ASoC USB support

Overview

In order to leverage the existing USB sound device support in ALSA, the ASoC USB APIs are introduced to allow the subsystems to exchange configuration information.

One potential use case would be to support USB audio offloading, which is an implementation that allows for an alternate power-optimized path in the audio subsystem to handle the transfer of audio data over the USB bus. This would let the main processor to stay in lower power modes for longer duration. The following is an example design of how the ASoC and ALSA pieces can be connected together to achieve this:

           USB                   |            ASoC
                                 |  _________________________
                                 | |   ASoC Platform card    |
                                 | |_________________________|
                                 |         |           |
                                 |      ___V____   ____V____
                                 |     |ASoC BE | |ASoC FE  |
                                 |     |DAI LNK | |DAI LNK  |
                                 |     |________| |_________|
                                 |         ^  ^        ^
                                 |         |  |________|
                                 |      ___V____    |
                                 |     |SoC-USB |   |
 ________       ________               |        |   |
|USB SND |<--->|USBSND  |<------------>|________|   |
|(card.c)|     |offld   |<----------                |
|________|     |________|___     | |                |
    ^               ^       |    | |    ____________V_________
    |               |       |    | |   |IPC                   |
 __ V_______________V_____  |    | |   |______________________|
|USB SND (endpoint.c)     | |    | |              ^
|_________________________| |    | |              |
            ^               |    | |   ___________V___________
            |               |    | |->|audio DSP              |
 ___________V_____________  |    |    |_______________________|
|XHCI HCD                 |<-    |
|_________________________|      |

SoC USB driver

Structures

struct snd_soc_usb

  • list: list head for SND SoC struct list

  • component: reference to ASoC component

  • connection_status_cb: callback to notify connection events

  • update_offload_route_info: callback to fetch selected USB sound card/PCM device

  • priv_data: driver data

The snd_soc_usb structure can be referenced using the ASoC platform card device, or a USB device (udev->dev). This is created by the ASoC BE DAI link, and the USB sound entity will be able to pass information to the ASoC BE DAI link using this structure.

struct snd_soc_usb_device

  • card_idx: sound card index associated with USB sound device

  • chip_idx: USB sound chip array index

  • cpcm_idx: capture pcm device indexes associated with the USB sound device

  • ppcm_idx: playback pcm device indexes associated with the USB sound device

  • num_playback: number of playback streams

  • num_capture: number of capture streams

  • list: list head for the USB sound device list

The struct snd_soc_usb_device is created by the USB sound offload driver. This will carry basic parameters/limitations that will be used to determine the possible offloading paths for this USB audio device.

Functions

int snd_soc_usb_find_supported_format(int card_idx,
                struct snd_pcm_hw_params *params, int direction)

  • card_idx: the index into the USB sound chip array.

  • params: Requested PCM parameters from the USB DPCM BE DAI link

  • direction: capture or playback

snd_soc_usb_find_supported_format() ensures that the requested audio profile being requested by the external DSP is supported by the USB device.

Returns 0 on success, and -EOPNOTSUPP on failure.

int snd_soc_usb_connect(struct device *usbdev, struct snd_soc_usb_device *sdev)

  • usbdev: the usb device that was discovered

  • sdev: capabilities of the device

snd_soc_usb_connect() notifies the ASoC USB DCPM BE DAI link of a USB audio device detection. This can be utilized in the BE DAI driver to keep track of available USB audio devices. This is intended to be called by the USB offload driver residing in USB SND.

Returns 0 on success, negative error code on failure.

int snd_soc_usb_disconnect(struct device *usbdev, struct snd_soc_usb_device *sdev)

  • usbdev: the usb device that was removed

  • sdev: capabilities to free

snd_soc_usb_disconnect() notifies the ASoC USB DCPM BE DAI link of a USB audio device removal. This is intended to be called by the USB offload driver that resides in USB SND.

void *snd_soc_usb_find_priv_data(struct device *usbdev)

  • usbdev: the usb device to reference to find private data

snd_soc_usb_find_priv_data() fetches the private data saved to the SoC USB device.

Returns pointer to priv_data on success, NULL on failure.

int snd_soc_usb_setup_offload_jack(struct snd_soc_component *component,
                                struct snd_soc_jack *jack)

  • component: ASoC component to add the jack

  • jack: jack component to populate

snd_soc_usb_setup_offload_jack() is a helper to add a sound jack control to the platform sound card. This will allow for consistent naming to be used on designs that support USB audio offloading. Additionally, this will enable the jack to notify of changes.

Returns 0 on success, negative otherwise.

int snd_soc_usb_update_offload_route(struct device *dev, int card, int pcm,
                                     int direction, enum snd_soc_usb_kctl path,
                                     long *route)

  • dev: USB device to look up offload path mapping

  • card: USB sound card index

  • pcm: USB sound PCM device index

  • direction: direction to fetch offload routing information

  • path: kcontrol selector - pcm device or card index

  • route: mapping of sound card and pcm indexes for the offload path. This is

    an array of two integers that will carry the card and pcm device indexes in that specific order. This can be used as the array for the kcontrol output.

snd_soc_usb_update_offload_route() calls a registered callback to the USB BE DAI link to fetch the information about the mapped ASoC devices for executing USB audio offload for the device. route may be a pointer to a kcontrol value output array, which carries values when the kcontrol is read.

