ALSA: introduce the snd-aloop module for the PCM loopback

The snd-aloop module allows redirecting of the PCM playback in the
kernel back to the user space using the standard ALSA PCM capture API.

The module also allows time synchronization with another timing source
and notifications of playback stream parameter changes.

Signed-off-by: Jaroslav Kysela <perex@perex.cz>

3 files changed, 1076 insertions, 0 deletions

diff --git a/sound/drivers/Kconfig b/sound/drivers/Kconfig
index 84714a65e5c..b6ae7628525 100644
--- a/sound/drivers/Kconfig
+++ b/sound/drivers/Kconfig
@@ -74,6 +74,25 @@ config SND_DUMMY
 	  To compile this driver as a module, choose M here: the module
 	  will be called snd-dummy.
 
+config SND_ALOOP
+        tristate "Generic loopback driver (PCM)"
+        select SND_PCM
+        help
+          Say 'Y' or 'M' to include support for the PCM loopback device.
+	  This module returns played samples back to the user space using
+	  the standard ALSA PCM device. The devices are routed 0->1 and
+          1->0, where first number is the playback PCM device and second
+	  number is the capture device. Module creates two PCM devices and
+	  configured number of substreams (see the pcm_substreams module
+          parameter).
+
+	  The looback device allow time sychronization with an external
+	  timing source using the time shift universal control (+-20%
+	  of system time).
+
+	  To compile this driver as a module, choose M here: the module
+	  will be called snd-aloop.
+
 config SND_VIRMIDI
 	tristate "Virtual MIDI soundcard"
 	depends on SND_SEQUENCER
diff --git a/sound/drivers/Makefile b/sound/drivers/Makefile
index d4a07f9ff2c..1a8440c8b13 100644
--- a/sound/drivers/Makefile
+++ b/sound/drivers/Makefile
@@ -4,6 +4,7 @@
 #
 
 snd-dummy-objs := dummy.o
+snd-aloop-objs := aloop.o
 snd-mtpav-objs := mtpav.o
 snd-mts64-objs := mts64.o
 snd-portman2x4-objs := portman2x4.o
@@ -13,6 +14,7 @@ snd-ml403-ac97cr-objs := ml403-ac97cr.o pcm-indirect2.o
 
 # Toplevel Module Dependency
 obj-$(CONFIG_SND_DUMMY) += snd-dummy.o
+obj-$(CONFIG_SND_ALOOP) += snd-aloop.o
 obj-$(CONFIG_SND_VIRMIDI) += snd-virmidi.o
 obj-$(CONFIG_SND_SERIAL_U16550) += snd-serial-u16550.o
 obj-$(CONFIG_SND_MTPAV) += snd-mtpav.o
diff --git a/sound/drivers/aloop.c b/sound/drivers/aloop.c
new file mode 100644
index 00000000000..3123a15d23f
--- /dev/null
+++ b/sound/drivers/aloop.c
@@ -0,0 +1,1055 @@
+/*
+ *  Loopback soundcard
+ *
+ *  Original code:
+ *  Copyright (c) by Jaroslav Kysela <perex@perex.cz>
+ *
+ *  More accurate positioning and full-duplex support:
+ *  Copyright (c) Ahmet İnan <ainan at mathematik.uni-freiburg.de>
+ *
+ *  Major (almost complete) rewrite:
+ *  Copyright (c) by Takashi Iwai <tiwai@suse.de>
+ *
+ *  A next major update in 2010 (separate timers for playback and capture):
+ *  Copyright (c) Jaroslav Kysela <perex@perex.cz>
+ *
+ *   This program is free software; you can redistribute it and/or modify
+ *   it under the terms of the GNU 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 General Public License for more details.
+ *
+ *   You should have received a copy of the GNU General Public License
+ *   along with this program; if not, write to the Free Software
+ *   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307 USA
+ *
+ */
+
+#include <linux/init.h>
+#include <linux/jiffies.h>
+#include <linux/slab.h>
+#include <linux/time.h>
+#include <linux/wait.h>
+#include <linux/moduleparam.h>
+#include <linux/platform_device.h>
+#include <sound/core.h>
+#include <sound/control.h>
+#include <sound/pcm.h>
+#include <sound/initval.h>
+
+MODULE_AUTHOR("Jaroslav Kysela <perex@perex.cz>");
+MODULE_DESCRIPTION("A loopback soundcard");
+MODULE_LICENSE("GPL");
+MODULE_SUPPORTED_DEVICE("{{ALSA,Loopback soundcard}}");
+
+#define MAX_PCM_SUBSTREAMS	8
+
+static int index[SNDRV_CARDS] = SNDRV_DEFAULT_IDX;	/* Index 0-MAX */
+static char *id[SNDRV_CARDS] = SNDRV_DEFAULT_STR;	/* ID for this card */
+static int enable[SNDRV_CARDS] = {1, [1 ... (SNDRV_CARDS - 1)] = 0};
+static int pcm_substreams[SNDRV_CARDS] = {[0 ... (SNDRV_CARDS - 1)] = 8};
+static int pcm_notify[SNDRV_CARDS];
