diff --git a/examples/device/uac2_speaker_fb/src/usb_descriptors.h b/examples/device/uac2_speaker_fb/src/usb_descriptors.h index 4ae6ab616..b0ec60ea1 100644 --- a/examples/device/uac2_speaker_fb/src/usb_descriptors.h +++ b/examples/device/uac2_speaker_fb/src/usb_descriptors.h @@ -132,6 +132,39 @@ /* Class-Specific AS Isochronous Audio Data Endpoint Descriptor(4.6.1.2) */\ TUD_AUDIO10_DESC_CS_AS_ISO_EP(/*_attr*/ AUDIO10_CS_AS_ISO_DATA_EP_ATT_SAMPLING_FRQ, /*_lockdelayunits*/ AUDIO10_CS_AS_ISO_DATA_EP_LOCK_DELAY_UNIT_UNDEFINED, /*_lockdelay*/ 0x0000),\ /* Standard AS Isochronous Synch Endpoint Descriptor (4.6.2.1) */\ - TUD_AUDIO10_DESC_STD_AS_ISO_SYNC_EP(/*_ep*/ _epfb, /*_bRefresh*/ 4) + TUD_AUDIO10_DESC_STD_AS_ISO_SYNC_EP(/*_ep*/ _epfb, /*_bRefresh*/ 0) + +//---------------------------------------------------------------------------+ +// UAC1 Isochronous Synch Endpoint bRefresh Workaround +// +// bRefresh value is set to 0, while UAC1 spec requires it to be between +// 1 (2 ms) and 9 (512 ms) +// +// This value has been tested to work with Windows, macOS and Linux. +// +// Rationale: +// Some USB device controllers (e.g. Synopsys DWC2) require a known transfer +// interval to manually schedule isochronous IN transfers. For data isochronous +// endpoints, the bInterval field in the endpoint descriptor is used. However, +// for synch endpoint it's unclear which field the host uses to determine the +// transfer interval. Windows and macOS use bRefresh, while Linux uses bInterval. +// +// Since bInterval is fixed to 1, if bRefresh is set to 2 then Windows and macOS +// will schedule the feedback transfer every 4 ms, but Linux will schedule it +// every 1 ms. DWC2 controller cannot handle this discrepancy without knwowing +// the actual interval, therefore we set bRefresh to 0 to let the transfer +// execute every 1 ms, which is the same as bInterval. +// +// Rant: +// WTF USB-IF? Why have two fields that mean the same thing? Why not just use +// bInterval for both data and synch endpoints? Why is bRefresh even necessary? +// +// Note: +// For the moment DWC2 driver doesn't have proper support for bInterval > 1 +// for isochronous IN endpoints. The implementation would be complex and CPU +// intensive (cfr. +// https://github.com/torvalds/linux/blob/master/drivers/usb/dwc2/gadget.c) +// It MAY work in some cases if you are lucky, but it's not guaranteed. +//---------------------------------------------------------------------------+ #endif