Fix cannot write to characteristic unless read property is set for Silabs's board

[silabs-ChatNguyen]

Hi Team,
We have an issue with characteristic properties:
When properties = (Characteristic.READ | Characteristic.WRITE_NO_RESPONSE), can change the value of characteristic normally
But when properties = (Characteristic.WRITE_NO_RESPONSE), cannot change the value of characteristic.
This pull request adds code handle for this case.
Thanks!

[jepler]

Thanks!

Two questions:

• Is it actually correct that a BT_GATT_CHRC_READ property (without _WRITE or _WRITE_WITHOUT_RESP) should be settable?
• if none of those bits is set, the sl_bt_gatt_server_write_attribute_value call is not made but sl_bt_gatt_server_send_notification / send_indication calls could occur if those bits are set. Should an exception be raised in this case? This is what the (incomplete) implementation of sl_bt_gatt_server_send_notification in the ble-hci implementation does.

[tannewt]

What BLE service requires that the server write a WRITE_NO_RESPONSE characteristic? Doesn't that say only the client is writing to the characteristic?

[silabs-ChatNguyen]

What BLE service requires that the server write a WRITE_NO_RESPONSE characteristic? Doesn't that say only the client is writing to the characteristic?

Hi @tannewt
This issue is raised by a user. He uses a custom BLE service.
With only Characteristic.WRITE_NO_RESPONSE, we can't set value for "TestSVC.TestValue = random.randint(0, 255)"
When properties = (Characteristic.READ | Characteristic.WRITE_NO_RESPONSE), The value can change normally.
You can see the below example.

-----------------------------------------------------------------------------------------------------------------
import random
from adafruit_ble import*
from adafruit_ble.services import*
from adafruit_ble.uuid import*
from adafruit_ble.characteristics.int import *
from time import sleep
 
class TestService(Service):
    uuid = StandardUUID(0x2A06)
    TestValue = Uint8Characteristic(
        uuid = StandardUUID(0x2a6e),
        properties=(Characteristic.WRITE_NO_RESPONSE),
    )
  
TestSVC = TestService()
ble = BLERadio()
adv = Advertisement()
adv.short_name="Silabs-BLE"
adv.connectable = True

while True:
    ble.start_advertising(adv)
    print("start_advertising")
    while not ble.connected:
        pass
    ble.stop_advertising()
    print("stop_advertising")

    while ble.connected:
        TestSVC.TestValue = random.randint(0, 255)
        print(f'TestValue = {TestSVC.TestValue}')
        sleep(1)
    print("disconnected")
-----------------------------------------------------------------------------------------------------------------------

So i added code handle for case with only WRITE_NO_RESPONSE.

[silabs-MartinL]

This is for the Alert Level characteristic in the Immediate Alert service part of the Find Me profile. The Alert Level should only have WRITE_NO_RESPONSE so can be written remotely, however the alert should also be able to be cancelled locally on the device, hence the need to update the characteristic value locally in addition to remotely.

Edit: see - https://www.bluetooth.com/specifications/specs/immediate-alert-service-1-0/