USB: gadget: f_audio_source: New gadget driver for audio output

This driver presents a standard USB audio class interface to the host
and an ALSA PCM device to userspace

Change-Id: If16b14a5ff27045f9cb2daaf1ae9195c5eeab7d0
Signed-off-by: Mike Lockwood <lockwood@google.com>

drivers/usb/gadget/f_audio_source.c

@@ -0,0 +1,828 @@
+/*
+ * Gadget Function Driver for USB audio source device
+ *
+ * Copyright (C) 2012 Google, Inc.
+ *
+ * This software is licensed under the terms of the GNU General Public
+ * License version 2, as published by the Free Software Foundation, and
+ * may be copied, distributed, and modified under those terms.
+ *
+ * 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.
+ *
+ */
+
+#include <linux/device.h>
+#include <linux/usb/audio.h>
+#include <linux/wait.h>
+#include <sound/core.h>
+#include <sound/initval.h>
+#include <sound/pcm.h>
+
+#define SAMPLE_RATE 44100
+#define FRAMES_PER_MSEC (SAMPLE_RATE / 1000)
+
+#define IN_EP_MAX_PACKET_SIZE 384
+
+/* Number of requests to allocate */
+#define IN_EP_REQ_COUNT 4
+
+#define AUDIO_AC_INTERFACE	0
+#define AUDIO_AS_INTERFACE	1
+#define AUDIO_NUM_INTERFACES	2
+
+/* B.3.1  Standard AC Interface Descriptor */
+static struct usb_interface_descriptor ac_interface_desc = {
+	.bLength =		USB_DT_INTERFACE_SIZE,
+	.bDescriptorType =	USB_DT_INTERFACE,
+	.bNumEndpoints =	0,
+	.bInterfaceClass =	USB_CLASS_AUDIO,
+	.bInterfaceSubClass =	USB_SUBCLASS_AUDIOCONTROL,
+};
+
+DECLARE_UAC_AC_HEADER_DESCRIPTOR(2);
+
+#define UAC_DT_AC_HEADER_LENGTH	UAC_DT_AC_HEADER_SIZE(AUDIO_NUM_INTERFACES)
+/* 1 input terminal, 1 output terminal and 1 feature unit */
+#define UAC_DT_TOTAL_LENGTH (UAC_DT_AC_HEADER_LENGTH \
+	+ UAC_DT_INPUT_TERMINAL_SIZE + UAC_DT_OUTPUT_TERMINAL_SIZE \
+	+ UAC_DT_FEATURE_UNIT_SIZE(0))
+/* B.3.2  Class-Specific AC Interface Descriptor */
+static struct uac1_ac_header_descriptor_2 ac_header_desc = {
+	.bLength =		UAC_DT_AC_HEADER_LENGTH,
+	.bDescriptorType =	USB_DT_CS_INTERFACE,
+	.bDescriptorSubtype =	UAC_HEADER,
+	.bcdADC =		__constant_cpu_to_le16(0x0100),
+	.wTotalLength =		__constant_cpu_to_le16(UAC_DT_TOTAL_LENGTH),
+	.bInCollection =	AUDIO_NUM_INTERFACES,
+	.baInterfaceNr = {
+		[0] =		AUDIO_AC_INTERFACE,
+		[1] =		AUDIO_AS_INTERFACE,
+	}
+};
+
+#define INPUT_TERMINAL_ID	1
+static struct uac_input_terminal_descriptor input_terminal_desc = {
+	.bLength =		UAC_DT_INPUT_TERMINAL_SIZE,
+	.bDescriptorType =	USB_DT_CS_INTERFACE,
+	.bDescriptorSubtype =	UAC_INPUT_TERMINAL,
+	.bTerminalID =		INPUT_TERMINAL_ID,
+	.wTerminalType =	UAC_INPUT_TERMINAL_MICROPHONE,
+	.bAssocTerminal =	0,
+	.wChannelConfig =	0x3,
+};
+
+DECLARE_UAC_FEATURE_UNIT_DESCRIPTOR(0);
+
+#define FEATURE_UNIT_ID		2
