Find the next acquiring processor according to the status. Will directly change the acquiring device if new one found. Cases: - BG should still be the acquiring processor (Return false): 1. Acquiring device has active BG bits: out_desired_dev = new acquiring device 2. No acquiring device, but BG active: out_desired_dev = randomly pick one device with active BG bits - BG should yield to the task (Return true): 3. Acquiring device has no active BG bits: out_desired_dev = new acquiring device 4. No acquiring device while no active BG bits: out_desired_dev=NULL Acquiring device task need to be resumed only when case 3. This scheduling can happen in either task or ISR, so `in_isr` or `bg_active` not touched.

spi_bus_lock_bg_entry()

BG (background operation) service

spi_bus_lock_bg_exit()

Handle the scheduling of other acquiring devices, and control of HW operation status. If no BG request is found, call with `wip=false`. This function will return false, indicating there is incoming BG requests for the current acquiring device (or for all devices if there is no acquiring device) and the ISR needs retry. Otherwise may schedule a new acquiring processor (unblock the task) if there is, and return true. Otherwise if a BG request is started in this ISR, call with `wip=true` and the function will enable the interrupt to make the ISR be called again when the request is done. This function is safe and should still be called when the ISR just lost its acquiring processor role, but hasn't quit.

spi_bus_lock_bg_check_dev_acq()

Check whether there is device asking for the acquiring device, and the desired device for the next operation is also recommended.

spi_bus_main_set_lock()

Bus attributes Set bus lock for the main bus, called by startup code.

lock_status_init()

try_acquire_free_dev()

update_pend_core()

bg_entry_core()

bg_exit_core()