Improve the accuracy of the baud rate generator by using round-to-closest instead of truncating when calculating baud rate divisors.

[Nikolaj Fogh <nikolajfogh@gmail.com>]

I have experienced that the ftdi_sio driver gives less-than-optimal baud rates as the driver truncates instead of rounds to nearest during baud rate divisor calculation.

This patch improves on the baud rate generation. The generated baud rate corresponds to the optimal baud rate achievable with the chip. This is what the windows driver gives as well.

Signed-off-by: Nikolaj Fogh <nikolajfogh@gmail.com>
---
 drivers/usb/serial/ftdi_sio.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/usb/serial/ftdi_sio.c b/drivers/usb/serial/ftdi_sio.c
index 609198d9594c..0edbd3427548 100644
--- a/drivers/usb/serial/ftdi_sio.c
+++ b/drivers/usb/serial/ftdi_sio.c
@@ -1130,7 +1130,7 @@ static unsigned short int ftdi_232am_baud_base_to_divisor(int baud, int base)
 {
 	unsigned short int divisor;
 	/* divisor shifted 3 bits to the left */
-	int divisor3 = base / 2 / baud;
+	int divisor3 = DIV_ROUND_CLOSEST(base, 2 * baud);
 	if ((divisor3 & 0x7) == 7)
 		divisor3++; /* round x.7/8 up to x+1 */
 	divisor = divisor3 >> 3;
@@ -1156,7 +1156,7 @@ static u32 ftdi_232bm_baud_base_to_divisor(int baud, int base)
 	static const unsigned char divfrac[8] = { 0, 3, 2, 4, 1, 5, 6, 7 };
 	u32 divisor;
 	/* divisor shifted 3 bits to the left */
-	int divisor3 = base / 2 / baud;
+	int divisor3 = DIV_ROUND_CLOSEST(base, 2 * baud);
 	divisor = divisor3 >> 3;
 	divisor |= (u32)divfrac[divisor3 & 0x7] << 14;
 	/* Deal with special cases for highest baud rates. */
@@ -1179,7 +1179,7 @@ static u32 ftdi_2232h_baud_base_to_divisor(int baud, int base)
 	int divisor3;
 
 	/* hi-speed baud rate is 10-bit sampling instead of 16-bit */
-	divisor3 = base * 8 / (baud * 10);
+	divisor3 = DIV_ROUND_CLOSEST(base * 8, baud * 10);
 
 	divisor = divisor3 >> 3;
 	divisor |= (u32)divfrac[divisor3 & 0x7] << 14;