+
+module_param_array(index, int, NULL, 0444);
+MODULE_PARM_DESC(index, "Index value for loopback soundcard.");
+module_param_array(id, charp, NULL, 0444);
+MODULE_PARM_DESC(id, "ID string for loopback soundcard.");
+module_param_array(enable, bool, NULL, 0444);
+MODULE_PARM_DESC(enable, "Enable this loopback soundcard.");
+module_param_array(pcm_substreams, int, NULL, 0444);
+MODULE_PARM_DESC(pcm_substreams, "PCM substreams # (1-8) for loopback driver.");
+module_param_array(pcm_notify, int, NULL, 0444);
+MODULE_PARM_DESC(pcm_notify, "Break capture when PCM format/rate/channels changes.");
+
+#define NO_PITCH 100000
+
+struct loopback_pcm;
+
+struct loopback_cable {
+	spinlock_t lock;
+	struct loopback_pcm *streams[2];
+	struct snd_pcm_hardware hw;
+	/* flags */
+	unsigned int valid;
+	unsigned int running;
+};
+
+struct loopback_setup {
+	unsigned int notify: 1;
+	unsigned int rate_shift;
+	unsigned int format;
+	unsigned int rate;
+	unsigned int channels;
+	struct snd_ctl_elem_id active_id;
+	struct snd_ctl_elem_id format_id;
+	struct snd_ctl_elem_id rate_id;
+	struct snd_ctl_elem_id channels_id;
+};
+
+struct loopback {
+	struct snd_card *card;
+	struct mutex cable_lock;
+	struct loopback_cable *cables[MAX_PCM_SUBSTREAMS][2];
+	struct snd_pcm *pcm[2];
+	struct loopback_setup setup[MAX_PCM_SUBSTREAMS][2];
+};
+
+struct loopback_pcm {
+	struct loopback *loopback;
+	struct snd_pcm_substream *substream;
+	struct loopback_cable *cable;
+	unsigned int pcm_buffer_size;
+	unsigned int buf_pos;	/* position in buffer */
+	unsigned int silent_size;
+	/* PCM parameters */
+	unsigned int pcm_period_size;
+	unsigned int pcm_bps;		/* bytes per second */
+	unsigned int pcm_salign;	/* bytes per sample * channels */
+	unsigned int pcm_rate_shift;	/* rate shift value */
+	/* flags */
+	unsigned int period_update_pending :1;
+	/* timer stuff */
+	unsigned int irq_pos;		/* fractional IRQ position */
+	unsigned int period_size_frac;
+	unsigned long last_jiffies;
+	struct timer_list timer;
+};
+
+static struct platform_device *devices[SNDRV_CARDS];
+
+static inline unsigned int byte_pos(struct loopback_pcm *dpcm, unsigned int x)
+{
+	if (dpcm->pcm_rate_shift == NO_PITCH) {
+		x /= HZ;
+	} else {
+		x = div_u64(NO_PITCH * (unsigned long long)x,
+			    HZ * (unsigned long long)dpcm->pcm_rate_shift);
+	}
+	return x - (x % dpcm->pcm_salign);
+}
+
+static inline unsigned int frac_pos(struct loopback_pcm *dpcm, unsigned int x)
+{
+	if (dpcm->pcm_rate_shift == NO_PITCH) {	/* no pitch */
+		return x * HZ;
+	} else {
+		x = div_u64(dpcm->pcm_rate_shift * (unsigned long long)x * HZ,
+			    NO_PITCH);
+	}
+	return x;
+}
+
+static inline struct loopback_setup *get_setup(struct loopback_pcm *dpcm)
+{
+	int device = dpcm->substream->pstr->pcm->device;
+	
+	if (dpcm->substream->stream == SNDRV_PCM_STREAM_PLAYBACK)
+		device ^= 1;
+	return &dpcm->loopback->setup[dpcm->substream->number][device];
+}
+
+static inline unsigned int get_notify(struct loopback_pcm *dpcm)
+{
+	return get_setup(dpcm)->notify;
+}
+
+static inline unsigned int get_rate_shift(struct loopback_pcm *dpcm)
+{
+	return get_setup(dpcm)->rate_shift;
+}
+
+static void loopback_timer_start(struct loopback_pcm *dpcm)
+{
+	unsigned long tick;
+	unsigned int rate_shift = get_rate_shift(dpcm);
+
+	if (rate_shift != dpcm->pcm_rate_shift) {
+		dpcm->pcm_rate_shift = rate_shift;
+		dpcm->period_size_frac = frac_pos(dpcm, dpcm->pcm_period_size);
+	}
+	tick = dpcm->period_size_frac - dpcm->irq_pos;
+	tick = (tick + dpcm->pcm_bps - 1) / dpcm->pcm_bps;
+	dpcm->timer.expires = jiffies + tick;
+	add_timer(&dpcm->timer);
+}
+
+static inline void loopback_timer_stop(struct loopback_pcm *dpcm)
+{
+	del_timer(&dpcm->timer);
+}
+
+#define CABLE_VALID_PLAYBACK	(1 << SNDRV_PCM_STREAM_PLAYBACK)
+#define CABLE_VALID_CAPTURE	(1 << SNDRV_PCM_STREAM_CAPTURE)
+#define CABLE_VALID_BOTH	(CABLE_VALID_PLAYBACK|CABLE_VALID_CAPTURE)
+
+static int loopback_check_format(struct loopback_cable *cable, int stream)
+{
+	struct snd_pcm_runtime *runtime;
+	struct loopback_setup *setup;
+	struct snd_card *card;
+	int check;
+
+	if (cable->valid != CABLE_VALID_BOTH) {
