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Support for Data Length Extension (DLE)?

DISCUSSION Christian Eggers 2 participants 2 messages Oct 17, 2022 Bluetooth drivers

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Christian Eggers

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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:

Quoted 1 line

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
...

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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)

...

Quoted 1 line

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

maintainer
#1 lore raw

Hi Christian,

Quoted 36 lines · Christian Eggers

When monitoring a Bluetooth connection between two Linux systems (kernel 5.14 / 5.15 + bluez-5.65), I noticed the following:

Quoted 1 line

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
...

Quoted 1 line

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)

...

Quoted 1 line

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
Quoted 2 lines

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?

We do programm the it earlier though with HCI_OP_LE_WRITE_DEF_DATA_LEN:

https://git.kernel.org/pub/scm/linux/kernel/git/bluetooth/bluetooth-next.git/tree/net/bluetooth/hci_sync.c#n4362

Quoted 1 line

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

I guess it would probably make sense to handle HCI_EV_LE_DATA_LEN_CHANGE, that said it seems to come after the L2CAP Connection Request in case you are suggesting to align the MTU with Max TX/RX length.

Quoted 5 lines

regards
Christian

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