USB: serial: ftdi_sio: Use rounding instead of truncating when calculating baud rate divisors.

Improve baud-rate generation by using rounding-to-closest instead of truncation in divisor calculation.

Results have been verified by logic analyzer on an FT232RT (232BM) chip. The following table shows the wanted baud rate, the baud rate obtained with the old method (truncation), with the new method (rounding) and the baud rate generated by the windows 10 driver. The numbers in parentheses is the error.

+- Wanted --+------ Old -------+------ New -------+------ Win -------+
|   9600    |   9600  (0.00%)  |   9604  (0.05%)  |   9605 (0.05%)  |
|   19200   |   19200 (0.00%)  |   19199 (0.01%)  |   19198 (0.01%)  |
|   38400   |   38395 (0.01%)  |   38431 (0.08%)  |   38394 (0.02%)  |
|   57600   |   57725 (0.22%)  |   57540 (0.10%)  |   57673 (0.13%)  |
|  115200   |  115307 (0.09%)  |  115330 (0.11%)  |  115320 (0.10%)  |
|  921600   |  919963 (0.18%)  |  920386 (0.13%)  |  920810 (0.09%)  |
|  961200   |  996512 (3.67%)  |  956480 (0.49%)  |  956937 (0.44%)  |
+-----------+------------------+------------------+------------------+

The error due to noise in the measurements is in the order of a few tenths of a %. As can be seen, the baud rate for 961200 is significantly improved for some rates, and corresponds to the output given by the windows driver.

The theoretical baud rate has been calculated for all baud rates from 1 to 3M, and as expected, the error is centered around 0, with a triangle shape instead of a sawtooth, so the maximum error is decreased to half.

Signed-off-by Nikolaj Fogh <[email protected]>

---
  drivers/usb/serial/ftdi_sio.c | 6 +++---
  1 file changed, 3 insertions(+), 3 deletions(-)

drivers/usb/serial/ftdi_sio.c +1 -1

index 609198d9594c..0edbd3427548 100644
@@ -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;