Hi Pierre,
On 2/13/2023 7:22 AM, Pierre-Louis Bossart wrote:
On 2/11/23 03:52, Wesley Cheng wrote:
Hi Pierre,
On 2/7/2023 5:29 AM, Pierre-Louis Bossart wrote:
On 2/6/23 19:15, Wesley Cheng wrote:
Hi Pierre,
On 1/26/2023 8:12 AM, Pierre-Louis Bossart wrote:
On 1/25/23 21:14, Wesley Cheng wrote:
With USB audio offloading, an audio session is started from the ASoC
platform sound card and PCM devices. Likewise, the USB SND path is
still
readily available for use, in case the non-offload path is
desired. In
order to prevent the two entities from attempting to use the USB bus,
introduce a flag that determines when either paths are in use.
If a PCM device is already in use, the check will return an error to
userspace notifying that the stream is currently busy. This ensures
that
only one path is using the USB substream.
It's good to maintain mutual exclusion, but it's still very hard for an
application to figure out which card can be used when.
Returning -EBUSY is not super helpful. There should be something like a
notification or connection status so that routing decisions can be made
without trial-and-error.
The USB offload driver does have access to the USB substream that is
being utilized/offloaded. Maybe in addition to this check, we can also
set the PCM runtime state as well (for that particular substream)? That
way userspace can fetch information about if the stream is busy or not.
You're missing the point. When a card is exposed but the PCM devices may
or may not be usable (consuming data with no sound rendered or returning
an error), it's much better to provide a clear connection status to
userspace.
Let me give you an example. Intel drivers can expose 3 HDMI/DP PCM
devices. Userspace has no idea which one to use, so there's a jack
control that tells userspace whether there is a receiver connected so
that the audio server can use the relevant PCM device.
Audio routing based on trial and error is really problematic, errors can
happen but they should be exceptional (e.g. xruns), not a means of
driver-userspace communication on the device status.
Thanks for clarifying. The example helped me understand a bit more on
how the potential use of the SND control interface. Since we're dealing
with multiple sound cards here (platform sound card (offload) and USB
SND card (legacy)), what do you think about creating a SND control on
both the USB backend (platform card) and the USB SND card listing the
PCM device status?
That way at least userspace can have the information about which PCM dev
(USB substream) is available (and not offloaded, or vice versa). So the
USB SND control will contain the PCM devices (exposed by the card) and
if any are offloaded (if so mark them as unavailable). Likewise, for
the USB backend, if the legacy path is being used, mark them as
unavailable for offloading.
We definitively need a control to indicate that a PCM offload device is
available or not.
There's still a very large open with the notion of having separate cards
for the same audio device. Not only would it duplicate the control parts
for e.g. volume control, but it would introduce the need to tag devices
across two cards are being the same physical device.
The volume control would still be done through the card that is exposed
by the USB SND card (even for the offload path)[no vol control option
for the USB device on the platform card].
In the last discussion, you did mention that maybe we can tag the
offload path as the "power saving" option for a particular USB stream.
Although I'm not sure how intricate the logic is, but if userspace marks
to use the power saving path, then would it already know which card and
PCM devices are involved?
Although, that part is missing, ie to select the card and pcm device
that we want to offload. It may be possible to do this with another
control on the USB ASoC backend driver. I believe the audio DSP can
support device selection.
I still think the least-bad option is to have a single card and an
optional PCM device for offload.
This is most likely the end goal, but as mentioned previously, its going
to be a large effort to slowly decouple some of the PCM related
operations from USB SND. IMO, that would most likely be another
significant patch series in itself.
Thanks
Wesley Cheng