+		if (stream == SNDRV_PCM_STREAM_PLAYBACK)
+			goto __notify;
+		return 0;
+	}
+	runtime = cable->streams[SNDRV_PCM_STREAM_PLAYBACK]->
+							substream->runtime;
+	check = cable->hw.formats != (1ULL << runtime->format) ||
+		cable->hw.rate_min != runtime->rate ||
+		cable->hw.rate_max != runtime->rate ||
+		cable->hw.channels_min != runtime->channels ||
+		cable->hw.channels_max != runtime->channels;
+	if (!check)
+		return 0;
+	if (stream == SNDRV_PCM_STREAM_CAPTURE) {
+		return -EIO;
+	} else {
+		snd_pcm_stop(cable->streams[SNDRV_PCM_STREAM_CAPTURE]->
+					substream, SNDRV_PCM_STATE_DRAINING);
+	      __notify:
+		runtime = cable->streams[SNDRV_PCM_STREAM_PLAYBACK]->
+							substream->runtime;
+		setup = get_setup(cable->streams[SNDRV_PCM_STREAM_PLAYBACK]);
+		card = cable->streams[SNDRV_PCM_STREAM_PLAYBACK]->loopback->card;
+		if (setup->format != runtime->format) {
+			snd_ctl_notify(card, SNDRV_CTL_EVENT_MASK_VALUE,
+							&setup->format_id);
+			setup->format = runtime->format;
+		}
+		if (setup->rate != runtime->rate) {
+			snd_ctl_notify(card, SNDRV_CTL_EVENT_MASK_VALUE,
+							&setup->rate_id);
+			setup->rate = runtime->rate;
+		}
+		if (setup->channels != runtime->channels) {
+			snd_ctl_notify(card, SNDRV_CTL_EVENT_MASK_VALUE,
+							&setup->channels_id);
+			setup->channels = runtime->channels;
+		}
+	}
+	return 0;
+}
+
+static void loopback_active_notify(struct loopback_pcm *dpcm)
+{
+	snd_ctl_notify(dpcm->loopback->card,
+		       SNDRV_CTL_EVENT_MASK_VALUE,
+		       &get_setup(dpcm)->active_id);
+}
+
+static int loopback_trigger(struct snd_pcm_substream *substream, int cmd)
+{
+	struct snd_pcm_runtime *runtime = substream->runtime;
+	struct loopback_pcm *dpcm = runtime->private_data;
+	struct loopback_cable *cable = dpcm->cable;
+	int err;
+
+	switch (cmd) {
+	case SNDRV_PCM_TRIGGER_START:
+		err = loopback_check_format(cable, substream->stream);
+		if (err < 0)
+			return err;
+		dpcm->last_jiffies = jiffies;
+		dpcm->pcm_rate_shift = 0;
+		loopback_timer_start(dpcm);
+		cable->running |= (1 << substream->stream);
+		if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK)
+			loopback_active_notify(dpcm);
+		break;
+	case SNDRV_PCM_TRIGGER_STOP:
+		cable->running &= ~(1 << substream->stream);
+		loopback_timer_stop(dpcm);
+		if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK)
+			loopback_active_notify(dpcm);
+		break;
+	default:
+		return -EINVAL;
+	}
+	return 0;
+}
+
+static int loopback_prepare(struct snd_pcm_substream *substream)
+{
+	struct snd_pcm_runtime *runtime = substream->runtime;
+	struct loopback_pcm *dpcm = runtime->private_data;
+	struct loopback_cable *cable = dpcm->cable;
+	unsigned int bps, salign;
+
+	salign = (snd_pcm_format_width(runtime->format) *
+						runtime->channels) / 8;
+	bps = salign * runtime->rate;
+	if (bps <= 0 || salign <= 0)
+		return -EINVAL;
+
+	dpcm->buf_pos = 0;
+	dpcm->pcm_buffer_size = frames_to_bytes(runtime, runtime->buffer_size);
+	if (substream->stream == SNDRV_PCM_STREAM_CAPTURE) {
+		/* clear capture buffer */
+		dpcm->silent_size = dpcm->pcm_buffer_size;
+		snd_pcm_format_set_silence(runtime->format, runtime->dma_area,
+					   runtime->buffer_size * runtime->channels);
+	}
+
+	dpcm->irq_pos = 0;
+	dpcm->period_update_pending = 0;
+	dpcm->pcm_bps = bps;
+	dpcm->pcm_salign = salign;
+	dpcm->pcm_period_size = frames_to_bytes(runtime, runtime->period_size);
+
+	mutex_lock(&dpcm->loopback->cable_lock);
+	if (!(cable->valid & ~(1 << substream->stream))) {
+		cable->hw.formats = (1ULL << runtime->format);
+		cable->hw.rate_min = runtime->rate;
+		cable->hw.rate_max = runtime->rate;
+		cable->hw.channels_min = runtime->channels;
+		cable->hw.channels_max = runtime->channels;
+	}
+	cable->valid |= 1 << substream->stream;
+	mutex_unlock(&dpcm->loopback->cable_lock);
+
+	return 0;
+}
+
+static void clear_capture_buf(struct loopback_pcm *dpcm, unsigned int bytes)
+{
+	struct snd_pcm_runtime *runtime = dpcm->substream->runtime;
+	char *dst = runtime->dma_area;
+	unsigned int dst_off = dpcm->buf_pos;
+
+	if (dpcm->silent_size >= dpcm->pcm_buffer_size)
+		return;
+	if (dpcm->silent_size + bytes > dpcm->pcm_buffer_size)
