From:   Iulia Tanasescu <iulia.tanasescu@nxp.com>
To:     "linux-bluetooth@vger.kernel.org" <linux-bluetooth@vger.kernel.org>
Subject: ISO socket bugs/lack of support
Thread-Topic: ISO socket bugs/lack of support
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Date:   Fri, 3 Mar 2023 14:52:07 +0000
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Hello,

Earlier this week I submitted a BlueZ patch ([1]) containing an initial imp=
lementation of the Broadcast Audio Scan Service.

In order to perform some of the BASS procedures, the BASS Server is require=
d to send specific HCI commands and to handle HCI events accordingly - for =
example, the "Set Broadcast_Code" operation of the Broadcast Audio Scan Con=
trol Point characteristic requires the BASS Server to issue the LE BIG Crea=
te Sync command with the Broadcast_Code parameter set to the characteristic=
 value written by the BASS Client.

My approach was to open an HCI socket and to send the command directly from=
 the BASS handler function. After my proposed implementation was reviewed, =
I was told I should use an ISO socket instead and let the Bluetooth kernel =
perform the BIG synchronization procedure. I investigated this solution and=
 the problem that I encountered was that the kernel sends HCI commands with=
 fixed parameters - calling the "listen" API on an ISO socket will always a=
ttempt to send the LE BIG Create Sync command with the Broadcast_Code param=
eter set to 0. I couldn't find a way to provide my desired parameters to th=
e HCI command.

I also discovered 2 kernel issues when calling "listen" on an ISO socket:

The first issue is that the "hci_pa_create_sync" function attempts to send =
the LE Periodic Advertising Create Sync command with the Sync_Timeout param=
eter set to 0 - according to the Bluetooth Core specification version 5.3, =
this parameter should be in the 0x000A - 0x4000 range. This is why the Cont=
roller will reject this command, as shown in the following btmon capture:


< HCI Command: LE Periodic Advertising Create Sync (0x08|0x0044) plen 14
        Options: 0x0000
        Use advertising SID, Advertiser Address Type and address
        Reporting initially enabled
        SID: 0x01
        Adv address type: Public (0x00)
        Adv address: C0:07:E8:8B:69:F2 (OUI C0-07-E8)
        Skip: 0x0000
        Sync timeout: 0 msec (0x0000)
        Sync CTE type: 0x0000

> HCI Event: Command Status (0x0f) plen 4
      LE Periodic Advertising Create Sync (0x08|0x0044) ncmd 1
        Status: Invalid HCI Command Parameters (0x12)

Updating the "hci_pa_create_sync" function and setting the "sync_timeout" f=
ield of the command parameters to a value in the required range solves the =
problem.

The same issue appears when the kernel attempts to send the LE BIG Create S=
ync command with the BIG_Sync_Timeout parameter set to 0:


< HCI Command: LE Broadcast Isochronous Group Create Sync (0x08|0x006b) ple=
n 25
        BIG Handle: 0x00
        BIG Sync Handle: 0x0000
        Encryption: Encrypted (0x00)
        Broadcast Code: 00000000000000000000000000000000
        Maximum Number Subevents: 0x00
        Timeout: 0 ms (0x0000)
        Number of BIS: 1
        BIS ID: 0x01

> HCI Event: Command Status (0x0f) plen 4
      LE Broadcast Isochronous Group Create Sync (0x08|0x006b) ncmd 1
        Status: Parameter Out Of Manadatory Range (0x30)

Updating the "hci_le_big_create_sync" function and setting the "timeout" fi=
eld of the command parameters to a valid value solves the issue.

Hopefully someone can confirm the kernel issues I described and the fixes c=
an be committed. I would also like to ask for some guidance regarding the I=
SO socket usage when it comes to personalizing HCI command parameters - is =
there a way to set the parameters from BlueZ? Or should this support be imp=
lemented in the Bluetooth kernel?

Thank you in advance

Iulia

[1]	[BlueZ,0/2] Add initial BASS support - Patchwork (kernel.org)