Hi Pierre,
On 1/31/2023 7:02 PM, Pierre-Louis Bossart wrote:
On 1/31/23 20:40, Wesley Cheng wrote:
Hi Pierre,
On 1/30/2023 3:59 PM, Pierre-Louis Bossart wrote:
On 1/30/23 16:54, Wesley Cheng wrote:
Hi Pierre,
On 1/26/2023 7:38 AM, Pierre-Louis Bossart wrote:
On 1/25/23 21:14, Wesley Cheng wrote:
The QC ADSP is able to support USB playback endpoints, so that the
main
application processor can be placed into lower CPU power modes. This
adds
the required AFE port configurations and port start command to
start an
audio session.
Specifically, the QC ADSP can support all potential endpoints that are
exposed by the audio data interface. This includes, feedback
endpoints
(both implicit and explicit) as well as the isochronous (data)
endpoints.
The size of audio samples sent per USB frame (microframe) will be
adjusted
based on information received on the feedback endpoint.
I think you meant "support all potential endpoint types"
It's likely that some USB devices have more endpoints than what the DSP
can handle, no?
True, as we discussed before, we only handle the endpoints for the audio
interface. Other endpoints, such as HID, or control is still handled by
the main processor.
The number of isoc/audio endpoints can be larger than 1 per direction,
it's not uncommon for a USB device to have multiple connectors on the
front side for instruments, mics, monitor speakers, you name it. Just
google 'motu' or 'rme usb' and you'll see examples of USB devices that
are very different from plain vanilla headsets.
Thanks for the reference.
I tried to do some research on the RME USB audio devices, and they
mentioned that they do have a "class compliant mode," which is for
compatibility w/ Linux hosts. I didn't see a vendor specific USB SND
driver matching the USB VID/PID either, so I am assuming that it uses
the USB SND driver as is.(and that Linux doesn't currently support their
vendor specific mode) In that case, the device should conform to the
UAC2.0 spec (same statement seen on UAC3.0), which states in Section
4.9.1 Standard AS Interface Descriptor Table 4-26:
"4 bNumEndpoints 1 Number Number of endpoints used by this
interface (excluding endpoint 0). Must be
either 0 (no data endpoint), 1 (data
endpoint) or 2 (data and explicit feedback
endpoint)."
So each audio streaming interface should only have 1 data and
potentially 1 feedback. However, this device does expose a large number
of channels (I saw up to 18 channels), which the USB backend won't be
able to support. I still need to check how ASoC behaves if I pass in a
profile that the backend can't support.
Maybe in the non-class compliant/vendor based class driver, they have
the support for multiple EPs per data interface? I don't have one of
these devices on hand, so I can't confirm that.
Look at Figure 3-1 in the UAC2 spec, it shows it's perfectly legal to
have multiple Audio Streaming interfaces - but one Audio Control
interface only.
The fact that there is a restriction to 1 or 2 endpoints per Audio
Streaming interface does not really matter if in the end there are
multiple endpoints and concurrent isoc transfers happening to/from the
same USB device.
So the reason I wanted to mention the max number of EPs within the audio
streaming descriptor is because the USB SND driver currently creates
streams based off of the number of AS desc:
static int snd_usb_create_streams(struct snd_usb_audio *chip, int ctrlif)
{
...
for (i = 0; i < assoc->bInterfaceCount; i++) {
int intf = assoc->bFirstInterface + i;
if (intf != ctrlif)
snd_usb_create_stream(chip, ctrlif, intf);
}
"assoc" is the audio control interface desc. In the end, when userspace
initiates a playback session, it operates on the streams created (which
contains at max 1 isoc and 1 feedback ep)
In short, the audio DSP doesn't need to consider handling more than 1
isoc ep (and potentially 1 feedback). I believe that each audio stream
creates a separate PCM device, so userspace is still free to attempt to
activate another audio stream. I believe # of PCM devices created
matches the # of streams, so when userspace does activate another
session, it would be on an entirely different substream, and can be
handled through the USB SND (non-offload) path. If attempted to open
the substream used by the offload path, then we would reject is based on
the new change.
Thanks
Wesley Cheng
