MTU size always 23

[vovagorodok]

Describe the bug
I'm trying to add to BLE OTA support of STM32. But have issue that MTU size always 23 even after acquireMTU. PR: vovagorodok/ArduinoBleOTA#29

To Reproduce
Complete source code which can be used to reproduce the issue. Please try to be as generic as possible (no extra code, extra hardware,...)
By running ./tools/upload.sh on branch https://github.com/vovagorodok/ArduinoBleOTA/tree/stm32_support

Steps to reproduce the behavior:
N/A

Expected behavior
Higher MTU than 23.

Screenshots
N/A

Desktop (please complete the following information):

• OS: [e.g. Windows] Fedora 43
• Arduino IDE version: [e.g. 1.8.8] Latest PlatformIO
• STM32 core version: [e.g. 1.4.0] N/A
• Library version: [e.g. 1.2.0] Latest 1.4.1
• Tools menu settings if not the default: [e.g. Newlib Standard, No Serial] N/A
• Upload method: [e.g. SWD] N/A

Hardware (please complete the following information):

• Board Name: [e.g. Nucleo F429ZI] Nucleo F401RE + X-NUCLEO-IDB05A2
• Hardware Revision: [e.g. Rev B] Rev C
• Extra hardware used if any: [e.g. X-Nucleo IKS01A1] X-NUCLEO-IDB05A2

Additional context
Maybe some additional config flag should be set? Lat me know what additional information is needed

[fpistm]

Hi @vovagorodok
I'm not an expert about this.
THe MTU is defined here:

STM32duinoBLE/src/utility/STM32Cube_FW/ble_bufsize.h

Line 26 in 23053fb

#define BLE_DEFAULT_ATT_MTU 23

But it seems to be the minimum value for ArduinoBLE part.
About the STM32 part:

STM32duinoBLE/src/utility/STM32_WPAN/app_conf_default.h

Line 179 in 1c3d607

#define CFG_BLE_MAX_ATT_MTU (156)

But as stated in our case it is ignored. As far as I understand MTU value is "negociated".
I guess your best bet is to try to debug and see how it is defined.

[vovagorodok]

Is here some logs that inform that MTU negotiation started/done?