Re: Passive scanning of iBeacons results in a "Data Buffer Overflow"

[Terry Hardie <terryh@orcas.net>]

On 2014-03-10 06:21, Anderson Lizardo wrote:
> Hi Terry,
> 
> On Sat, Mar 8, 2014 at 3:19 AM, Terry Hardie <terryh@orcas.net> wrote:
>> The problem is when running a lescan (hcitool lescan) with a LE device 
>> in
>> paring mode, which is transmitting a lot of LE Advertising report 
>> packets,
>> the HCI drivers eventually loses sync. I've traced it down to a 
>> duplicate
>> USB fragment.
> 
> I think your logs show missing USB packets (not duplicated)... More
> details below.

Interesting that it always loses 2 packets, and never 1, that's why I 
assumed duplication...

> In summary, it is expected that for devices that don't change address,
> the only byte to change is the RSSI (last byte of the last fragment).
> That's why the 2 first fragments are usually the same for all
> advertising packets.

You asked in another email if I tried a powered hub. Yes, I tried a 
powered hub. I get the same results.

Also, although it's not common, I also see these lost frames outside of 
a scan. A scan is just the easiest way to reproduce it. If I leave a tag 
broadcasting once per second, 2 receivers on the same USB bus, after an 
hour or so, the same problem happens (and this is only 1 broadcast per 
second from the PROX tag, received by 2 receivers simultaneously, 
resulting in each receiver sending 3 USB transfers to the host - 6 USB 
transfer total), so this is not just limited to high packet rates. I 
think there's some problem with USB bus contention, especially since 
both receivers try to send data to the host at EXACTLY the same time.

Terry