opentitan i2c: zero-length requests hang #680
Description
The micropython machine_i2c_scan command performs zero-length writes with a STOP condition.
static mp_obj_t machine_i2c_scan(mp_obj_t self_in) {
mp_obj_base_t *self = MP_OBJ_TO_PTR(self_in);
mp_obj_t list = mp_obj_new_list(0, NULL);
// 7-bit addresses 0b0000xxx and 0b1111xxx are reserved
for (int addr = 0x08; addr < 0x78; ++addr) {
int ret = mp_machine_i2c_writeto(self, addr, NULL, 0, true);
if (ret == 0) {
mp_obj_list_append(list, MP_OBJ_NEW_SMALL_INT(addr));
}
// This scan loop may run for some time, so process any pending events/exceptions,
// or allow the port to run any necessary background tasks. But do it as fast as
// possible, in particular we are not waiting on any events.
mp_event_handle_nowait();
}
return list;
}
MP_DEFINE_CONST_FUN_OBJ_1(machine_i2c_scan_obj, machine_i2c_scan);In the case of the Opentitan i2c driver, this results in it hanging.
We should:
- Fix this
- Add sDDF tests to verify this case is OK