Re: [PATCH v24 12/34] ASoC: doc: Add documentation for SOC USB

Pierre-Louis Bossart
2024-08-01T09:11:59Z

> +
> +::
> +
> + 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 |<- |
> + |_________________________| |
> +

It wouldn't hurt to describe what you mean by 'port' in this diagram...

> +SOC USB driver
> +==============
> +Structures
> +----------
> +``struct snd_soc_usb``
> +
> + - ``list``: list head for SND SOC struct list
> + - ``component``: reference to ASoC component
> + - ``num_supported_streams``: number of supported concurrent sessions
> + - ``connection_status_cb``: callback to notify connection events
> + - ``get_offload_dev``: callback to fetch selected USB sound card/PCM device

I think you meant fetch offloaded sound card and PCM device information
for a given USB card:device pair?

> +Functions
> +---------
> +.. code-block:: rst
> +
> + const char *snd_soc_usb_get_components_tag(bool playback);
> +..
> +
> + - ``playback``: direction of audio stream

why not use the usual direction 0: playback and 1: capture?

> +
> +**snd_soc_usb_get_components_tag()** returns the tag used for describing if USB
> +offloading is supported for appending to a sound card's components string.

How does this work if the ASoC part is probe after the USB card? The
component string would be modified after the creation of the card?

A control is more dynamic by nature, not sure about this component
string. Jaroslav?

>
> +**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.
> +
> +.. code-block:: rst
> +
> + 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.

Not clear to me if the notion of 'port' helps, why not just
snd_soc_usb_add_device() and remove_device()?

> +
> +USB Offload Related Kcontrols
> +=============================
> +Details
> +-------
> +A set of kcontrols can be utilized by applications to help select the proper sound
> +devices to enable USB audio offloading. SOC USB exposes the get_offload_dev()
> +callback that designs can use to ensure that the proper indices are returned to the
> +application.
> +
> +Implementation
> +--------------
> +
> +**Example:**
> +
> + **Sound Cards**:
> +
> + ::
> +
> + 0 [SM8250MTPWCD938]: sm8250 - SM8250-MTP-WCD9380-WSA8810-VA-D
> + SM8250-MTP-WCD9380-WSA8810-VA-DMIC
> + 1 [Seri ]: USB-Audio - Plantronics Blackwire 3225 Seri
> + Plantronics Plantronics Blackwire 3225 Seri at usb-xhci-hcd.1.auto-1.1, full sp
> + 2 [C320M ]: USB-Audio - Plantronics C320-M
> + Plantronics Plantronics C320-M at usb-xhci-hcd.1.auto-1.2, full speed
> +
> + **USB Sound Card** - card#1:
> +
> + ::
> +
> + USB Offload Playback Route PCM#0 -1, -1 (range -1->255)
> +
> + **USB Sound Card** - card#2:
> +
> + ::
> +
> + USB Offload Playback Route PCM#0 0, 1 (range -1->255)
> +
> +The above example shows a scenario where the system has one ASoC platform card
> +(card#0) and two USB sound devices connected (card#1 and card#2). When reading
> +the available kcontrols for each USB audio device, the following kcontrol lists
> +the mapped offload path for the specific device:
> +
> + "USB Offload Playback Route#*"
> +
> +The kcontrol is indexed, because a USB audio device could potentially have
> +several PCM devices. The above kcontrols are defined as:
> +
> + - ``USB Offload Playback Route PCM`` **(R)**: Returns the ASoC platform sound
> + card and PCM device index. The output "0, 1" (card index, PCM device index)
> + signifies that there is an available offload path for the USB SND device
> + through card#0-PCM device#1. If "-1, -1" is seen, then no offload path is
> + available for the USB SND device.
> +
> +USB Offload Playback Route Kcontrol
> +-----------------------------------
> +In order to allow for vendor specific implementations on audio offloading device
> +selection, the SOC USB layer exposes the following:
> +
> +.. code-block:: rst
> +
> + int (*get_offload_dev)(struct snd_kcontrol *kcontrol,
> + struct snd_ctl_elem_value *ucontrol);
> +..
> +
> +These are specific for the **USB Offload Playback Route PCM#** kcontrol.
> +
> +When users issue get calls to the kcontrol, the registered SOC USB callbacks will
> +execute the registered function calls to the DPCM BE DAI link.

Oh man, now I get what 'get_offload_dev" means: it really means
"update_offload_info' or 'update_info_kcontrol".
The 'get' routines usually provide a handle on something to another part
of the kernel.
Not here, it's an update of something to be looked-up by userspace...

> +**Callback Registration:**
> +
> +.. code-block:: rst
> +
> + static int q6usb_component_probe(struct snd_soc_component *component)
> + {
> + ...
> + usb = snd_soc_usb_allocate_port(component, 1, &data->priv);
> + if (IS_ERR(usb))
> + return -ENOMEM;
> +
> + usb->connection_status_cb = q6usb_alsa_connection_cb;
> + usb->get_offload_dev = q6usb_get_offload_dev;
> +
> + ret = snd_soc_usb_add_port(usb);
> +..