+		bytes = dpcm->pcm_buffer_size - dpcm->silent_size;
+
+	for (;;) {
+		unsigned int size = bytes;
+		if (dst_off + size > dpcm->pcm_buffer_size)
+			size = dpcm->pcm_buffer_size - dst_off;
+		snd_pcm_format_set_silence(runtime->format, dst + dst_off,
+					   bytes_to_frames(runtime, size) *
+					   	runtime->channels);
+		dpcm->silent_size += size;
+		bytes -= size;
+		if (!bytes)
+			break;
+		dst_off = 0;
+	}
+}
+
+static void copy_play_buf(struct loopback_pcm *play,
+			  struct loopback_pcm *capt,
+			  unsigned int bytes)
+{
+	struct snd_pcm_runtime *runtime = play->substream->runtime;
+	char *src = play->substream->runtime->dma_area;
+	char *dst = capt->substream->runtime->dma_area;
+	unsigned int src_off = play->buf_pos;
+	unsigned int dst_off = capt->buf_pos;
+	unsigned int clear_bytes = 0;
+
+	/* check if playback is draining, trim the capture copy size
+	 * when our pointer is at the end of playback ring buffer */
+	if (runtime->status->state == SNDRV_PCM_STATE_DRAINING &&
+	    snd_pcm_playback_hw_avail(runtime) < runtime->buffer_size) { 
+	    	snd_pcm_uframes_t appl_ptr, appl_ptr1, diff;
+		appl_ptr = appl_ptr1 = runtime->control->appl_ptr;
+		appl_ptr1 -= appl_ptr1 % runtime->buffer_size;
+		appl_ptr1 += play->buf_pos / play->pcm_salign;
+		if (appl_ptr < appl_ptr1)
+			appl_ptr1 -= runtime->buffer_size;
+		diff = (appl_ptr - appl_ptr1) * play->pcm_salign;
+		if (diff < bytes) {
+			clear_bytes = bytes - diff;
+			bytes = diff;
+		}
+	}
+
+	for (;;) {
+		unsigned int size = bytes;
+		if (src_off + size > play->pcm_buffer_size)
+			size = play->pcm_buffer_size - src_off;
+		if (dst_off + size > capt->pcm_buffer_size)
+			size = capt->pcm_buffer_size - dst_off;
+		memcpy(dst + dst_off, src + src_off, size);
+		capt->silent_size = 0;
+		bytes -= size;
+		if (!bytes)
+			break;
+		src_off = (src_off + size) % play->pcm_buffer_size;
+		dst_off = (dst_off + size) % capt->pcm_buffer_size;
+	}
+
+	if (clear_bytes > 0)
+		clear_capture_buf(capt, clear_bytes);
+}
+
+#define BYTEPOS_UPDATE_POSONLY	0
+#define BYTEPOS_UPDATE_CLEAR	1
+#define BYTEPOS_UPDATE_COPY	2
+
+static void loopback_bytepos_update(struct loopback_pcm *dpcm,
+				    unsigned int delta,
+				    unsigned int cmd)
+{
+	unsigned int count;
+	unsigned long last_pos;
+
+	last_pos = byte_pos(dpcm, dpcm->irq_pos);
+	dpcm->irq_pos += delta * dpcm->pcm_bps;
+	count = byte_pos(dpcm, dpcm->irq_pos) - last_pos;
+	if (!count)
+		return;
+	if (cmd == BYTEPOS_UPDATE_CLEAR)
+		clear_capture_buf(dpcm, count);
+	else if (cmd == BYTEPOS_UPDATE_COPY)
+		copy_play_buf(dpcm->cable->streams[SNDRV_PCM_STREAM_PLAYBACK],
+			      dpcm->cable->streams[SNDRV_PCM_STREAM_CAPTURE],
+			      count);
+	dpcm->buf_pos += count;
+	dpcm->buf_pos %= dpcm->pcm_buffer_size;
+	if (dpcm->irq_pos >= dpcm->period_size_frac) {
+		dpcm->irq_pos %= dpcm->period_size_frac;
+		dpcm->period_update_pending = 1;
+	}
+}
+
+static void loopback_pos_update(struct loopback_cable *cable)
+{
+	struct loopback_pcm *dpcm_play =
+			cable->streams[SNDRV_PCM_STREAM_PLAYBACK];
+	struct loopback_pcm *dpcm_capt =
+			cable->streams[SNDRV_PCM_STREAM_CAPTURE];
+	unsigned long delta_play = 0, delta_capt = 0;
+
+	spin_lock(&cable->lock);	
+	if (cable->running & (1 << SNDRV_PCM_STREAM_PLAYBACK)) {
+		delta_play = jiffies - dpcm_play->last_jiffies;
+		dpcm_play->last_jiffies += delta_play;
+	}
+
+	if (cable->running & (1 << SNDRV_PCM_STREAM_CAPTURE)) {
+		delta_capt = jiffies - dpcm_capt->last_jiffies;
+		dpcm_capt->last_jiffies += delta_capt;
+	}
+
+	if (delta_play == 0 && delta_capt == 0) {
+		spin_unlock(&cable->lock);
+		return;
+	}
+		
+	if (delta_play > delta_capt) {
+		loopback_bytepos_update(dpcm_play, delta_play - delta_capt,
+					BYTEPOS_UPDATE_POSONLY);
+		delta_play = delta_capt;
+	} else if (delta_play < delta_capt) {
+		loopback_bytepos_update(dpcm_capt, delta_capt - delta_play,
+					BYTEPOS_UPDATE_CLEAR);
+		delta_capt = delta_play;
+	}
+
+	if (delta_play == 0 && delta_capt == 0) {
+		spin_unlock(&cable->lock);
+		return;
+	}
+	/* note delta_capt == delta_play at this moment */
+	loopback_bytepos_update(dpcm_capt, delta_capt, BYTEPOS_UPDATE_COPY);
