
 	mulaw_f func;
 	int cvt_endian;			/* need endian conversion? */
 	unsigned int native_ofs;	/* byte offset in native format */
 	unsigned int copy_ofs;		/* byte offset in s16 format */
 	unsigned int native_bytes;	/* byte size of the native format */
 	unsigned int copy_bytes;	/* bytes to copy per conversion */
 	u16 flip; /* MSB flip for signedness, done after endian conversion */
 };

 static inline void cvt_s16_to_native(struct mulaw_priv *data,
 				     unsigned char *dst, u16 sample)
 {
 	sample ^= data->flip;
 	if (data->cvt_endian)
 		sample = swab16(sample);
 	if (data->native_bytes > data->copy_bytes)
 		memset(dst, 0, data->native_bytes);
 	memcpy(dst + data->native_ofs, (char *)&sample + data->copy_ofs,
 	       data->copy_bytes);
 }

 static void mulaw_decode(struct snd_pcm_plugin *plugin,
 			const struct snd_pcm_plugin_channel *src_channels,
 			struct snd_pcm_plugin_channel *dst_channels,
 			snd_pcm_uframes_t frames)
 {
 	struct mulaw_priv *data = (struct mulaw_priv *)plugin->extra_data;
 	int channel;
 	int nchannels = plugin->src_format.channels;
 	for (channel = 0; channel < nchannels; ++channel) {
 		char *src;
 		char *dst;
 		int src_step, dst_step;
 		snd_pcm_uframes_t frames1;
 		if (!src_channels[channel].enabled) {
 			if (dst_channels[channel].wanted)
 				snd_pcm_area_silence(&dst_channels[channel].area, 0, frames, plugin->dst_format.format);
 			dst_channels[channel].enabled = 0;
 			continue;
 		}
 		dst_channels[channel].enabled = 1;
 		src = src_channels[channel].area.addr + src_channels[channel].area.first / 8;
 		dst = dst_channels[channel].area.addr + dst_channels[channel].area.first / 8;
 		src_step = src_channels[channel].area.step / 8;
 		dst_step = dst_channels[channel].area.step / 8;
 		frames1 = frames;
 		while (frames1-- > 0) {
 			signed short sample = ulaw2linear(*src);
 			cvt_s16_to_native(data, dst, sample);
 			src += src_step;
 			dst += dst_step;
 		}
 	}
 }

 static inline signed short cvt_native_to_s16(struct mulaw_priv *data,
 					     unsigned char *src)
 {
 	u16 sample = 0;
 	memcpy((char *)&sample + data->copy_ofs, src + data->native_ofs,
 	       data->copy_bytes);
 	if (data->cvt_endian)
 		sample = swab16(sample);
 	sample ^= data->flip;
 	return (signed short)sample;
 }

 static void mulaw_encode(struct snd_pcm_plugin *plugin,
 			const struct snd_pcm_plugin_channel *src_channels,
 			struct snd_pcm_plugin_channel *dst_channels,
 			snd_pcm_uframes_t frames)
 {
 	struct mulaw_priv *data = (struct mulaw_priv *)plugin->extra_data;
 	int channel;
 	int nchannels = plugin->src_format.channels;
 	for (channel = 0; channel < nchannels; ++channel) {
 		char *src;
 		char *dst;
 		int src_step, dst_step;
 		snd_pcm_uframes_t frames1;
 		if (!src_channels[channel].enabled) {
 			if (dst_channels[channel].wanted)
 				snd_pcm_area_silence(&dst_channels[channel].area, 0, frames, plugin->dst_format.format);
 			dst_channels[channel].enabled = 0;
 			continue;
 		}
 		dst_channels[channel].enabled = 1;
 		src = src_channels[channel].area.addr + src_channels[channel].area.first / 8;
 		dst = dst_channels[channel].area.addr + dst_channels[channel].area.first / 8;
 		src_step = src_channels[channel].area.step / 8;
 		dst_step = dst_channels[channel].area.step / 8;
 		frames1 = frames;
 		while (frames1-- > 0) {
 			signed short sample = cvt_native_to_s16(data, src);
 			*dst = linear2ulaw(sample);
 			src += src_step;
 			dst += dst_step;
 		}
 	}
 }

