BlueZ: No default controller available - embedded-linux

Question

I'm setting up Linux device to act as BLE host, and ESP32 to work as a BLE controller over UART.
I'm using BlueZ v5.55, with btattach and bluetoothctl utilities, and bluetoothd running in the background. However, after finishing the setup bluetoothctl still says No default controller available.
# /usr/bin/btattach -B /dev/ttymxc1 -S 921600 --protocol h4 &
# Attaching Primary controller to /dev/ttymxc1
Switched line discipline from 0 to 15
Device index 0 attached
# /usr/libexec/bluetooth/bluetoothd
# bluetoothctl
Agent registered
[bluetooth]# power on
No default controller available
Output of btmon:
# btmon
Bluetooth monitor ver 5.55
= Note: Linux version 5.4.8 (armv7l) 0.258161
= Note: Bluetooth subsystem version 2.22 0.258269
= New Index: 00:00:00:00:00:00 (Primary,UART,hci0) [hci0] 0.258314
# MGMT Open: bluetoothd (privileged) version 1.14 {0x0001} 0.258357
Output of dmesg during kernel start:
# dmesg | grep Blu
[ 0.643821] Bluetooth: Core ver 2.22
[ 0.644222] Bluetooth: HCI device and connection manager initialized
[ 0.644528] Bluetooth: HCI socket layer initialized
[ 0.644622] Bluetooth: L2CAP socket layer initialized
[ 0.645069] Bluetooth: SCO socket layer initialized
[ 4.678559] Bluetooth: HCI UART driver ver 2.3
[ 4.683346] Bluetooth: HCI UART protocol H4 registered
[ 4.688821] Bluetooth: HCI UART protocol LL registered
The ESP32 is running UART HCI controller firmware: https://github.com/espressif/esp-idf/tree/master/examples/bluetooth/hci/controller_hci_uart
The host is running a Buildroot-created Linux with BR2_PACKAGE_BLUEZ_TOOLS=y and a kernel with CONFIG_BT=y, CONFIG_BT_LE=y, CONFIG_BT_HCIUART=y, CONFIG_BT_HCIUART_H4=y.
I have 4 UART lines connected TX->RX, RX->TX, RTS->CTS and CTS->RTS.
I have checked the UART communication with the same HW setup - I can exchange data over UART with the ESP32 (in both directions).
The commands are run as root.
What am I missing here?
If you have any decent documentation on BlueZ HCI setup, I'll be happy to see it.
EDIT: I also installed hcidump and hciconfig (normally deprecated in BLueZ v5.55). Here is a log from hcidump:
# hcidump
HCI sniffer - Bluetooth packet analyzer ver 5.55
device: hci0 snap_len: 1500 filter: 0xffffffff
< HCI Command: Read Local Supported Features (0x04|0x0003) plen 0
< HCI Command: Read Local Version Information (0x04|0x0001) plen 0
< HCI Command: Read BD ADDR (0x04|0x0009) plen 0
(in 2nd terminal)
# hciconfig hci0 up
Can't init device hci0: Connection timed out (110)

Answer 1

After a few days, I have found a solution of this problem.
I have also used nRF52 DK as an alternative to better understand if the problem lies on the side of ESP32 controller, or on the side of Linux host. Because the nRF HCI UART worked on Ubuntu 20.04, but didn't work on my embedded Linux, I looked deeper on the Linux host side.
After starting BlueZ daemon with
bluetoothd -n
it showed more details:
bluetoothd[593]: src/adapter.c:get_static_addr() Failed to open crypto
bluetoothd[593]: No Bluetooth address for index 0
Which led me to this topic: https://github.com/raspberrypi/linux/issues/3628
It turns out, when attaching the UART HCI controller with btattach, BlueZ was not able to create a static address, because of missing Linux kernel crypto interface in user-space.
I compiled the kernel with additional configs:
CONFIG_CRYPTO_USER
CONFIG_CRYPTO_USER_API
CONFIG_CRYPTO_USER_API_AEAD
CONFIG_CRYPTO_USER_API_HASH
CONFIG_CRYPTO_AES
CONFIG_CRYPTO_CCM
CONFIG_CRYPTO_AEAD
CONFIG_CRYPTO_CMAC
After that change, I was able to attach the Bluetooth controller to my Embedded Linux host.