The Qualcomm GENI serial driver did not handle buffer flushing and used
to print discarded characters when the circular buffer was cleared.
Since commit 1788cf6a ("tty: serial: switch from circ_buf to kfifo")
this instead resulted in a hard lockup due to
qcom_geni_serial_send_chunk_fifo() spinning indefinitely in the
interrupt handler.

The underlying bugs have now been fixed, but make sure to output NUL
characters instead of killing the machine if a similar driver bug is
ever reintroduced.
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Link: https://lore.kernel.org/r/20240704101805.30612-4-johan+linaro@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>