 static snd_pcm_sframes_t mulaw_transfer(struct snd_pcm_plugin *plugin,
 			      const struct snd_pcm_plugin_channel *src_channels,
 			      struct snd_pcm_plugin_channel *dst_channels,
 			      snd_pcm_uframes_t frames)
 {
 	struct mulaw_priv *data;

 	if (snd_BUG_ON(!plugin || !src_channels || !dst_channels))
 		return -ENXIO;
 	if (frames == 0)
 		return 0;
 #ifdef CONFIG_SND_DEBUG
 	{
 		unsigned int channel;
 		for (channel = 0; channel < plugin->src_format.channels; channel++) {
 			if (snd_BUG_ON(src_channels[channel].area.first % 8 ||
 				       src_channels[channel].area.step % 8))
 				return -ENXIO;
 			if (snd_BUG_ON(dst_channels[channel].area.first % 8 ||
 				       dst_channels[channel].area.step % 8))
 				return -ENXIO;
 		}
 	}
 #endif
 	data = (struct mulaw_priv *)plugin->extra_data;
 	data->func(plugin, src_channels, dst_channels, frames);
 	return frames;
 }

 static void init_data(struct mulaw_priv *data, snd_pcm_format_t format)
 {
 #ifdef SNDRV_LITTLE_ENDIAN
 	data->cvt_endian = snd_pcm_format_big_endian(format) > 0;
 #else
 	data->cvt_endian = snd_pcm_format_little_endian(format) > 0;
 #endif
 	if (!snd_pcm_format_signed(format))
 		data->flip = 0x8000;
 	data->native_bytes = snd_pcm_format_physical_width(format) / 8;
 	data->copy_bytes = data->native_bytes < 2 ? 1 : 2;
 	if (snd_pcm_format_little_endian(format)) {
 		data->native_ofs = data->native_bytes - data->copy_bytes;
 		data->copy_ofs = 2 - data->copy_bytes;
 	} else {
 		/* S24 in 4bytes need an 1 byte offset */
 		data->native_ofs = data->native_bytes -
 			snd_pcm_format_width(format) / 8;
 	}
 }

 int snd_pcm_plugin_build_mulaw(struct snd_pcm_substream *plug,
 			       struct snd_pcm_plugin_format *src_format,
 			       struct snd_pcm_plugin_format *dst_format,
 			       struct snd_pcm_plugin **r_plugin)
 {
 	int err;
 	struct mulaw_priv *data;
 	struct snd_pcm_plugin *plugin;
 	struct snd_pcm_plugin_format *format;
 	mulaw_f func;

 	if (snd_BUG_ON(!r_plugin))
 		return -ENXIO;
 	*r_plugin = NULL;

 	if (snd_BUG_ON(src_format->rate != dst_format->rate))
 		return -ENXIO;
 	if (snd_BUG_ON(src_format->channels != dst_format->channels))
 		return -ENXIO;

 	if (dst_format->format == SNDRV_PCM_FORMAT_MU_LAW) {
 		format = src_format;
 		func = mulaw_encode;
 	}
 	else if (src_format->format == SNDRV_PCM_FORMAT_MU_LAW) {
 		format = dst_format;
 		func = mulaw_decode;
 	}
 	else {
 		snd_BUG();
 		return -EINVAL;
 	}
 	if (snd_BUG_ON(!snd_pcm_format_linear(format->format)))
 		return -ENXIO;

 	err = snd_pcm_plugin_build(plug, "Mu-Law<->linear conversion",
 				   src_format, dst_format,
 				   sizeof(struct mulaw_priv), &plugin);
 	if (err < 0)
 		return err;
 	data = (struct mulaw_priv *)plugin->extra_data;
 	data->func = func;
 	init_data(data, format->format);
 	plugin->transfer = mulaw_transfer;
 	*r_plugin = plugin;
 	return 0;
 }
	/*
	* Mu-Law conversion Plug-In Interface
	* Copyright (c) 1999 by Jaroslav Kysela <perex@perex.cz>
	* Uros Bizjak <uros@kss-loka.si>
	*
	* Based on reference implementation by Sun Microsystems, Inc.
	*
	* This library is free software; you can redistribute it and/or modify
	* it under the terms of the GNU Library General Public License as
	* published by the Free Software Foundation; either version 2 of
	* the License, or (at your option) any later version.
	*
	* 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 Library General Public License for more details.
	*
	* You should have received a copy of the GNU Library General Public
	* License along with this library; if not, write to the Free Software
	* Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
	*
	*/

