Re: [Bluez-devel] Questions about correctness of hci_usb sco support.

[Marcel Holtmann <marcel@holtmann.org>]

Hi James,

> In hci_usb.c file, line 604 ish
> static inline int __recv_frame(struct hci_usb *husb, int type, void 
> *data, int count)
> 
> Contains: -
> case HCI_SCODATA_PKT:
>         if (count >= HCI_SCO_HDR_SIZE) {
>                   struct hci_sco_hdr *h = data;
>                   len = HCI_SCO_HDR_SIZE + h->dlen;
>         } else
>                   return -EILSEQ;
>         break;
> 
> With a SCO HCI packet, it lasts 3 air frames for a bluetooth headset.
> How do we know that the first frame we receive from the usb bluetooth 
> device is the SCO HCI header?

	if (!skb) {
		/* Start of the frame */

> What happens if the first SCO HCI frame we receive is actually the 
> second or third frame in the SCO HCI packet ?
> Surely some validation checks need to be done.
> For example, depending on the sample format we are using, we should 
> already know what the SCO HCI length should be, so we could check this 
> against the length in the SCO HCI header, and only accept the frame if 
> they match, if they don't match, drop the frame, and wait for the next 
> frame.
> I would expect similar problems with HCI int/bulk frames, but I don't 
> actually see any corrupt int/bulk frames, so I was wondering whether the 
> usb bluetooth dongle somehow ensures that the first air frame we receive 
> is actually the start of an HCI frame. Maybe it is just luck, as 
> int/bulk frames normally have a lot of blank invalid frames in between, 
> so maybe as soon as it sees a valid frame, it is always the start of the 
> int/bulk frame. I don't think we can make this assumtion all the time, 
> in case we start filling the air entirely with bulk frames, and some air 
> frames get lost. We will have to drop the hci frame, and then resync 
> when the next hci frame arrives.

I actually don't get your point, because the USB INT, BULK and ISOC
URB's has nothing to do with the frames on the air. It is the HCI of the
Bluetooth chip.

Regards

Marcel




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