Re: [RFC V1 04/16] Bluetooth: hci_ldisc: Add HCI RESET comment to hci_unregister_dev() call

[Marcel Holtmann <marcel@holtmann.org>]

Hi Dean,

> This commit relates to the HCI UART quirk HCI_QUIRK_RESET_ON_CLOSE
> which is enabled by default and can be disabled by setting hdev_flags
> flag HCI_UART_RESET_ON_INIT via ioctl HCIUARTSETFLAGS from userland.
> 
> The implementation of HCI_QUIRK_RESET_ON_CLOSE is broken for the BCSP
> Data Link protocol layer because it can be seen that
> hci_unregister_dev() takes 2 seconds to run due to BCSP communications
> failure. Suspect that sending of the HCI RESET command is broken
> for the other Data Link protocols as well.
> 
> Therefore, add a comment to inform that the call to
> hci_unregister_dev() in hci_uart_tty_close() may send a HCI RESET
> command. This transmission needs the HCI UART driver to remain
> operational including having the Data Link protocol being bound to
> the HCI UART driver. If the transmission fails, then
> hci_unregister_dev() waits HCI_CMD_TIMEOUT (2) seconds for the
> timeout to occur which is undesirable.
> 
> The current software has a call to hci_unregister_dev(hdev) in
> hci_uart_tty_close() which can cause an attempt at sending the
> command HCI RESET to occur through the following callstack:
> 
> hci_uart_tty_close()
> hci_unregister_dev()
> hci_dev_do_close()
> __hci_req_sync() to queue up command HCI RESET
> which adds a work-item to the hdev->workqueue and waits 2 seconds
> for a response
> 
> Subsequently
> hdev->workqueue runs and processes the work-item by calling
> hci_cmd_work()
> hci_send_frame()
> hci_uart_send_frame() to enqueue into the Data Link protocol layer
> 
> No protocol response comes so hci_unregister_dev() takes 2 seconds to
> run!
> 
> In fact, this hidden transmission of the HCI RESET command is most
> likely the root cause of why hci_uart_tty_close() crashed in the past
> and was "fixed" with multiple workarounds in the past.
> 
> The underlying design flaw is that hci_uart_tty_close() is interfering
> with various aspects of transmitting and receiving HCI messages
> whilst hci_unregister_dev() is trying to use the Data Link protocol
> to send the HCI RESET command. Consequently, the design flaw
> causes the Data Link protocol to retransmit as no reply comes from
> the Bluetooth Radio module over the UART link.
> 
> Over the many years of "fixes", this caused the logic in
> hci_uart_tty_close() to become over-complex.
> 
> Signed-off-by: Dean Jenkins <Dean_Jenkins@mentor.com>
> ---
> drivers/bluetooth/hci_ldisc.c | 6 ++++++
> 1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/bluetooth/hci_ldisc.c b/drivers/bluetooth/hci_ldisc.c
> index 1887ad4..c78c9dc 100644
> --- a/drivers/bluetooth/hci_ldisc.c
> +++ b/drivers/bluetooth/hci_ldisc.c
> @@ -499,6 +499,12 @@ static void hci_uart_tty_close(struct tty_struct *tty)
> 	if (test_and_clear_bit(HCI_UART_PROTO_READY, &hu->flags)) {
> 		if (hdev) {
> 			if (test_bit(HCI_UART_REGISTERED, &hu->flags))
> +				/* Note hci_unregister_dev() may try to send
> +				 * a HCI RESET command. If the transmission
> +				 * fails then hci_unregister_dev() waits
> +				 * HCI_CMD_TIMEOUT (2) seconds for the timeout
> +				 * to occur.
> +				 */
> 				hci_unregister_dev(hdev);

lets put { } around this if statement. I know it is no needed, but we generally do that for clarity.

Regards

Marcel