	#include <linux/time.h>
	#include <sound/core.h>
	#include <sound/pcm.h>
	#include "pcm_plugin.h"

	#define SIGN_BIT (0x80) /* Sign bit for a u-law byte. */
	#define QUANT_MASK (0xf) /* Quantization field mask. */
	#define NSEGS (8) /* Number of u-law segments. */
	#define SEG_SHIFT (4) /* Left shift for segment number. */
	#define SEG_MASK (0x70) /* Segment field mask. */

	static inline int val_seg(int val)
	{
	int r = 0;
	val >>= 7;
	if (val & 0xf0) {
	val >>= 4;
	r += 4;
	}
	if (val & 0x0c) {
	val >>= 2;
	r += 2;
	}
	if (val & 0x02)
	r += 1;
	return r;
	}

	#define BIAS (0x84) /* Bias for linear code. */

	/*
	* linear2ulaw() - Convert a linear PCM value to u-law
	*
	* In order to simplify the encoding process, the original linear magnitude
	* is biased by adding 33 which shifts the encoding range from (0 - 8158) to
	* (33 - 8191). The result can be seen in the following encoding table:
	*
	* Biased Linear Input Code Compressed Code
	* ------------------------ ---------------
	* 00000001wxyza 000wxyz
	* 0000001wxyzab 001wxyz
	* 000001wxyzabc 010wxyz
	* 00001wxyzabcd 011wxyz
	* 0001wxyzabcde 100wxyz
	* 001wxyzabcdef 101wxyz
	* 01wxyzabcdefg 110wxyz
	* 1wxyzabcdefgh 111wxyz
	*
	* Each biased linear code has a leading 1 which identifies the segment
	* number. The value of the segment number is equal to 7 minus the number
	* of leading 0's. The quantization interval is directly available as the
	* four bits wxyz. * The trailing bits (a - h) are ignored.
	*
	* Ordinarily the complement of the resulting code word is used for
	* transmission, and so the code word is complemented before it is returned.
	*
	* For further information see John C. Bellamy's Digital Telephony, 1982,
	* John Wiley & Sons, pps 98-111 and 472-476.
	*/
	static unsigned char linear2ulaw(int pcm_val) /* 2's complement (16-bit range) */
	{
	int mask;
	int seg;
	unsigned char uval;

	/* Get the sign and the magnitude of the value. */
	if (pcm_val < 0) {
	pcm_val = BIAS - pcm_val;
	mask = 0x7F;
	} else {
	pcm_val += BIAS;
	mask = 0xFF;
	}
	if (pcm_val > 0x7FFF)
	pcm_val = 0x7FFF;

	/* Convert the scaled magnitude to segment number. */
	seg = val_seg(pcm_val);

	/*
	* Combine the sign, segment, quantization bits;
	* and complement the code word.
	*/
	uval = (seg << 4) \| ((pcm_val >> (seg + 3)) & 0xF);
	return uval ^ mask;
	}

	/*
	* ulaw2linear() - Convert a u-law value to 16-bit linear PCM
	*
	* First, a biased linear code is derived from the code word. An unbiased
	* output can then be obtained by subtracting 33 from the biased code.
	*
	* Note that this function expects to be passed the complement of the
	* original code word. This is in keeping with ISDN conventions.
	*/
	static int ulaw2linear(unsigned char u_val)
	{
	int t;

	/* Complement to obtain normal u-law value. */
	u_val = ~u_val;