+	loopback_bytepos_update(dpcm_play, delta_play, BYTEPOS_UPDATE_POSONLY);
+	spin_unlock(&cable->lock);
+}
+
+static void loopback_timer_function(unsigned long data)
+{
+	struct loopback_pcm *dpcm = (struct loopback_pcm *)data;
+	int stream;
+
+	loopback_pos_update(dpcm->cable);
+	stream = dpcm->substream->stream;
+	if (dpcm->cable->running & (1 << stream))
+		loopback_timer_start(dpcm);
+	if (dpcm->period_update_pending) {
+		dpcm->period_update_pending = 0;
+		if (dpcm->cable->running & (1 << stream))
+			snd_pcm_period_elapsed(dpcm->substream);
+	}
+}
+
+static snd_pcm_uframes_t loopback_pointer(struct snd_pcm_substream *substream)
+{
+	struct snd_pcm_runtime *runtime = substream->runtime;
+	struct loopback_pcm *dpcm = runtime->private_data;
+
+	loopback_pos_update(dpcm->cable);
+	return bytes_to_frames(runtime, dpcm->buf_pos);
+}
+
+static struct snd_pcm_hardware loopback_pcm_hardware =
+{
+	.info =		(SNDRV_PCM_INFO_INTERLEAVED | SNDRV_PCM_INFO_MMAP |
+			 SNDRV_PCM_INFO_MMAP_VALID),
+	.formats =	(SNDRV_PCM_FMTBIT_S16_LE | SNDRV_PCM_FMTBIT_S16_BE |
+			 SNDRV_PCM_FMTBIT_S32_LE | SNDRV_PCM_FMTBIT_S32_BE |
+			 SNDRV_PCM_FMTBIT_FLOAT_LE | SNDRV_PCM_FMTBIT_FLOAT_BE),
+	.rates =	SNDRV_PCM_RATE_CONTINUOUS | SNDRV_PCM_RATE_8000_192000,
+	.rate_min =		8000,
+	.rate_max =		192000,
+	.channels_min =		1,
+	.channels_max =		32,
+	.buffer_bytes_max =	2 * 1024 * 1024,
+	.period_bytes_min =	64,
+	.period_bytes_max =	2 * 1024 * 1024,
+	.periods_min =		1,
+	.periods_max =		1024,
+	.fifo_size =		0,
+};
+
+static void loopback_runtime_free(struct snd_pcm_runtime *runtime)
+{
+	struct loopback_pcm *dpcm = runtime->private_data;
+	kfree(dpcm);
+}
+
+static int loopback_hw_params(struct snd_pcm_substream *substream,
+			      struct snd_pcm_hw_params *params)
+{
+	return snd_pcm_lib_malloc_pages(substream, params_buffer_bytes(params));
+}
+
+static int loopback_hw_free(struct snd_pcm_substream *substream)
+{
+	struct snd_pcm_runtime *runtime = substream->runtime;
+	struct loopback_pcm *dpcm = runtime->private_data;
+	struct loopback_cable *cable = dpcm->cable;
+
+	mutex_lock(&dpcm->loopback->cable_lock);
+	cable->valid &= ~(1 << substream->stream);
+	mutex_unlock(&dpcm->loopback->cable_lock);
+	return snd_pcm_lib_free_pages(substream);
+}
+
+static unsigned int get_cable_index(struct snd_pcm_substream *substream)
+{
+	if (!substream->pcm->device)
+		return substream->stream;
+	else
+		return !substream->stream;
+}
+
+static int loopback_open(struct snd_pcm_substream *substream)
+{
+	struct snd_pcm_runtime *runtime = substream->runtime;
+	struct loopback *loopback = substream->private_data;
+	struct loopback_pcm *dpcm;
+	struct loopback_cable *cable;
+	int err = 0;
+	int dev = get_cable_index(substream);
+
+	mutex_lock(&loopback->cable_lock);
+	dpcm = kzalloc(sizeof(*dpcm), GFP_KERNEL);
+	if (!dpcm) {
+		err = -ENOMEM;
+		goto unlock;
+	}
+	dpcm->loopback = loopback;
+	dpcm->substream = substream;
+	setup_timer(&dpcm->timer, loopback_timer_function,
+		    (unsigned long)dpcm);
+
+	cable = loopback->cables[substream->number][dev];
+	if (!cable) {
+		cable = kzalloc(sizeof(*cable), GFP_KERNEL);
+		if (!cable) {
+			kfree(dpcm);
+			err = -ENOMEM;
+			goto unlock;
+		}
+		spin_lock_init(&cable->lock);
+		cable->hw = loopback_pcm_hardware;
+		loopback->cables[substream->number][dev] = cable;
+	}
+	dpcm->cable = cable;
+	cable->streams[substream->stream] = dpcm;
+
+	snd_pcm_hw_constraint_integer(runtime, SNDRV_PCM_HW_PARAM_PERIODS);
+
+	runtime->private_data = dpcm;
+	runtime->private_free = loopback_runtime_free;
+	if (get_notify(dpcm) &&
+	    substream->stream == SNDRV_PCM_STREAM_PLAYBACK) {
+		runtime->hw = loopback_pcm_hardware;
+	} else {
+		runtime->hw = cable->hw;
+	}
+ unlock:
+	mutex_unlock(&loopback->cable_lock);
+	return err;
+}
+
+static int loopback_close(struct snd_pcm_substream *substream)
+{
+	struct loopback *loopback = substream->private_data;
+	struct loopback_pcm *dpcm = substream->runtime->private_data;
+	struct loopback_cable *cable;
+	int dev = get_cable_index(substream);
+
+	loopback_timer_stop(dpcm);
+	mutex_lock(&loopback->cable_lock);
