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From:   Luiz Augusto von Dentz <luiz.dentz@gmail.com>
Date:   Mon, 17 Oct 2022 13:11:15 -0700
Message-ID: <CABBYNZKmKz40WAD9fZ-UixDGe7isQxmrrf_7L54yRrLm6P-0XQ@mail.gmail.com>
Subject: Re: Support for Data Length Extension (DLE)?
To:     Christian Eggers <ceggers@arri.de>
Cc:     linux-bluetooth@vger.kernel.org
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Hi Christian,

On Mon, Oct 17, 2022 at 7:57 AM Christian Eggers <ceggers@arri.de> wrote:
>
> 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?

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

> 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.

> regards
> Christian
>
>
>


-- 
Luiz Augusto von Dentz