	/*
	* Extract and bias the quantization bits. Then
	* shift up by the segment number and subtract out the bias.
	*/
	t = ((u_val & QUANT_MASK) << 3) + BIAS;
	t <<= ((unsigned)u_val & SEG_MASK) >> SEG_SHIFT;

	return ((u_val & SIGN_BIT) ? (BIAS - t) : (t - BIAS));
	}

	/*
	* Basic Mu-Law plugin
	*/

	typedef void (mulaw_f)(struct snd_pcm_plugin plugin,
	const struct snd_pcm_plugin_channel *src_channels,
	struct snd_pcm_plugin_channel *dst_channels,
	snd_pcm_uframes_t frames);

	struct mulaw_priv {
	mulaw_f func;
	int cvt_endian; /* need endian conversion? */
	unsigned int native_ofs; /* byte offset in native format */
	unsigned int copy_ofs; /* byte offset in s16 format */
	unsigned int native_bytes; /* byte size of the native format */
	unsigned int copy_bytes; /* bytes to copy per conversion */
	u16 flip; /* MSB flip for signedness, done after endian conversion */
	};

	static inline void cvt_s16_to_native(struct mulaw_priv *data,
	unsigned char *dst, u16 sample)
	{
	sample ^= data->flip;
	if (data->cvt_endian)
	sample = swab16(sample);
	if (data->native_bytes > data->copy_bytes)
	memset(dst, 0, data->native_bytes);
	memcpy(dst + data->native_ofs, (char *)&sample + data->copy_ofs,
	data->copy_bytes);
	}

	static void mulaw_decode(struct snd_pcm_plugin *plugin,
	const struct snd_pcm_plugin_channel *src_channels,
	struct snd_pcm_plugin_channel *dst_channels,
	snd_pcm_uframes_t frames)
	{
	struct mulaw_priv data = (struct mulaw_priv )plugin->extra_data;
	int channel;
	int nchannels = plugin->src_format.channels;
	for (channel = 0; channel < nchannels; ++channel) {
	char *src;
	char *dst;
	int src_step, dst_step;
	snd_pcm_uframes_t frames1;
	if (!src_channels[channel].enabled) {
	if (dst_channels[channel].wanted)
	snd_pcm_area_silence(&dst_channels[channel].area, 0, frames, plugin->dst_format.format);
	dst_channels[channel].enabled = 0;
	continue;
	}
	dst_channels[channel].enabled = 1;
	src = src_channels[channel].area.addr + src_channels[channel].area.first / 8;
	dst = dst_channels[channel].area.addr + dst_channels[channel].area.first / 8;
	src_step = src_channels[channel].area.step / 8;
	dst_step = dst_channels[channel].area.step / 8;
	frames1 = frames;
	while (frames1-- > 0) {
	signed short sample = ulaw2linear(*src);
	cvt_s16_to_native(data, dst, sample);
	src += src_step;
	dst += dst_step;
	}
	}
	}

	static inline signed short cvt_native_to_s16(struct mulaw_priv *data,
	unsigned char *src)
	{
	u16 sample = 0;
	memcpy((char *)&sample + data->copy_ofs, src + data->native_ofs,
	data->copy_bytes);
	if (data->cvt_endian)
	sample = swab16(sample);
	sample ^= data->flip;
	return (signed short)sample;
	}

	static void mulaw_encode(struct snd_pcm_plugin *plugin,
	const struct snd_pcm_plugin_channel *src_channels,
	struct snd_pcm_plugin_channel *dst_channels,
	snd_pcm_uframes_t frames)
	{
	struct mulaw_priv data = (struct mulaw_priv )plugin->extra_data;
	int channel;
	int nchannels = plugin->src_format.channels;
	for (channel = 0; channel < nchannels; ++channel) {
	char *src;
	char *dst;
	int src_step, dst_step;
	snd_pcm_uframes_t frames1;
	if (!src_channels[channel].enabled) {
	if (dst_channels[channel].wanted)
	snd_pcm_area_silence(&dst_channels[channel].area, 0, frames, plugin->dst_format.format);
	dst_channels[channel].enabled = 0;
	continue;
	}
	dst_channels[channel].enabled = 1;
	src = src_channels[channel].area.addr + src_channels[channel].area.first / 8;
	dst = dst_channels[channel].area.addr + dst_channels[channel].area.first / 8;
	src_step = src_channels[channel].area.step / 8;
	dst_step = dst_channels[channel].area.step / 8;
	frames1 = frames;
	while (frames1-- > 0) {
	signed short sample = cvt_native_to_s16(data, src);
	*dst = linear2ulaw(sample);
	src += src_step;
	dst += dst_step;
	}
	}
	}

