[PATCH v4 2/8] tty: serial: Add uart_fifo_timeout_ms()

[Douglas Anderson]

The current uart_fifo_timeout() returns jiffies, which is not always
the most convenient for callers. Add a variant uart_fifo_timeout_ms()
that returns the timeout in milliseconds.

NOTES:
- msecs_to_jiffies() rounds up, unlike nsecs_to_jiffies(). This is
  because msecs_to_jiffies() is actually intended for device drivers
  to calculate timeout value. This means we don't need to take the max
  of the timeout and "1" since the timeout will always be > 0 ms (we
  add 20 ms of slop).
- uart_fifo_timeout_ms() returns "unsigned int" but we leave
  uart_fifo_timeout() returning "unsigned long". This matches the
  types of msecs_to_jiffies().

Suggested-by: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxxxxxx>
Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
---

Changes in v4:
- New

 include/linux/serial_core.h | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index 8cb65f50e830..97968acfd564 100644
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -889,14 +889,21 @@ unsigned int uart_get_divisor(struct uart_port *port, unsigned int baud);
 /*
  * Calculates FIFO drain time.
  */
-static inline unsigned long uart_fifo_timeout(struct uart_port *port)
+static inline unsigned int uart_fifo_timeout_ms(struct uart_port *port)
 {
 	u64 fifo_timeout = (u64)READ_ONCE(port->frame_time) * port->fifosize;
+	unsigned int fifo_timeout_ms = div_u64(fifo_timeout, NSEC_PER_MSEC);
 
-	/* Add .02 seconds of slop */
-	fifo_timeout += 20 * NSEC_PER_MSEC;
+	/*
+	 * Add .02 seconds of slop. This also helps account for the fact that
+	 * when we converted from ns to ms that we didn't round up.
+	 */
+	return fifo_timeout_ms + 20;
+}
 
-	return max(nsecs_to_jiffies(fifo_timeout), 1UL);
+static inline unsigned long uart_fifo_timeout(struct uart_port *port)
+{
+	return msecs_to_jiffies(uart_fifo_timeout_ms(port));
 }
 
 /* Base timer interval for polling */
-- 
2.45.2.505.gda0bf45e8d-goog