Reasons not to enable CONFIG_BT_HCI_ACL_FLOW_CONTROL

RE: Reasons not to enable CONFIG_BT_HCI_ACL_FLOW_CONTROL

Vidar Berg — Wed, 18 Mar 2026 07:19:11 GMT

Choden , please create a new thread and try to include as much relevant details as possible about your setup. For example, what SDK versions,etc and why you suspect the wd timeout is caused by a deadlock in the bt host.

RE: Reasons not to enable CONFIG_BT_HCI_ACL_FLOW_CONTROL

Choden — Wed, 18 Mar 2026 07:15:41 GMT

I’m having a similar problem with the nRF52832. I’m running NUS in dual-role (central + peripheral) mode, and it randomly resets due to the watchdog—sometimes after a few minutes, sometimes after several hours or even 10+ hours.

RE: Reasons not to enable CONFIG_BT_HCI_ACL_FLOW_CONTROL

BertL — Fri, 29 Aug 2025 11:55:22 GMT

Okay, the default for CONFIG_BT_BUF_ACL_TX_COUNT seems to be 3. So my configuration should work then.

I will let you know the results!

RE: Reasons not to enable CONFIG_BT_HCI_ACL_FLOW_CONTROL

Vidar Berg — Fri, 29 Aug 2025 11:37:02 GMT

Sorry, It was supposed to be CONFIG_BT_BUF_ACL_TX_COUNT not CONFIG_BT_BUF_EVT_TX_COUNT. I've edited my first post.

RE: Reasons not to enable CONFIG_BT_HCI_ACL_FLOW_CONTROL

BertL — Fri, 29 Aug 2025 11:32:32 GMT

Okay clear! We have thousands of devices and only see the reset for a couple of them (and only sometimes), so it is really rare in our situation as well. Let's hope it is indeed this deadlock and fixed by the workaround. It is always nice if there are no unexpected behaviors anymore. ;)

CONFIG_BT_BUF_EVT_TX_COUNT does not exist by the way. Do you mean CONFIG_BT_BUF_CMD_TX_COUNT?

We now use this config:

CONFIG_BT_HCI_ACL_FLOW_CONTROL=y

CONFIG_BT_BUF_CMD_TX_COUNT=10

CONFIG_BT_BUF_EVT_RX_COUNT=11

Let us know if this is indeed correct.

RE: Reasons not to enable CONFIG_BT_HCI_ACL_FLOW_CONTROL

Vidar Berg — Fri, 29 Aug 2025 11:26:39 GMT

[quote userid="72920" url="~/f/nordic-q-a/123994/reasons-not-to-enable-config_bt_hci_acl_flow_control/547185"]So it is indeed possible that this deadlock occurs on a single core nrf52840?[/quote]

The developers confirmed that it can occur in certain scenarios, but I have not experienced it myself, so I would consider it rare. If it can happen or not depends on the application and the configuration.

Please let me know how the testing goes.

Best regards,

Vidar

RE: Reasons not to enable CONFIG_BT_HCI_ACL_FLOW_CONTROL

BertL — Fri, 29 Aug 2025 11:23:46 GMT

Hi Vidar, thanks for your useful response!

So it is indeed possible that this deadlock occurs on a single core nrf52840?

Either way, we will implement your suggestions and do some testing.

RE: Reasons not to enable CONFIG_BT_HCI_ACL_FLOW_CONTROL

Vidar Berg — Fri, 29 Aug 2025 11:16:17 GMT

Hello,

Enabling CONFIG_BT_HCI_ACL_FLOW_CONTROL may increase RAM usage because buffer requirements (CMD, ACL, and EVT buffers) are calculated differently with this option enabled (you should get a build assert if your buffers need to be adjusted)

Suggestions:

- Enable CONFIG_BT_HCI_ACL_FLOW_CONTROL and adjust buffer settings according to the build error if an assert is raised.

- Increase CONFIG_BT_BUF_EVT_RX_COUNT and ensure it is set to a value at least '1' more than the TX buffer count (CONFIG_BT_BUF_EVT_RX_COUNT > CONFIG_BT_BUF_EVT_TX_COUNT CONFIG_BT_BUF_ACL_TX_COUNT + 1).

This will hopefully resolve the problem you are experiencing. We are actively working on mitigating the risk of deadlocks in the stack.

Best regards,

Vidar