+static struct uac_feature_unit_descriptor_0 feature_unit_desc = {
+	.bLength		= UAC_DT_FEATURE_UNIT_SIZE(0),
+	.bDescriptorType	= USB_DT_CS_INTERFACE,
+	.bDescriptorSubtype	= UAC_FEATURE_UNIT,
+	.bUnitID		= FEATURE_UNIT_ID,
+	.bSourceID		= INPUT_TERMINAL_ID,
+	.bControlSize		= 2,
+};
+
+#define OUTPUT_TERMINAL_ID	3
+static struct uac1_output_terminal_descriptor output_terminal_desc = {
+	.bLength		= UAC_DT_OUTPUT_TERMINAL_SIZE,
+	.bDescriptorType	= USB_DT_CS_INTERFACE,
+	.bDescriptorSubtype	= UAC_OUTPUT_TERMINAL,
+	.bTerminalID		= OUTPUT_TERMINAL_ID,
+	.wTerminalType		= UAC_TERMINAL_STREAMING,
+	.bAssocTerminal		= FEATURE_UNIT_ID,
+	.bSourceID		= FEATURE_UNIT_ID,
+};
+
+/* B.4.1  Standard AS Interface Descriptor */
+static struct usb_interface_descriptor as_interface_alt_0_desc = {
+	.bLength =		USB_DT_INTERFACE_SIZE,
+	.bDescriptorType =	USB_DT_INTERFACE,
+	.bAlternateSetting =	0,
+	.bNumEndpoints =	0,
+	.bInterfaceClass =	USB_CLASS_AUDIO,
+	.bInterfaceSubClass =	USB_SUBCLASS_AUDIOSTREAMING,
+};
+
+static struct usb_interface_descriptor as_interface_alt_1_desc = {
+	.bLength =		USB_DT_INTERFACE_SIZE,
+	.bDescriptorType =	USB_DT_INTERFACE,
+	.bAlternateSetting =	1,
+	.bNumEndpoints =	1,
+	.bInterfaceClass =	USB_CLASS_AUDIO,
+	.bInterfaceSubClass =	USB_SUBCLASS_AUDIOSTREAMING,
+};
+
+/* B.4.2  Class-Specific AS Interface Descriptor */
+static struct uac1_as_header_descriptor as_header_desc = {
+	.bLength =		UAC_DT_AS_HEADER_SIZE,
+	.bDescriptorType =	USB_DT_CS_INTERFACE,
+	.bDescriptorSubtype =	UAC_AS_GENERAL,
+	.bTerminalLink =	INPUT_TERMINAL_ID,
+	.bDelay =		1,
+	.wFormatTag =		UAC_FORMAT_TYPE_I_PCM,
+};
+
+DECLARE_UAC_FORMAT_TYPE_I_DISCRETE_DESC(1);
+
+static struct uac_format_type_i_discrete_descriptor_1 as_type_i_desc = {
+	.bLength =		UAC_FORMAT_TYPE_I_DISCRETE_DESC_SIZE(1),
+	.bDescriptorType =	USB_DT_CS_INTERFACE,
+	.bDescriptorSubtype =	UAC_FORMAT_TYPE,
+	.bFormatType =		UAC_FORMAT_TYPE_I,
+	.bSubframeSize =	2,
+	.bBitResolution =	16,
+	.bSamFreqType =		1,
+};
+
+/* Standard ISO IN Endpoint Descriptor for highspeed */
+static struct usb_endpoint_descriptor hs_as_in_ep_desc  = {
+	.bLength =		USB_DT_ENDPOINT_AUDIO_SIZE,
+	.bDescriptorType =	USB_DT_ENDPOINT,
+	.bEndpointAddress =	USB_DIR_IN,
+	.bmAttributes =		USB_ENDPOINT_SYNC_SYNC
+				| USB_ENDPOINT_XFER_ISOC,
+	.wMaxPacketSize =	__constant_cpu_to_le16(IN_EP_MAX_PACKET_SIZE),
+	.bInterval =		4, /* poll 1 per millisecond */
+};
+
+/* Standard ISO IN Endpoint Descriptor for highspeed */
+static struct usb_endpoint_descriptor fs_as_in_ep_desc  = {
+	.bLength =		USB_DT_ENDPOINT_AUDIO_SIZE,
+	.bDescriptorType =	USB_DT_ENDPOINT,
+	.bEndpointAddress =	USB_DIR_IN,
+	.bmAttributes =		USB_ENDPOINT_SYNC_SYNC
