Max MTU for characteristic read not honoured in some cases

Question

Hello, I have an nRF52 device with NRF_SDH_BLE_GATT_MAX_MTU_SIZE set to 512 and it accepts connections and the client sets the max mtu to 512. It is on SDK 15 
 When 
 
 an Android based central does a read characteristic, data of 100 or more bytes is returned i.e. working correctly 
 a Linux based central (bluez/pygatt) does the same read only 22 bytes are returned, as if the max mtu change is not recognized 
 
 The Linux based central can successfully do reads on the increased mtu size when communicating with a Raspberry Pi based peripheral 
 I have compared sniffer traces in Wireshark and one difference I see is that the Android app does a GATT Primary Service Declaration whereas the Linux one does not. I have no idea if this is a factor, just a difference that I noticed. 
 Any ideas on debugging or otherwise are appreciated and please let me know if there is more information I can provide. 
 Thank you, 
 Eric

haakonsh · Accepted Answer

I've seen an issue with an MTU exchange with an android phone not yielding the requested amount when the peripheral GATT server initiated the MTU request instead of letting the Central GATT Client (Android phone) do it. If you're using the GATT library I suggest you comment out the following lines starting at line 150 of nrf_ble_gatt.c in SDK 15: // Begin an ATT MTU exchange if necessary.
 if (p_link->att_mtu_desired > p_link->att_mtu_effective)
 {
 NRF_LOG_DEBUG("Requesting to update ATT MTU to %u bytes on connection 0x%x.",
 p_link->att_mtu_desired, conn_handle);

 err_code = sd_ble_gattc_exchange_mtu_request(conn_handle, p_link->att_mtu_desired);

 if (err_code == NRF_SUCCESS)
 {
 p_link->att_mtu_exchange_requested = true;
 }
 else if (err_code == NRF_ERROR_BUSY)
 {
 p_link->att_mtu_exchange_pending = true;
 NRF_LOG_DEBUG("sd_ble_gattc_exchange_mtu_request()"
 " on connection 0x%x returned busy, will retry.", conn_handle);
 }
 else
 {
 NRF_LOG_ERROR("sd_ble_gattc_exchange_mtu_request() returned %s.",
 nrf_strerror_get(err_code));
 }
 } This solved the problem by letting the Central GATT Client issue the MTU request instead. Also note that the code you're removing should be present when using the GATT library for a Client role. You should perhaps encapsulate the snippet in an if( p_ble_evt-> evt . gap_evt . params . connected . role == BLE_GAP_ROLE_CENTRAL), or just move it into the switch-case above.

Max MTU for characteristic read not honoured in some cases

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