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From:   Christian Eggers <ceggers@arri.de>
To:     <linux-bluetooth@vger.kernel.org>
Subject: Support for Data Length Extension (DLE)?
Date:   Mon, 17 Oct 2022 16:54:04 +0200
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When monitoring a Bluetooth connection between two Linux systems
(kernel 5.14 / 5.15 + bluez-5.65), I noticed the following:

> HCI Event: LE Meta Event (0x3e) plen 12                                                                                                                          #21108 [hci0] 1650.255887
      LE Read Remote Used Features (0x04)
        Status: Success (0x00)
        Handle: 25 Address: 00:1A:7D:F0:21:40 (OUI 00-1A-7D)
        Features: 0xbd 0x5f 0x66 0x00 0x00 0x00 0x00 0x00
...
          LE Data Packet Length Extension
...

> ACL Data RX: Handle 25 flags 0x02 dlen 18                                                                                                                        #21109 [hci0] 1650.300224
      LE L2CAP: LE Connection Request (0x14) ident 1 len 10
        PSM: 128 (0x0080)
        Source CID: 64
        MTU: 672
        MPS: 23
        Credits: 30
< ACL Data TX: Handle 25 flags 0x00 dlen 18                                                                                                                        #21110 [hci0] 1650.300431
      LE L2CAP: LE Connection Response (0x15) ident 1 len 10
        Destination CID: 64
        MTU: 672
        MPS: 23
        Credits: 30
        Result: Connection successful (0x0000)
...
> HCI Event: LE Meta Event (0x3e) plen 11                                                                                                                          #21113 [hci0] 1650.345822
      LE Data Length Change (0x07)
        Handle: 25 Address: 00:1A:7D:F0:21:40 (OUI 00-1A-7D)
        Max TX octets: 251
        Max TX time: 2120
        Max RX octets: 251
        Max RX time: 2120

I can't find that HCI_EV_LE_DATA_LEN_CHANGE is implemented anywhere in the kernel nor in bluez userspace.
So why do I get this event? Is it generated by controller firmware itself?

I think that both adapters support DLE. Is there there already support for using it with Linux?

regards
Christian