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Add note about bRefresh value

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Signed-off-by: HiFiPhile <admin@hifiphile.com>
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HiFiPhile
2025-10-01 21:21:04 +02:00
parent 1ee113e0e7
commit cd9266bf3d
1 changed files with 34 additions and 1 deletions
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35
examples/device/uac2_speaker_fb/src/usb_descriptors.h
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@ -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