+				| USB_ENDPOINT_XFER_ISOC,
+	.wMaxPacketSize =	__constant_cpu_to_le16(IN_EP_MAX_PACKET_SIZE),
+	.bInterval =		1, /* poll 1 per millisecond */
+};
+
+/* Class-specific AS ISO OUT Endpoint Descriptor */
+static struct uac_iso_endpoint_descriptor as_iso_in_desc = {
+	.bLength =		UAC_ISO_ENDPOINT_DESC_SIZE,
+	.bDescriptorType =	USB_DT_CS_ENDPOINT,
+	.bDescriptorSubtype =	UAC_EP_GENERAL,
+	.bmAttributes =		1,
+	.bLockDelayUnits =	1,
+	.wLockDelay =		__constant_cpu_to_le16(1),
+};
+
+static struct usb_descriptor_header *hs_audio_desc[] = {
+	(struct usb_descriptor_header *)&ac_interface_desc,
+	(struct usb_descriptor_header *)&ac_header_desc,
+
+	(struct usb_descriptor_header *)&input_terminal_desc,
+	(struct usb_descriptor_header *)&output_terminal_desc,
+	(struct usb_descriptor_header *)&feature_unit_desc,
+
+	(struct usb_descriptor_header *)&as_interface_alt_0_desc,
+	(struct usb_descriptor_header *)&as_interface_alt_1_desc,
+	(struct usb_descriptor_header *)&as_header_desc,
+
+	(struct usb_descriptor_header *)&as_type_i_desc,
+
+	(struct usb_descriptor_header *)&hs_as_in_ep_desc,
+	(struct usb_descriptor_header *)&as_iso_in_desc,
+	NULL,
+};
+
+static struct usb_descriptor_header *fs_audio_desc[] = {
+	(struct usb_descriptor_header *)&ac_interface_desc,
+	(struct usb_descriptor_header *)&ac_header_desc,
+
+	(struct usb_descriptor_header *)&input_terminal_desc,
+	(struct usb_descriptor_header *)&output_terminal_desc,
+	(struct usb_descriptor_header *)&feature_unit_desc,
+
+	(struct usb_descriptor_header *)&as_interface_alt_0_desc,
+	(struct usb_descriptor_header *)&as_interface_alt_1_desc,
+	(struct usb_descriptor_header *)&as_header_desc,
+
+	(struct usb_descriptor_header *)&as_type_i_desc,
+
+	(struct usb_descriptor_header *)&fs_as_in_ep_desc,
+	(struct usb_descriptor_header *)&as_iso_in_desc,
+	NULL,
+};
+
+static struct snd_pcm_hardware audio_hw_info = {
+	.info =			SNDRV_PCM_INFO_MMAP |
+				SNDRV_PCM_INFO_MMAP_VALID |
+				SNDRV_PCM_INFO_BATCH |
+				SNDRV_PCM_INFO_INTERLEAVED |
+				SNDRV_PCM_INFO_BLOCK_TRANSFER,
+
+	.formats		= SNDRV_PCM_FMTBIT_S16_LE,
+	.channels_min		= 2,
+	.channels_max		= 2,
+	.rate_min		= SAMPLE_RATE,
+	.rate_max		= SAMPLE_RATE,
+
+	.buffer_bytes_max =	1024 * 1024,
+	.period_bytes_min =	64,
+	.period_bytes_max =	512 * 1024,
+	.periods_min =		2,
+	.periods_max =		1024,
+};
+
+/*-------------------------------------------------------------------------*/
+
+struct audio_source_config {
+	int	card;
+	int	device;
+};
+
+struct audio_dev {
+	struct usb_function		func;
+	struct snd_card			*card;
+	struct snd_pcm			*pcm;
+	struct snd_pcm_substream *substream;
+
+	struct list_head		idle_reqs;
+	struct usb_ep			*in_ep;