+	cable = loopback->cables[substream->number][dev];
+	if (cable->streams[!substream->stream]) {
+		/* other stream is still alive */
+		cable->streams[substream->stream] = NULL;
+	} else {
+		/* free the cable */
+		loopback->cables[substream->number][dev] = NULL;
+		kfree(cable);
+	}
+	mutex_unlock(&loopback->cable_lock);
+	return 0;
+}
+
+static struct snd_pcm_ops loopback_playback_ops = {
+	.open =		loopback_open,
+	.close =	loopback_close,
+	.ioctl =	snd_pcm_lib_ioctl,
+	.hw_params =	loopback_hw_params,
+	.hw_free =	loopback_hw_free,
+	.prepare =	loopback_prepare,
+	.trigger =	loopback_trigger,
+	.pointer =	loopback_pointer,
+};
+
+static struct snd_pcm_ops loopback_capture_ops = {
+	.open =		loopback_open,
+	.close =	loopback_close,
+	.ioctl =	snd_pcm_lib_ioctl,
+	.hw_params =	loopback_hw_params,
+	.hw_free =	loopback_hw_free,
+	.prepare =	loopback_prepare,
+	.trigger =	loopback_trigger,
+	.pointer =	loopback_pointer,
+};
+
+static int __devinit loopback_pcm_new(struct loopback *loopback,
+				      int device, int substreams)
+{
+	struct snd_pcm *pcm;
+	int err;
+
+	err = snd_pcm_new(loopback->card, "Loopback PCM", device,
+			  substreams, substreams, &pcm);
+	if (err < 0)
+		return err;
+	snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK, &loopback_playback_ops);
+	snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE, &loopback_capture_ops);
+
+	pcm->private_data = loopback;
+	pcm->info_flags = 0;
+	strcpy(pcm->name, "Loopback PCM");
+
+	loopback->pcm[device] = pcm;
+
+	snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_CONTINUOUS,
+			snd_dma_continuous_data(GFP_KERNEL),
+			0, 2 * 1024 * 1024);
+	return 0;
+}
+
+static int loopback_rate_shift_info(struct snd_kcontrol *kcontrol,   
+				    struct snd_ctl_elem_info *uinfo) 
+{
+	uinfo->type = SNDRV_CTL_ELEM_TYPE_INTEGER;
+	uinfo->count = 1;
+	uinfo->value.integer.min = 80000;
+	uinfo->value.integer.max = 120000;
+	uinfo->value.integer.step = 1;
+	return 0;
+}                                  
+
+static int loopback_rate_shift_get(struct snd_kcontrol *kcontrol,
+				   struct snd_ctl_elem_value *ucontrol)
+{
+	struct loopback *loopback = snd_kcontrol_chip(kcontrol);
+	
+	ucontrol->value.integer.value[0] =
+		loopback->setup[kcontrol->id.subdevice]
+			       [kcontrol->id.device].rate_shift;
+	return 0;
+}
+
+static int loopback_rate_shift_put(struct snd_kcontrol *kcontrol,
+				   struct snd_ctl_elem_value *ucontrol)
+{
+	struct loopback *loopback = snd_kcontrol_chip(kcontrol);
+	unsigned int val;
+	int change = 0;
+
+	val = ucontrol->value.integer.value[0];
+	if (val < 80000)
+		val = 80000;
+	if (val > 120000)
+		val = 120000;	
+	mutex_lock(&loopback->cable_lock);
+	if (val != loopback->setup[kcontrol->id.subdevice]
+				  [kcontrol->id.device].rate_shift) {
+		loopback->setup[kcontrol->id.subdevice]
+			       [kcontrol->id.device].rate_shift = val;
+		change = 1;
+	}
+	mutex_unlock(&loopback->cable_lock);
+	return change;
+}
+
+static int loopback_notify_get(struct snd_kcontrol *kcontrol,
+			       struct snd_ctl_elem_value *ucontrol)
+{
+	struct loopback *loopback = snd_kcontrol_chip(kcontrol);
+	
+	ucontrol->value.integer.value[0] =
+		loopback->setup[kcontrol->id.subdevice]
+			       [kcontrol->id.device].notify;
+	return 0;
+}
+
+static int loopback_notify_put(struct snd_kcontrol *kcontrol,
+			       struct snd_ctl_elem_value *ucontrol)
+{
+	struct loopback *loopback = snd_kcontrol_chip(kcontrol);
+	unsigned int val;
+	int change = 0;
+
+	val = ucontrol->value.integer.value[0] ? 1 : 0;
+	if (val != loopback->setup[kcontrol->id.subdevice]
+				[kcontrol->id.device].notify) {
+		loopback->setup[kcontrol->id.subdevice]
+			[kcontrol->id.device].notify = val;
+		change = 1;
+	}
+	return change;
+}
+
+static int loopback_active_get(struct snd_kcontrol *kcontrol,
+			       struct snd_ctl_elem_value *ucontrol)
+{
+	struct loopback *loopback = snd_kcontrol_chip(kcontrol);
+	struct loopback_cable *cable = loopback->cables
+				[kcontrol->id.subdevice][kcontrol->id.device];
+	unsigned int val = 0;
+
+	if (cable != NULL)
+		val = (cable->running & (1 << SNDRV_PCM_STREAM_PLAYBACK)) ?
