On Thu, 23 May 2013, David Herrmann wrote: > While l2cap_user callbacks are running, the whole hci_dev is locked. Even > if we would add more fine-grained locking to HCI core, it would still be > called from the non-reentrant rx work-queue and thus block the event > processing. > > However, if we want to perform synchronous I/O during HID device > registration (eg., to perform device-detection), we need the HCI core > to be able to dispatch incoming data. > > Therefore, we now move device-registration to a separate worker. The HCI > core can continue running and we add devices asynchronously in another > kernel thread. Device removal is synchronized and waits for the worker > to exit before calling the usual device removal functions. > > If l2cap_user->remove is called before the thread registered the devices, > we set "terminate" to true and the thread will skip it. If > l2cap_user->remove is called after it, we notice this as the device > is no longer in HIDP_SESSION_PREPARING state and simply unregister the > device as we did before. > There is no new deadlock as we now call hidp_session_add_dev() with > one lock less held (the HCI lock) and it cannot itself call back into > HCI as it was called with the HCI-lock held before. > > One might wonder whether this can block during device unregistration. > But we set "terminate" to true and wake the HIDP thread up _before_ > unregistering the HID/input devices. Therefore, all pending HID I/O > operations are canceled. All further I/O attempts will fail with ENODEV > or EIO. So all latency we can get are few context-switches, but no > timeouts or blocking I/O waits! > > This change also prepares for a long standing HID bug. All HID devices > that register power_supply devices need to be able to handle callbacks > during registration (a power_supply oddity that cannot easily be fixed). > So with this patch available, we can allow HID I/O during registration > by calling the recently introduced hid_device_io_start/stop helpers, > which currently are a no-op for bluetooth due to this locking. > > Note that we cannot do the same for input devices. input-core doesn't > allow us to call input_event() asynchronously to input_register_device(), > which HID-core kindly allows (for good reasons). > Fixing input-core to allow this isn't as easy as it sounds and is, > beside simplifying HIDP, not really an improvement. Hence, we still > register input devices synchronously as we did before. Only HID devices > are registered asynchronously. > > Signed-off-by: David Herrmann <dh.herrmann@xxxxxxxxx> Acked-by: Jiri Kosina <jkosina@xxxxxxx> Gustavo, I think I'd like to take this patch together with the ENODATA change for hid-input, as they, in some sense, stick together. If you are OK with that, could you please provide me with your Acked-by, and I'll take it through my tree? Thanks. -- Jiri Kosina SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe linux-input" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