+
+	spinlock_t			lock;
+
+	/* beginning, end and current position in our buffer */
+	void				*buffer_start;
+	void				*buffer_end;
+	void				*buffer_pos;
+
+	/* byte size of a "period" */
+	unsigned int			period;
+	/* bytes sent since last call to snd_pcm_period_elapsed */
+	unsigned int			period_offset;
+	/* time we started playing */
+	ktime_t				start_time;
+	/* number of frames sent since start_time */
+	s64				frames_sent;
+};
+
+static inline struct audio_dev *func_to_audio(struct usb_function *f)
+{
+	return container_of(f, struct audio_dev, func);
+}
+
+/*-------------------------------------------------------------------------*/
+
+static struct usb_request *audio_request_new(struct usb_ep *ep, int buffer_size)
+{
+	struct usb_request *req = usb_ep_alloc_request(ep, GFP_KERNEL);
+	if (!req)
+		return NULL;
+
+	req->buf = kmalloc(buffer_size, GFP_KERNEL);
+	if (!req->buf) {
+		usb_ep_free_request(ep, req);
+		return NULL;
+	}
+	req->length = buffer_size;
+	return req;
+}
+
+static void audio_request_free(struct usb_request *req, struct usb_ep *ep)
+{
+	if (req) {
+		kfree(req->buf);
+		usb_ep_free_request(ep, req);
+	}
+}
+
+static void audio_req_put(struct audio_dev *audio, struct usb_request *req)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&audio->lock, flags);
+	list_add_tail(&req->list, &audio->idle_reqs);
+	spin_unlock_irqrestore(&audio->lock, flags);
+}
+
+static struct usb_request *audio_req_get(struct audio_dev *audio)
+{
+	unsigned long flags;
+	struct usb_request *req;
+
+	spin_lock_irqsave(&audio->lock, flags);
+	if (list_empty(&audio->idle_reqs)) {
+		req = 0;
+	} else {
+		req = list_first_entry(&audio->idle_reqs, struct usb_request,
+				list);
+		list_del(&req->list);
+	}
+	spin_unlock_irqrestore(&audio->lock, flags);
+	return req;
+}
+
+/* send the appropriate number of packets to match our bitrate */
+static void audio_send(struct audio_dev *audio)
+{
+	struct snd_pcm_runtime *runtime;
+	struct usb_request *req;
+	int length, length1, length2, ret;
+	s64 msecs;
+	s64 frames;
+	ktime_t now;
+
+	/* audio->substream will be null if we have been closed */
+	if (!audio->substream)
+		return;
+	/* audio->buffer_pos will be null if we have been stopped */
+	if (!audio->buffer_pos)
+		return;
+
+	runtime = audio->substream->runtime;
+
+	/* compute number of frames to send */
+	now = ktime_get();
+	msecs = ktime_to_ns(now) - ktime_to_ns(audio->start_time);
+	do_div(msecs, 1000000);
+	frames = msecs * SAMPLE_RATE;
+	do_div(frames, 1000);
+
+	/* Readjust our frames_sent if we fall too far behind.
+	 * If we get too far behind it is better to drop some frames than
+	 * to keep sending data too fast in an attempt to catch up.