Returns 0 on success, negative otherwise.

struct snd_soc_usb *snd_soc_usb_allocate_port(struct snd_soc_component *component,
                void *data);

  • component: DPCM BE DAI link component

  • data: private data

snd_soc_usb_allocate_port() allocates a SoC USB device and populates standard parameters that is used for further operations.

Returns a pointer to struct soc_usb on success, negative on error.

void snd_soc_usb_free_port(struct snd_soc_usb *usb);

  • usb: SoC USB device to free

snd_soc_usb_free_port() frees a SoC USB device.

void snd_soc_usb_add_port(struct snd_soc_usb *usb);

  • usb: SoC USB device to add

snd_soc_usb_add_port() add an allocated SoC USB device to the SOC USB framework. Once added, this device can be referenced by further operations.

void snd_soc_usb_remove_port(struct snd_soc_usb *usb);

  • usb: SoC USB device to remove

snd_soc_usb_remove_port() removes a SoC USB device from the SoC USB framework. After removing a device, any SOC USB operations would not be able to reference the device removed.

How to Register to SoC USB

The ASoC DPCM USB BE DAI link is the entity responsible for allocating and registering the SoC USB device on the component bind. Likewise, it will also be responsible for freeing the allocated resources. An example can be shown below:

static int q6usb_component_probe(struct snd_soc_component *component)
{
        ...
        data->usb = snd_soc_usb_allocate_port(component, 1, &data->priv);
        if (!data->usb)
                return -ENOMEM;

        usb->connection_status_cb = q6usb_alsa_connection_cb;

        ret = snd_soc_usb_add_port(usb);
        if (ret < 0) {
                dev_err(component->dev, "failed to add usb port\n");
                goto free_usb;
        }
        ...
}

static void q6usb_component_remove(struct snd_soc_component *component)
{
        ...
        snd_soc_usb_remove_port(data->usb);
        snd_soc_usb_free_port(data->usb);
}

static const struct snd_soc_component_driver q6usb_dai_component = {
        .probe = q6usb_component_probe,
        .remove = q6usb_component_remove,
        .name = "q6usb-dai-component",
        ...
};

BE DAI links can pass along vendor specific information as part of the call to allocate the SoC USB device. This will allow any BE DAI link parameters or settings to be accessed by the USB offload driver that resides in USB SND.

USB Audio Device Connection Flow

USB devices can be hotplugged into the USB ports at any point in time. The BE DAI link should be aware of the current state of the physical USB port, i.e. if there are any USB devices with audio interface(s) connected. connection_status_cb() can be used to notify the BE DAI link of any change.

This is called whenever there is a USB SND interface bind or remove event, using snd_soc_usb_connect() or snd_soc_usb_disconnect():

static void qc_usb_audio_offload_probe(struct snd_usb_audio *chip)
{
        ...
        snd_soc_usb_connect(usb_get_usb_backend(udev), sdev);
        ...
}

static void qc_usb_audio_offload_disconnect(struct snd_usb_audio *chip)
{
        ...
        snd_soc_usb_disconnect(usb_get_usb_backend(chip->dev), dev->sdev);
        ...
}

In order to account for conditions where driver or device existence is not guaranteed, USB SND exposes snd_usb_rediscover_devices() to resend the connect events for any identified USB audio interfaces. Consider the the following situation:

usb_audio_probe()
--> USB audio streams allocated and saved to usb_chip[]
--> Propagate connect event to USB offload driver in USB SND
--> snd_soc_usb_connect() exits as USB BE DAI link is not ready
BE DAI link component probe
--> DAI link is probed and SoC USB port is allocated
--> The USB audio device connect event is missed

To ensure connection events are not missed, snd_usb_rediscover_devices() is executed when the SoC USB device is registered. Now, when the BE DAI link component probe occurs, the following highlights the sequence:

BE DAI link component probe
--> DAI link is probed and SoC USB port is allocated
--> SoC USB device added, and snd_usb_rediscover_devices() runs
snd_usb_rediscover_devices()
--> Traverses through usb_chip[] and for non-NULL entries issue
connection_status_cb()

In the case where the USB offload driver is unbound, while USB SND is ready, the snd_usb_rediscover_devices() is called during module init. This allows for the offloading path to also be enabled with the following flow:

usb_audio_probe()
--> USB audio streams allocated and saved to usb_chip[]
--> Propagate connect event to USB offload driver in USB SND
--> USB offload driver NOT ready!
BE DAI link component probe
--> DAI link is probed and SoC USB port is allocated
--> No USB connect event due to missing USB offload driver
USB offload driver probe
--> qc_usb_audio_offload_init()
--> Calls snd_usb_rediscover_devices() to notify of devices