[Bluez-devel] SCO packet processing

[Jose Vasconcellos <vasmac@gmail.com>]

Hello,

I've been studying the SCO packet processing to understand how it works.
There are a few things that I noticed that don't seem quite right. I'll 
attempt
to explain.

In the SCO transmit path, data gets put in a skb and then queued. There's
a tasklet that will dequeue and send the skb to the hci driver. This tasklet
is scheduled when data needs to be sent or when there's a HCI event
HCI_EV_NUM_COMP_PKTS. This works fine for an ACL link and it could work
for a non-USB SCO transport. However, for USB HCI, the SCO data is carried
by the isochronous transport that doesn't require explicit flow control. The
problem is that the current code (hci_sched_sco in hci_core.c) will send all
packets to the hci layer with no flow control. That explains why application
kludges have been introduced to avoid loosing packets.

Note that for non-USB links (i.e. UART), the current code is not correct
either as it doesn't enable HCI_EV_NUM_COMP_PKTS anywhere. Also,
the value of hdev->sco_cnt is never decremented.

Last May, Fabien Chevalier posted some patches to avoid this problem.
His approach of using timers didn't work for me but I think this is the 
wrong
way to go about it. What is needed is a proper flow control mechanism 
between
the bluetooth module and the hci layer for the case of USB HCI transport.
I'm thinking that a callback is necessary that is invoked by hci_usb to
kickstart the process.

I also have issues with hci_sched_sco. I think it should interleave skb from
different active SCO sockets. Currently, it sends too many skbs from the
same socket flooding the dongle's buffers and potentially starving other
SCO channels.

I realize SCO handling has not been a priority and it sort of works.
Is there much interest in getting it right?

Regards,

Jose

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