+	 */
+	if (frames - audio->frames_sent > 10 * FRAMES_PER_MSEC)
+		audio->frames_sent = frames - FRAMES_PER_MSEC;
+
+	frames -= audio->frames_sent;
+
+	/* We need to send something to keep the pipeline going */
+	if (frames <= 0)
+		frames = FRAMES_PER_MSEC;
+
+	while (frames > 0) {
+		req = audio_req_get(audio);
+		if (!req)
+			break;
+
+		length = frames_to_bytes(runtime, frames);
+		if (length > IN_EP_MAX_PACKET_SIZE)
+			length = IN_EP_MAX_PACKET_SIZE;
+
+		if (audio->buffer_pos + length > audio->buffer_end)
+			length1 = audio->buffer_end - audio->buffer_pos;
+		else
+			length1 = length;
+		memcpy(req->buf, audio->buffer_pos, length1);
+		if (length1 < length) {
+			/* Wrap around and copy remaining length
+			 * at beginning of buffer.
+			 */
+			length2 = length - length1;
+			memcpy(req->buf + length1, audio->buffer_start,
+					length2);
+			audio->buffer_pos = audio->buffer_start + length2;
+		} else {
+			audio->buffer_pos += length1;
+			if (audio->buffer_pos >= audio->buffer_end)
+				audio->buffer_pos = audio->buffer_start;
+		}
+
+		req->length = length;
+		ret = usb_ep_queue(audio->in_ep, req, GFP_ATOMIC);
+		if (ret < 0) {
+			pr_err("usb_ep_queue failed ret: %d\n", ret);
+			audio_req_put(audio, req);
+			break;
+		}
+
+		frames -= bytes_to_frames(runtime, length);
+		audio->frames_sent += bytes_to_frames(runtime, length);
+	}
+}
+
+static void audio_control_complete(struct usb_ep *ep, struct usb_request *req)
+{
+	/* nothing to do here */
+}
+
+static void audio_data_complete(struct usb_ep *ep, struct usb_request *req)
+{
+	struct audio_dev *audio = req->context;
+
+	pr_debug("audio_data_complete req->status %d req->actual %d\n",
+		req->status, req->actual);
+
+	audio_req_put(audio, req);
+
+	if (!audio->buffer_start || req->status)
+		return;
+
+	audio->period_offset += req->actual;
+	if (audio->period_offset >= audio->period) {
+		snd_pcm_period_elapsed(audio->substream);
+		audio->period_offset = 0;
+	}
+	audio_send(audio);
+}
+
+static int audio_set_endpoint_req(struct usb_function *f,
+		const struct usb_ctrlrequest *ctrl)
+{
+	int value = -EOPNOTSUPP;
+	u16 ep = le16_to_cpu(ctrl->wIndex);
+	u16 len = le16_to_cpu(ctrl->wLength);
+	u16 w_value = le16_to_cpu(ctrl->wValue);
+
+	pr_debug("bRequest 0x%x, w_value 0x%04x, len %d, endpoint %d\n",
+			ctrl->bRequest, w_value, len, ep);
+
+	switch (ctrl->bRequest) {
+	case UAC_SET_CUR:
+	case UAC_SET_MIN:
+	case UAC_SET_MAX:
+	case UAC_SET_RES:
+		value = len;
+		break;
+	default:
+		break;
+	}
+
+	return value;
+}
+
+static int audio_get_endpoint_req(struct usb_function *f,
+		const struct usb_ctrlrequest *ctrl)
+{
+	struct usb_composite_dev *cdev = f->config->cdev;
+	int value = -EOPNOTSUPP;
+	u8 ep = ((le16_to_cpu(ctrl->wIndex) >> 8) & 0xFF);
+	u16 len = le16_to_cpu(ctrl->wLength);
+	u16 w_value = le16_to_cpu(ctrl->wValue);
+	u8 *buf = cdev->req->buf;
+
+	pr_debug("bRequest 0x%x, w_value 0x%04x, len %d, endpoint %d\n",
+			ctrl->bRequest, w_value, len, ep);
+
+	if (w_value == UAC_EP_CS_ATTR_SAMPLE_RATE << 8) {
+		switch (ctrl->bRequest) {
+		case UAC_GET_CUR:
+		case UAC_GET_MIN:
+		case UAC_GET_MAX:
+		case UAC_GET_RES:
+			/* return our sample rate */
+			buf[0] = (u8)SAMPLE_RATE;
+			buf[1] = (u8)(SAMPLE_RATE >> 8);
+			buf[2] = (u8)(SAMPLE_RATE >> 16);
+			value = 3;
+			break;
+		default:
+			break;
+		}
+	}
+
+	return value;
+}
+
+static int
+audio_setup(struct usb_function *f, const struct usb_ctrlrequest *ctrl)
+{
+	struct usb_composite_dev *cdev = f->config->cdev;
+	struct usb_request *req = cdev->req;
+	int value = -EOPNOTSUPP;
+	u16 w_index = le16_to_cpu(ctrl->wIndex);
+	u16 w_value = le16_to_cpu(ctrl->wValue);
+	u16 w_length = le16_to_cpu(ctrl->wLength);
+
+	/* composite driver infrastructure handles everything; interface
+	 * activation uses set_alt().
