Bluetooth: Disabling `BT_DEBUG_LOG` doesn't disable logs

[JordanYates]

Describe the bug
When CONFIG_BT_DEBUG_LOG is not enabled, the log level (CONFIG_BT_HCI_DRIVER_LOG_LEVEL) of Bluetooth drivers can actually increase.

Disabling CONFIG_BT_DEBUG_LOG results in BT_DEBUG not being defined, resulting in BT_HCI_DRIVER not being defined.

zephyr/subsys/bluetooth/common/Kconfig

Lines 382 to 385 in bc5fbc9

	module = BT_HCI_DRIVER
	legacy-debug-sym = BT_DEBUG_HCI_DRIVER
	module-str = "Bluetooth HCI driver"
	source "subsys/bluetooth/common/Kconfig.template.log_config_bt"

This logging level is used in quite a few locations, for example:

zephyr/subsys/bluetooth/controller/hci/hci.c

Lines 97 to 99 in bc5fbc9

	#define LOG_LEVEL CONFIG_BT_HCI_DRIVER_LOG_LEVEL
	#include <zephyr/logging/log.h>
	LOG_MODULE_REGISTER(bt_ctlr_hci);

Because of the behaviour of the logging subsystem when log levels are undefined, in the general case the logs will go to LOG_LEVEL_INF:

zephyr/include/zephyr/logging/log.h

Lines 290 to 293 in bc5fbc9

	#define _LOG_LEVEL_RESOLVE(...) \
	Z_LOG_EVAL(LOG_LEVEL, \
	(GET_ARG_N(2, __VA_ARGS__, LOG_LEVEL)), \
	(GET_ARG_N(2, __VA_ARGS__, CONFIG_LOG_DEFAULT_LEVEL)))

zephyr/subsys/logging/Kconfig.filtering

Lines 13 to 19 in bc5fbc9

	config LOG_DEFAULT_LEVEL
	int "Default log level"
	default 3
	range 0 4
	help
	Sets log level for modules which don't specify it explicitly. When
	set to 0 it means log will not be activated for those modules.

To Reproduce

Modify samples/bluetooth/scan_adv/prj.conf with

CONFIG_LOG=y
# CONFIG_BT_DEBUG_LOG=y

west build -b nrf52840dk_nrf52840 zephyr/samples/bluetooth/scan_adv/

Expected behavior

Disabling CONFIG_BT_DEBUG_LOG should not result in more logs being printed.
I'm not sure if CONFIG_BT_DEBUG_LOG even makes sense now in the context of logging transitioning to standard LOG_* calls in #51897

Impact
Annoyance

Environment (please complete the following information):

• Zephyr v3.3

[alwa-nordic]

I confirm; this is undesirable behavior. The logging macros LOG_INF and the like should ultimately be controlled by the templated log configs. I suggest the following resolution:

• Remove config BT_DEBUG. Assume BT_DEBUG=y when removing uses.
• Move config BT_DEBUG_LOG out of choice BT_DEBUG_TYPE. config BT_DEBUG_LOG becomes the legacy symbol for templated log config "BT". Make choice BT_DEBUG_LOG select DEPRECATED. (EDIT: Ignore this. This would make it enable DBG-verbosity as default. That's not what it used to do.)
• Change/define meaning of choice BT_DEBUG_TYPE to "BT_MONITOR output destination".
• Change/define meaning of config BT_DEBUG_NONE to "No BT_MONITOR".

[alwa-nordic]

I think we should just remove config BT_DEBUG_LOG as well. It has no purpose after select BT_DEBUG is removed.

[theob-pro]

Hi @JordanYates, with the PR #56183 and #56321 BT_DEBUG has been removed and BT_DEBUG_LOG has been deprecated.

If you disable BT_DEBUG_LOG and enable LOG, the same way you have done it in "To Reproduce", it will no longer result in more log being printed. In fact, it will change nothing because BT_DEBUG_LOG is selecting LOG.

Did I missed something or does those change fix your issue?

---

Some details on the current status of the Bluetooth logging system:

By default, if you enable BT and LOG, the Bluetooth logging will be set to LOG_LEVEL_INF. You can set CONFIG_BT_LOG_LEVEL_OFF=y if you want to disable all Bluetooth logging.
You can override the inherited value by setting the log level of the module you want to enable the logging for (for example CONFIG_BT_HCI_DRIVER_LOG_LEVEL_DBG=y).

Note that for project like the scan_adv, that only set BT_DEBUG_LOG and not LOG, they should in the end be updated to use LOG instead of BT_DEBUG_LOG.