On 18 May 2004, at 11:34, Lars Grunewaldt wrote:
there's a kernel patch in snd-bt-sco, and it would be great if someone could explain what it does:
***** - --- linux-2.6.4-orig/drivers/bluetooth/hci_usb.c 2004-03-11 02:55:44.000000000 +0000 +++ linux-2.6.4/drivers/bluetooth/hci_usb.c 2004-03-13 23:52:27.000000000 +0000 @@ -851,7 +851,12 @@
~ #ifdef CONFIG_BT_HCIUSB_SCO ~ case USB_ENDPOINT_XFER_ISOC: - - if (ep->desc.wMaxPacketSize < size || a > 2) + /* Use only the 9 byte + "One voice channel with 8 bit encoding" + endpoint until there is support for changing + the endpoint dynamically. See + Bluetooth 1.1 Part H:2, section 2.1 */ + if (ep->desc.wMaxPacketSize != 9) ~ break; ~ size = ep->desc.wMaxPacketSize; ****
There are different voice encodings over the isochronous USB channel that is used for the SCO connection. Which encoding is used is selected by running 'hciconfig hci0 voice <somenumber>', where <somenumber> is, for example, 0x0040 for one 8 bit 8000Hz stream and 0x0060 for one 16 bit 8000Hz stream.
Depending on how many SCO channels are open and what format is used, different isoc USB frames are used by the bluetooth device. The possibilities are enumerated in the documentation that is referenced in that comment.
What it boils down to is that for one 8000Hz 8 bit channel, the USB packets are 9 bytes each. That check ensures that the appropriate endpoint is selected. Otherwise, the wrong endpoint probably gets chosen and the audio doesn't work at all or it sounds very bad.
The isoc streams ideally would be dynamically selected as required, depending on the type and number of SCO connections. Since there wasn't infrastructure in place when I wrote that patch, I hard-coded that choice to coincide with that required by the alsa driver.
Hope that sheds some light on the matter!
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