+	 */
+	switch (ctrl->bRequestType) {
+	case USB_DIR_OUT | USB_TYPE_CLASS | USB_RECIP_ENDPOINT:
+		value = audio_set_endpoint_req(f, ctrl);
+		break;
+
+	case USB_DIR_IN | USB_TYPE_CLASS | USB_RECIP_ENDPOINT:
+		value = audio_get_endpoint_req(f, ctrl);
+		break;
+	}
+
+	/* respond with data transfer or status phase? */
+	if (value >= 0) {
+		pr_debug("audio req%02x.%02x v%04x i%04x l%d\n",
+			ctrl->bRequestType, ctrl->bRequest,
+			w_value, w_index, w_length);
+		req->zero = 0;
+		req->length = value;
+		req->complete = audio_control_complete;
+		value = usb_ep_queue(cdev->gadget->ep0, req, GFP_ATOMIC);
+		if (value < 0)
+			pr_err("audio response on err %d\n", value);
+	}
+
+	/* device either stalls (value < 0) or reports success */
+	return value;
+}
+
+static int audio_set_alt(struct usb_function *f, unsigned intf, unsigned alt)
+{
+	struct audio_dev *audio = func_to_audio(f);
+	struct usb_composite_dev *cdev = f->config->cdev;
+	int ret;
+
+	pr_debug("audio_set_alt intf %d, alt %d\n", intf, alt);
+
+	ret = config_ep_by_speed(cdev->gadget, f, audio->in_ep);
+	if (ret)
+		return ret;
+
+	usb_ep_enable(audio->in_ep);
+	return 0;
+}
+
+static void audio_disable(struct usb_function *f)
+{
+	struct audio_dev	*audio = func_to_audio(f);
+
+	pr_debug("audio_disable\n");
+	usb_ep_disable(audio->in_ep);
+}
+
+/*-------------------------------------------------------------------------*/
+
+static void audio_build_desc(struct audio_dev *audio)
+{
+	u8 *sam_freq;
+	int rate;
+
+	/* Set channel numbers */
+	input_terminal_desc.bNrChannels = 2;
+	as_type_i_desc.bNrChannels = 2;
+
+	/* Set sample rates */
+	rate = SAMPLE_RATE;
+	sam_freq = as_type_i_desc.tSamFreq[0];
+	memcpy(sam_freq, &rate, 3);
+}
+
+/* audio function driver setup/binding */
+static int
+audio_bind(struct usb_configuration *c, struct usb_function *f)
+{
+	struct usb_composite_dev *cdev = c->cdev;
+	struct audio_dev *audio = func_to_audio(f);
+	int status;
+	struct usb_ep *ep;
+	struct usb_request *req;
+	int i;
+
+	audio_build_desc(audio);
+
+	/* allocate instance-specific interface IDs, and patch descriptors */
+	status = usb_interface_id(c, f);
+	if (status < 0)
+		goto fail;
+	ac_interface_desc.bInterfaceNumber = status;
+
+	status = usb_interface_id(c, f);
+	if (status < 0)
+		goto fail;
+	as_interface_alt_0_desc.bInterfaceNumber = status;
+	as_interface_alt_1_desc.bInterfaceNumber = status;
+
+	status = -ENODEV;
+
+	/* allocate our endpoint */
+	ep = usb_ep_autoconfig(cdev->gadget, &fs_as_in_ep_desc);
+	if (!ep)
+		goto fail;
+	audio->in_ep = ep;
+	ep->driver_data = audio; /* claim */
+
+	if (gadget_is_dualspeed(c->cdev->gadget))
+		hs_as_in_ep_desc.bEndpointAddress =
+			fs_as_in_ep_desc.bEndpointAddress;
+
+	f->descriptors = fs_audio_desc;
+	f->hs_descriptors = hs_audio_desc;