+									1 : 0;
+	ucontrol->value.integer.value[0] = val;
+	return 0;
+}
+
+static int loopback_format_info(struct snd_kcontrol *kcontrol,   
+				struct snd_ctl_elem_info *uinfo) 
+{
+	uinfo->type = SNDRV_CTL_ELEM_TYPE_INTEGER;
+	uinfo->count = 1;
+	uinfo->value.integer.min = 0;
+	uinfo->value.integer.max = SNDRV_PCM_FORMAT_LAST;
+	uinfo->value.integer.step = 1;
+	return 0;
+}                                  
+
+static int loopback_format_get(struct snd_kcontrol *kcontrol,
+			       struct snd_ctl_elem_value *ucontrol)
+{
+	struct loopback *loopback = snd_kcontrol_chip(kcontrol);
+	
+	ucontrol->value.integer.value[0] =
+		loopback->setup[kcontrol->id.subdevice]
+			       [kcontrol->id.device].format;
+	return 0;
+}
+
+static int loopback_rate_info(struct snd_kcontrol *kcontrol,   
+			      struct snd_ctl_elem_info *uinfo) 
+{
+	uinfo->type = SNDRV_CTL_ELEM_TYPE_INTEGER;
+	uinfo->count = 1;
+	uinfo->value.integer.min = 0;
+	uinfo->value.integer.max = 192000;
+	uinfo->value.integer.step = 1;
+	return 0;
+}                                  
+
+static int loopback_rate_get(struct snd_kcontrol *kcontrol,
+			     struct snd_ctl_elem_value *ucontrol)
+{
+	struct loopback *loopback = snd_kcontrol_chip(kcontrol);
+	
+	ucontrol->value.integer.value[0] =
+		loopback->setup[kcontrol->id.subdevice]
+			       [kcontrol->id.device].rate;
+	return 0;
+}
+
+static int loopback_channels_info(struct snd_kcontrol *kcontrol,   
+				  struct snd_ctl_elem_info *uinfo) 
+{
+	uinfo->type = SNDRV_CTL_ELEM_TYPE_INTEGER;
+	uinfo->count = 1;
+	uinfo->value.integer.min = 1;
+	uinfo->value.integer.max = 1024;
+	uinfo->value.integer.step = 1;
+	return 0;
+}                                  
+
+static int loopback_channels_get(struct snd_kcontrol *kcontrol,
+				 struct snd_ctl_elem_value *ucontrol)
+{
+	struct loopback *loopback = snd_kcontrol_chip(kcontrol);
+	
+	ucontrol->value.integer.value[0] =
+		loopback->setup[kcontrol->id.subdevice]
+			       [kcontrol->id.device].rate;
+	return 0;
+}
+
+static struct snd_kcontrol_new loopback_controls[]  __devinitdata = {
+{
+	.iface =        SNDRV_CTL_ELEM_IFACE_PCM,
+	.name =         "PCM Rate Shift 100000",
+	.info =         loopback_rate_shift_info,
+	.get =          loopback_rate_shift_get,
+	.put =          loopback_rate_shift_put,
+},
+{
+	.iface =        SNDRV_CTL_ELEM_IFACE_PCM,
+	.name =         "PCM Notify",
+	.info =         snd_ctl_boolean_mono_info,
+	.get =          loopback_notify_get,
+	.put =          loopback_notify_put,
+},
+#define ACTIVE_IDX 2
+{
+	.access =	SNDRV_CTL_ELEM_ACCESS_READ,
+	.iface =        SNDRV_CTL_ELEM_IFACE_PCM,
+	.name =         "PCM Slave Active",
+	.info =         snd_ctl_boolean_mono_info,
+	.get =          loopback_active_get,
+},
+#define FORMAT_IDX 3
+{
+	.access =	SNDRV_CTL_ELEM_ACCESS_READ,
+	.iface =        SNDRV_CTL_ELEM_IFACE_PCM,
+	.name =         "PCM Slave Format",
+	.info =         loopback_format_info,
+	.get =          loopback_format_get
+},
+#define RATE_IDX 4
+{
+	.access =	SNDRV_CTL_ELEM_ACCESS_READ,
+	.iface =        SNDRV_CTL_ELEM_IFACE_PCM,
+	.name =         "PCM Slave Rate",
+	.info =         loopback_rate_info,
+	.get =          loopback_rate_get
+},
+#define CHANNELS_IDX 5
+{
+	.access =	SNDRV_CTL_ELEM_ACCESS_READ,
+	.iface =        SNDRV_CTL_ELEM_IFACE_PCM,
+	.name =         "PCM Slave Channels",
+	.info =         loopback_channels_info,
+	.get =          loopback_channels_get
+}
+};
+
+static int __devinit loopback_mixer_new(struct loopback *loopback, int notify)
+{
+	struct snd_card *card = loopback->card;
+	struct snd_pcm *pcm;
+	struct snd_kcontrol *kctl;
+	struct loopback_setup *setup;
+	int err, dev, substr, substr_count, idx;
+
+	strcpy(card->mixername, "Loopback Mixer");
+	for (dev = 0; dev < 2; dev++) {
+		pcm = loopback->pcm[dev];
+		substr_count =
+		    pcm->streams[SNDRV_PCM_STREAM_CAPTURE].substream_count;
+		for (substr = 0; substr < substr_count; substr++) {
+			setup = &loopback->setup[substr][dev];
+			setup->notify = notify;
+			setup->rate_shift = NO_PITCH;
