tty: serial: omap-serial: wakeup latency constraint is in microseconds, not milliseconds

The receive FIFO wakeup latency estimate in the omap-serial driver is
three orders of magnitude too small.  This effectively prevents the
MPU from going to a low-power state when CONFIG_CPU_IDLE=y.  This is a
major power management regression and masks some other FIFO-related
bugs in the driver.

Fix by correcting the most egregious problem in the RX wakeup latency
estimate.  There are several other flaws in the estimator; these will
be fixed by a separate patch series intended for 3.4.

The difference in low-power states with this patch can be observed via
debugfs in pm_debug/count.

This estimate does not have any effect when CONFIG_CPU_IDLE=n.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Alan Cox <alan@linux.intel.com>
Acked-by: Govindraj.R <govindraj.raja@ti.com>
Reviewed-by: Kevin Hilman <khilman@ti.com>
Tested-by: Kevin Hilman <khilman@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 1 insertions, 2 deletions

diff --git a/drivers/tty/serial/omap-serial.c b/drivers/tty/serial/omap-serial.c
index 18d13248d9ba..f80904145fd4 100644
--- a/drivers/tty/serial/omap-serial.c
+++ b/drivers/tty/serial/omap-serial.c
@@ -740,8 +740,7 @@ serial_omap_set_termios(struct uart_port *port, struct ktermios *termios,
         quot = serial_omap_get_divisor(port, baud);
 
         /* calculate wakeup latency constraint */
-        up->calc_latency = (1000000 * up->port.fifosize) /
-                                (1000 * baud / 8);
+        up->calc_latency = (USEC_PER_SEC * up->port.fifosize) / (baud / 8);
         up->latency = up->calc_latency;
         schedule_work(&up->qos_work);