Re: [edk2-devel] EFI_AUDIO_OUTPUT_PROTOCOL: assistance with VirtIO initialization

["Michael Brown" <mcb30@ipxe.org>]

On 02/07/2021 10:41, Michael Brown wrote:
> UsbIo->UsbControlTransfer(UsbIo, &Req, EfiUsbDataIn,
>                            PcdGet32 (PcdUsbTransferTimeoutValue),
>                            &Header, sizeof(Header), &Status);
> 
> (Error handling etc omitted for brevity)
> 
> That would get you the first 8 bytes of the class-specific AC interface 
> header descriptor.  You would then need to extract the TotalLength 
> field, allocate that length of memory, and repeat the 
> UsbControlTransfer() call to fetch the full-length descriptor into the 
> newly allocated block.
> 
> Hope that helps,

BTW, in case you aren't already aware of this: wireshark is pretty good 
at dissecting USB traffic.  You can capture it by doing a "modprobe 
usbmon", after which you'll see a number of usbmonN devices show up in 
the wireshark interface list.  Try them each in turn until you find 
which one corresponds to the host controller to which your device is 
attached.

My normal method for developing or debugging iPXE USB drivers will 
typically involve using wireshark to capture the USB traffic that 
happens when the device is being used by a known-working driver (e.g. 
the Linux driver for that device) and comparing it to the traffic that 
happens when I'm using my own driver (via USB pass-through in a VM). 
This is often a lot faster than trying to pull together all of the 
information from the multiple USB spec documents.

Good luck!

Michael