+
+	for (i = 0, status = 0; i < IN_EP_REQ_COUNT && status == 0; i++) {
+		req = audio_request_new(ep, IN_EP_MAX_PACKET_SIZE);
+		if (req) {
+			req->context = audio;
+			req->complete = audio_data_complete;
+			audio_req_put(audio, req);
+		} else
+			status = -ENOMEM;
+	}
+
+fail:
+	return status;
+}
+
+static void
+audio_unbind(struct usb_configuration *c, struct usb_function *f)
+{
+	struct audio_dev *audio = func_to_audio(f);
+	struct usb_request *req;
+
+	while ((req = audio_req_get(audio)))
+		audio_request_free(req, audio->in_ep);
+
+	snd_card_free_when_closed(audio->card);
+	audio->card = NULL;
+	audio->pcm = NULL;
+	audio->substream = NULL;
+	audio->in_ep = NULL;
+}
+
+static void audio_pcm_playback_start(struct audio_dev *audio)
+{
+	audio->start_time = ktime_get();
+	audio->frames_sent = 0;
+	audio_send(audio);
+}
+
+static void audio_pcm_playback_stop(struct audio_dev *audio)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&audio->lock, flags);
+	audio->buffer_start = 0;
+	audio->buffer_end = 0;
+	audio->buffer_pos = 0;
+	spin_unlock_irqrestore(&audio->lock, flags);
+}
+
+static int audio_pcm_open(struct snd_pcm_substream *substream)
+{
+	struct snd_pcm_runtime *runtime = substream->runtime;
+	struct audio_dev *audio = substream->private_data;
+
+	runtime->private_data = audio;
+	runtime->hw = audio_hw_info;
+	snd_pcm_limit_hw_rates(runtime);
+	runtime->hw.channels_max = 2;
+
+	audio->substream = substream;
+	return 0;
+}
+
+static int audio_pcm_close(struct snd_pcm_substream *substream)
+{
+	struct audio_dev *audio = substream->private_data;
+	unsigned long flags;
+
+	spin_lock_irqsave(&audio->lock, flags);
+	audio->substream = NULL;
+	spin_unlock_irqrestore(&audio->lock, flags);
+
+	return 0;
+}
+
+static int audio_pcm_hw_params(struct snd_pcm_substream *substream,
+				struct snd_pcm_hw_params *params)
+{
+	unsigned int channels = params_channels(params);
+	unsigned int rate = params_rate(params);
+
+	if (rate != SAMPLE_RATE)
+		return -EINVAL;
+	if (channels != 2)
+		return -EINVAL;
+
+	return snd_pcm_lib_alloc_vmalloc_buffer(substream,
+		params_buffer_bytes(params));
+}
+
+static int audio_pcm_hw_free(struct snd_pcm_substream *substream)
+{
+	return snd_pcm_lib_free_vmalloc_buffer(substream);
+}
+
+static int audio_pcm_prepare(struct snd_pcm_substream *substream)
+{
+	struct snd_pcm_runtime *runtime = substream->runtime;
+	struct audio_dev *audio = runtime->private_data;
+
+	audio->period = snd_pcm_lib_period_bytes(substream);
+	audio->period_offset = 0;
+	audio->buffer_start = runtime->dma_area;
+	audio->buffer_end = audio->buffer_start
+		+ snd_pcm_lib_buffer_bytes(substream);
+	audio->buffer_pos = audio->buffer_start;
+
+	return 0;
+}
+
+static snd_pcm_uframes_t audio_pcm_pointer(struct snd_pcm_substream *substream)