	static snd_pcm_sframes_t mulaw_transfer(struct snd_pcm_plugin *plugin,
	const struct snd_pcm_plugin_channel *src_channels,
	struct snd_pcm_plugin_channel *dst_channels,
	snd_pcm_uframes_t frames)
	{
	struct mulaw_priv *data;

	if (snd_BUG_ON(!plugin \|\| !src_channels \|\| !dst_channels))
	return -ENXIO;
	if (frames == 0)
	return 0;
	#ifdef CONFIG_SND_DEBUG
	{
	unsigned int channel;
	for (channel = 0; channel < plugin->src_format.channels; channel++) {
	if (snd_BUG_ON(src_channels[channel].area.first % 8 \|\|
	src_channels[channel].area.step % 8))
	return -ENXIO;
	if (snd_BUG_ON(dst_channels[channel].area.first % 8 \|\|
	dst_channels[channel].area.step % 8))
	return -ENXIO;
	}
	}
	#endif
	data = (struct mulaw_priv *)plugin->extra_data;
	data->func(plugin, src_channels, dst_channels, frames);
	return frames;
	}

	static void init_data(struct mulaw_priv *data, snd_pcm_format_t format)
	{
	#ifdef SNDRV_LITTLE_ENDIAN
	data->cvt_endian = snd_pcm_format_big_endian(format) > 0;
	#else
	data->cvt_endian = snd_pcm_format_little_endian(format) > 0;
	#endif
	if (!snd_pcm_format_signed(format))
	data->flip = 0x8000;
	data->native_bytes = snd_pcm_format_physical_width(format) / 8;
	data->copy_bytes = data->native_bytes < 2 ? 1 : 2;
	if (snd_pcm_format_little_endian(format)) {
	data->native_ofs = data->native_bytes - data->copy_bytes;
	data->copy_ofs = 2 - data->copy_bytes;
	} else {
	/* S24 in 4bytes need an 1 byte offset */
	data->native_ofs = data->native_bytes -
	snd_pcm_format_width(format) / 8;
	}
	}

	int snd_pcm_plugin_build_mulaw(struct snd_pcm_substream *plug,
	struct snd_pcm_plugin_format *src_format,
	struct snd_pcm_plugin_format *dst_format,
	struct snd_pcm_plugin **r_plugin)
	{
	int err;
	struct mulaw_priv *data;
	struct snd_pcm_plugin *plugin;
	struct snd_pcm_plugin_format *format;
	mulaw_f func;

	if (snd_BUG_ON(!r_plugin))
	return -ENXIO;
	*r_plugin = NULL;

	if (snd_BUG_ON(src_format->rate != dst_format->rate))
	return -ENXIO;
	if (snd_BUG_ON(src_format->channels != dst_format->channels))
	return -ENXIO;

	if (dst_format->format == SNDRV_PCM_FORMAT_MU_LAW) {
	format = src_format;
	func = mulaw_encode;
	}
	else if (src_format->format == SNDRV_PCM_FORMAT_MU_LAW) {
	format = dst_format;
	func = mulaw_decode;
	}
	else {
	snd_BUG();
	return -EINVAL;
	}
	if (snd_BUG_ON(!snd_pcm_format_linear(format->format)))
	return -ENXIO;

	err = snd_pcm_plugin_build(plug, "Mu-Law<->linear conversion",
	src_format, dst_format,
	sizeof(struct mulaw_priv), &plugin);
	if (err < 0)
	return err;
	data = (struct mulaw_priv *)plugin->extra_data;
	data->func = func;
	init_data(data, format->format);
	plugin->transfer = mulaw_transfer;
	*r_plugin = plugin;
	return 0;
	}