EFI_AUDIO_OUTPUT_PROTOCOL: assistance with VirtIO initialization

["Ethin Probst" <harlydavidsen@gmail.com>]

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Hi all,
So Leif and I have been working on USB Audio but we've run into a snag. We've encountered a problem -- neither of us knows enough about USB to figure out how to get the class-specific AC interface descriptors, and those contain vital information that I need to be able to control the audio device. The audio specification was quite useless. So I have two projects running in parallel: the USB audio one and the VirtIO audio (We managed to get the virtio-snd patchset in qemu and I've got a built version of it over here and, though its not a fully working implementation, as long as I get something -- no matter how bad it sounds -- I'll be happy). Whichever project we get working first is the one we move forward with.
The VirtIO sound device specification contains four virtqueues: the control queue for controlling the device; the event queue for receiving notifications from the device; the tx queue for transmitting audio data to the device; and the rx queue for receiving audio data from the device. Thus far I've been following the VirtioRngDxe code as a guide on how to get a VirtIO device initialized using the VirtioLib in OVMF, but I've reached an impasse regarding the queues and descriptors.

The VirtioRngDxe DXE uses a single descriptor and a single queue for operation. However, the RNG device works vastly differently to the audio device and is far simpler. The virtio-snd specification says that, to initialize the device, I must (copied from the spec):

* Configure the control, event, tx and rx queues.
* Read the jacks field and send a control request to query information about the available jacks.
* Read the streams field and send a control request to query information about the available PCM streams.
* Read the chmaps field and send a control request to query information about the available channel maps.
* Populate the event queue with empty buffers.

Steps 2, 3, 4, and 5 are a bit confusing and raise some questions:

1. Does the device automatically send notifications in the event queue about the jacks, streams, and channel maps, or do I have to explicitly ask for them?
2. How many buffers would we need to populate the event queue with, and what VirtioLib function (if any) specifically accomplishes that?

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