31 Aug
2005
31 Aug
'05
9:57 a.m.
"CLASS" means that the meaning of the
bRequest field
is class-specific
-- in this case, defined in the USB audio
specification.
Aha.
To determine what the other fields mean, get the
unit ID from the
upper byte of the wIndex field (sixth byte in the
setup packet),
search for the unit in the descriptors, and then
look up that unit
type in the USB audio spec.
OK - its starting to make sense.
Can I send it
using CONTROL_TRANSFER URBs or is
yet another
extension of the alsa driver necessary ?
With USB audio devices, all transfers (except the
audio data itself)
are control transfers. SetupPacket:
0000: 21 01 00 01 00 04 01 00
bRequestType: 0x21 CLASS_INTERFACE, out
bRequest: 0x01 SET_CUR
wValue: 0x0100 control/channel? #1 (MSB) of
this unit
wIndex: 0x0400 interface #0 (LSB), unit #4
(MSB)
wLength: 0x0001
I guess that unit 4 is a Selector Unit, because most
other types would require a two-byte buffer.