Re: [Bluez-devel] [PATCH] Updated sco flow control feature

[Jose Vasconcellos <jose@vasmac.com>]

Fabien Chevalier wrote:
> Hello Jose,
>
>   
>> Hi, Fabien,
>>
>> I'll take a look at your patch. I've been working on one myself.
>> As I mentioned earlier, I've taken a different approach that works
>> well. I use the standard HCI synchronous flow control. Now since
>> USB dongles don't generate this, I have the hci_usb_tx_complete
>> generate a HCI_EV_NUM_COMP_PKTS for transfer. Then the existing
>> system works fine. No timers are necessary. Only a small change
>> to hci_sched_sco in hci_core.c was necessary.
>>     
>
> Yes this will work. The only issue i see with this approach is that it
> is hci USB centric. This works with USB because there is a dedicated USB
> channel to send SCO packets, and the bandwidth of this channel is
> precisely the one of the SCO stream. So when the USB controller
> acknowledges the transmitted packet, you know you can precisely send the
> next one, which is great. :-)
>
> Now, for RS232 hci devices, there is only one physical channel, the
> RS232 link, which is shared for SCO/ACL/CMD packets. The bandwidth is
> typically as high as 921 kbit/s. This means when the R232 controller
> acknowledges the SCO packet, you have to wait a bit to send the next SCO
> packet, and thus rely a way or another on a timer. It's too bad, but i
> see no other way for now :-(
>
> Thus, if we take into account the fact completed packets events are not
> send by bt controllers, and that the flow control stuff should work
> whatever controller driver is, it think we have to rely on timers ;-)
>
>   
Hi Fabien,

So your issue is supporting synchronous channels on the UART
interface. I think it's best to enable synchronous flow control.
and have the hci_uart manage the transmit queue to minimize delay.
This means that hci_uart can peek at the HCI_EV_NUM_COMP_PKTS
received to determine if another synchronous packet needs
to be sent.

Regards,

Jose

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