serial: qcom-geni: fix fifo polling timeout

serial: qcom-geni: fix fifo polling timeout

The qcom_geni_serial_poll_bit() can be used to wait for events like
command completion and is supposed to wait for the time it takes to
clear a full fifo before timing out.

As noted by Doug, the current implementation does not account for start,
stop and parity bits when determining the timeout. The helper also does
not currently account for the shift register and the two-word
intermediate transfer register.

A too short timeout can specifically lead to lost characters when
waiting for a transfer to complete as the transfer is cancelled on
timeout.

Instead of determining the poll timeout on every call, store the fifo
timeout when updating it in set_termios() and make sure to take the
shift and intermediate registers into account. Note that serial core has
already added a 20 ms margin to the fifo timeout.

Also note that the current uart_fifo_timeout() interface does
unnecessary calculations on every call and did not exist in earlier
kernels so only store its result once. This facilitates backports too as
earlier kernels can derive the timeout from uport->timeout, which has
since been removed.

Fixes: c4f528795d1a ("tty: serial: msm_geni_serial: Add serial driver support for GENI based QUP")
Cc: stable@vger.kernel.org	# 4.17
Reported-by: Douglas Anderson <dianders@chromium.org>
Tested-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Link: https://lore.kernel.org/r/20240906131336.23625-2-johan+linaro@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>