Re: [RFC PATCH v2 07/22] ASoC: Add SOC USB APIs for adding an USB backend

[Pierre-Louis Bossart]

On 1/25/23 21:14, Wesley Cheng wrote:
> Some platforms may want to register its USB port to be handled by the ASoC
> framework.  Audio playback/capture support is also handled entirely by the
> vendor ASoC drivers.

Can you clarify what you mean by 'port'?

> +/**
> + * snd_soc_usb_add_port() - Add a USB backend port
> + * @dev: USB backend device
> + * @connection_cb: connection status callback
> + *
> + * Register a USB backend device to the SND USB SOC framework.  Memory is
> + * allocated as part of the USB backend device.
> + *
> + */
> +struct snd_soc_usb *snd_soc_usb_add_port(struct device *dev,
> +			int (*connection_cb)(struct snd_soc_usb *usb, int card_idx,
> +			int connected))
> +{
> +	struct snd_soc_usb *usb;
> +
> +	usb = devm_kzalloc(dev, sizeof(*usb), GFP_KERNEL);
> +	if (!usb)
> +		return ERR_PTR(-ENOMEM);
> +
> +	usb->connection_status_cb = connection_cb;
> +	usb->dev = dev;
> +
> +	mutex_lock(&ctx_mutex);
> +	list_add_tail(&usb->list, &usb_ctx_list);
> +	mutex_unlock(&ctx_mutex);
> +
> +	return usb;
> +}
> +EXPORT_SYMBOL_GPL(snd_soc_usb_add_port);

Can a backend have more than one ports?

Is there any relationship between port and USB endpoint, and if yes
where is this determined?

> +
> +/**
> + * snd_soc_usb_remove_port() - Remove a USB backend port
> + * @dev: USB backend device
> + *
> + * Remove a USB backend device from USB SND SOC.  Memory is freed when USB
> + * backend is removed.
> + *
> + */
> +int snd_soc_usb_remove_port(struct device *dev)
> +{
> +	struct snd_soc_usb *ctx, *tmp;
> +
> +	mutex_lock(&ctx_mutex);
> +	list_for_each_entry_safe(ctx, tmp, &usb_ctx_list, list) {
> +		if (ctx->dev == dev) {
> +			list_del(&ctx->list);
> +			break;
> +		}
> +	}
> +	mutex_unlock(&ctx_mutex);
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(snd_soc_usb_remove_port);

What happens if the ASoC driver probes/initialize AFTER the USB device
is plugged?

Likewise, can the ASoC driver be removed 'safely' with a fallback to
normal non-offloaded operation happening on remove?

> +
> +/**
> + * snd_soc_usb_connect() - Notification of USB device connection
> + * @usbdev: USB bus device
> + * @card_idx: USB SND card instance
> + *
> + * Notify of a new USB SND device connection.  The card_idx can be used to
> + * handle how the USB backend selects, which device to enable offloading on.
> + *
> + */
> +int snd_soc_usb_connect(struct device *usbdev, int card_idx)
> +{
> +	struct snd_soc_usb *ctx;
> +
> +	if (!usbdev)
> +		return -ENODEV;
> +
> +	ctx = snd_soc_find_usb_ctx(usbdev);
> +	if (!ctx)
> +		return -ENODEV;
> +
> +	if (ctx->connection_status_cb)
> +		ctx->connection_status_cb(ctx, card_idx, 1);
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(snd_soc_usb_connect);
> +
> +/**
> + * snd_soc_usb_connect() - Notification of USB device connection
> + * @usbdev: USB bus device
> + *
> + * Notify of a new USB SND device disconnection to the USB backend.
> + *
> + */
> +int snd_soc_usb_disconnect(struct device *usbdev)
> +{
> +	struct snd_soc_usb *ctx;
> +
> +	if (!usbdev)
> +		return -ENODEV;
> +
> +	ctx = snd_soc_find_usb_ctx(usbdev);
> +	if (!ctx)
> +		return -ENODEV;
> +
> +	if (ctx->connection_status_cb)
> +		ctx->connection_status_cb(ctx, -1, 0);
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(snd_soc_usb_disconnect);

Similar concern on connect/disconnect, does this assume any specific
order for the driver probe?