Current UAC2 gadget implements capture/sync paths as two USB ISO ASYNC endpoints (IN and OUT). This violates USB spec which says that ISO ASYNC OUT endpoint should have feedback companion endpoint. See USB2.0 spec "5.12.4.1 Synchronization Type": asynchronous sink provides explicit feedback (isochronous pipe). Interesting that for ISO ASYNC *IN* endpoint respective feedback isn't required since source provides implicit feedforward (data stream). While it's not an issue if UAC2 Gadget is connected to Linux host (Linux ignores missing feedback endpoint), with other hosts like Windows or MacOS the UAC2 Gadget isn't enumerated due to missing feedback endpoint. This patch series adds feedback endpoint support to UAC2 function, new control to UAC2 mixer which can be used by userspace tools (like alsaloop from alsa-utils) for updating feedback frequency reported to the host. This is useful for usecases when UAC2 Gadget's audio samples are played to another codec or audio card with its own internal freerunning clock so host can be notified that more/less samples are required. The alsaloop tool requires some (relatively small) modifications in order to start support driving feedback frequency through UAC2 mixer control. That change will be sent as a separate patch to ALSA community. Also added ability to switch ISO ASYNC OUT endpoint into adaptive endpoint which doesn't require feedback endpoint (as per USB spec). Ruslan Bilovol (3): usb: gadget: f_uac2/u_audio: add feedback endpoint support usb: gadget: f_uac2: add adaptive sync support for capture usb: gadget: u_audio: add real feedback implementation Documentation/ABI/testing/configfs-usb-gadget-uac2 | 1 + Documentation/usb/gadget-testing.rst | 1 + drivers/usb/gadget/function/f_uac2.c | 118 ++++++++++- drivers/usb/gadget/function/u_audio.c | 217 ++++++++++++++++++++- drivers/usb/gadget/function/u_audio.h | 10 + drivers/usb/gadget/function/u_uac2.h | 2 + 6 files changed, 345 insertions(+), 4 deletions(-) -- 1.9.1