+			setup->format = SNDRV_PCM_FORMAT_S16_LE;
+			setup->rate = 48000;
+			setup->channels = 2;
+			for (idx = 0; idx < ARRAY_SIZE(loopback_controls);
+									idx++) {
+				kctl = snd_ctl_new1(&loopback_controls[idx],
+						    loopback);
+				if (!kctl)
+					return -ENOMEM;
+				kctl->id.device = dev;
+				kctl->id.subdevice = substr;
+				switch (idx) {
+				case ACTIVE_IDX:
+					setup->active_id = kctl->id;
+					break;
+				case FORMAT_IDX:
+					setup->format_id = kctl->id;
+					break;
+				case RATE_IDX:
+					setup->rate_id = kctl->id;
+					break;
+				case CHANNELS_IDX:
+					setup->channels_id = kctl->id;
+					break;
+				default:
+					break;
+				}
+				err = snd_ctl_add(card, kctl);
+				if (err < 0)
+					return err;
+			}
+		}
+	}
+	return 0;
+}
+
+static int __devinit loopback_probe(struct platform_device *devptr)
+{
+	struct snd_card *card;
+	struct loopback *loopback;
+	int dev = devptr->id;
+	int err;
+
+	err = snd_card_create(index[dev], id[dev], THIS_MODULE,
+			      sizeof(struct loopback), &card);
+	if (err < 0)
+		return err;
+	loopback = card->private_data;
+
+	if (pcm_substreams[dev] < 1)
+		pcm_substreams[dev] = 1;
+	if (pcm_substreams[dev] > MAX_PCM_SUBSTREAMS)
+		pcm_substreams[dev] = MAX_PCM_SUBSTREAMS;
+	
+	loopback->card = card;
+	mutex_init(&loopback->cable_lock);
+
+	err = loopback_pcm_new(loopback, 0, pcm_substreams[dev]);
+	if (err < 0)
+		goto __nodev;
+	err = loopback_pcm_new(loopback, 1, pcm_substreams[dev]);
+	if (err < 0)
+		goto __nodev;
+	err = loopback_mixer_new(loopback, pcm_notify[dev] ? 1 : 0);
+	if (err < 0)
+		goto __nodev;
+	strcpy(card->driver, "Loopback");
+	strcpy(card->shortname, "Loopback");
+	sprintf(card->longname, "Loopback %i", dev + 1);
+	err = snd_card_register(card);
+	if (!err) {
+		platform_set_drvdata(devptr, card);
+		return 0;
+	}
+      __nodev:
+	snd_card_free(card);
+	return err;
+}
+
+static int __devexit loopback_remove(struct platform_device *devptr)
+{
+	snd_card_free(platform_get_drvdata(devptr));
+	platform_set_drvdata(devptr, NULL);
+	return 0;
+}
+
+#ifdef CONFIG_PM
+static int loopback_suspend(struct platform_device *pdev,
+				pm_message_t state)
+{
+	struct snd_card *card = platform_get_drvdata(pdev);
+	struct loopback *loopback = card->private_data;
+
+	snd_power_change_state(card, SNDRV_CTL_POWER_D3hot);
+
+	snd_pcm_suspend_all(loopback->pcm[0]);
+	snd_pcm_suspend_all(loopback->pcm[1]);
+	return 0;
+}
+	
+static int loopback_resume(struct platform_device *pdev)
+{
+	struct snd_card *card = platform_get_drvdata(pdev);
+
+	snd_power_change_state(card, SNDRV_CTL_POWER_D0);
+	return 0;
+}
+#endif
+
+#define SND_LOOPBACK_DRIVER	"snd_aloop"
+
+static struct platform_driver loopback_driver = {
+	.probe		= loopback_probe,
+	.remove		= __devexit_p(loopback_remove),
+#ifdef CONFIG_PM
+	.suspend	= loopback_suspend,
+	.resume		= loopback_resume,
+#endif
+	.driver		= {
+		.name	= SND_LOOPBACK_DRIVER
+	},
+};
+
+static void loopback_unregister_all(void)
+{
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(devices); ++i)
+		platform_device_unregister(devices[i]);
+	platform_driver_unregister(&loopback_driver);
+}
+
+static int __init alsa_card_loopback_init(void)
+{
+	int i, err, cards;
+
+	err = platform_driver_register(&loopback_driver);
+	if (err < 0)
+		return err;
+
+
+	cards = 0;
+	for (i = 0; i < SNDRV_CARDS; i++) {
+		struct platform_device *device;
+		if (!enable[i])
+			continue;
+		device = platform_device_register_simple(SND_LOOPBACK_DRIVER,
+							 i, NULL, 0);
+		if (IS_ERR(device))
+			continue;
+		if (!platform_get_drvdata(device)) {
+			platform_device_unregister(device);
+			continue;
+		}
+		devices[i] = device;
+		cards++;
+	}
+	if (!cards) {
+#ifdef MODULE
+		printk(KERN_ERR "aloop: No loopback enabled\n");
+#endif
+		loopback_unregister_all();
+		return -ENODEV;
+	}
+	return 0;
+}
+
+static void __exit alsa_card_loopback_exit(void)
+{
+	loopback_unregister_all();
+}
+
+module_init(alsa_card_loopback_init)
+module_exit(alsa_card_loopback_exit)