+{
+	struct snd_pcm_runtime *runtime = substream->runtime;
+	struct audio_dev *audio = runtime->private_data;
+	ssize_t bytes = audio->buffer_pos - audio->buffer_start;
+
+	/* return offset of next frame to fill in our buffer */
+	return bytes_to_frames(runtime, bytes);
+}
+
+static int audio_pcm_playback_trigger(struct snd_pcm_substream *substream,
+					int cmd)
+{
+	struct audio_dev *audio = substream->runtime->private_data;
+	int ret = 0;
+
+	switch (cmd) {
+	case SNDRV_PCM_TRIGGER_START:
+	case SNDRV_PCM_TRIGGER_RESUME:
+		audio_pcm_playback_start(audio);
+		break;
+
+	case SNDRV_PCM_TRIGGER_STOP:
+	case SNDRV_PCM_TRIGGER_SUSPEND:
+		audio_pcm_playback_stop(audio);
+		break;
+
+	default:
+		ret = -EINVAL;
+	}
+
+	return ret;
+}
+
+static struct audio_dev _audio_dev = {
+	.func = {
+		.name = "audio_source",
+		.bind = audio_bind,
+		.unbind = audio_unbind,
+		.set_alt = audio_set_alt,
+		.setup = audio_setup,
+		.disable = audio_disable,
+	},
+	.lock = __SPIN_LOCK_UNLOCKED(_audio_dev.lock),
+	.idle_reqs = LIST_HEAD_INIT(_audio_dev.idle_reqs),
+};
+
+static struct snd_pcm_ops audio_playback_ops = {
+	.open		= audio_pcm_open,
+	.close		= audio_pcm_close,
+	.ioctl		= snd_pcm_lib_ioctl,
+	.hw_params	= audio_pcm_hw_params,
+	.hw_free	= audio_pcm_hw_free,
+	.prepare	= audio_pcm_prepare,
+	.trigger	= audio_pcm_playback_trigger,
+	.pointer	= audio_pcm_pointer,
+};
+
+int audio_source_bind_config(struct usb_configuration *c,
+		struct audio_source_config *config)
+{
+	struct audio_dev *audio;
+	struct snd_card *card;
+	struct snd_pcm *pcm;
+	int err;
+
+	config->card = -1;
+	config->device = -1;
+
+	audio = &_audio_dev;
+
+	err = snd_card_create(SNDRV_DEFAULT_IDX1, SNDRV_DEFAULT_STR1,
+			THIS_MODULE, 0, &card);
+	if (err)
+		return err;
+
+	snd_card_set_dev(card, &c->cdev->gadget->dev);
+
+	err = snd_pcm_new(card, "USB audio source", 0, 1, 0, &pcm);
+	if (err)
+		goto pcm_fail;
+	pcm->private_data = audio;
+	pcm->info_flags = 0;
+	audio->pcm = pcm;
+
+	strlcpy(pcm->name, "USB gadget audio", sizeof(pcm->name));
+
+	snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK, &audio_playback_ops);
+	snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_DEV,
+				NULL, 0, 64 * 1024);
+
+	strlcpy(card->driver, "audio_source", sizeof(card->driver));
+	strlcpy(card->shortname, card->driver, sizeof(card->shortname));
+	strlcpy(card->longname, "USB accessory audio source",
+		sizeof(card->longname));
+
+	err = snd_card_register(card);
+	if (err)
+		goto register_fail;
+
+	err = usb_add_function(c, &audio->func);
+	if (err)
+		goto add_fail;
+
+	config->card = pcm->card->number;
+	config->device = pcm->device;
+	audio->card = card;
+	return 0;
+
+add_fail:
+register_fail:
+pcm_fail:
+	snd_card_free(audio->card);
+	return err;
+}