USB: serial: ftdi_sio: use rounding 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 is significantly improved
for some rates (e.g. 961200), 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 <nikolajfogh@gmail.com>
[ johan: edit commit message slightly ]
Signed-off-by: Johan Hovold <johan@kernel.org>

1 files changed, 3 insertions, 3 deletions

diff --git a/drivers/usb/serial/ftdi_sio.c b/drivers/usb/serial/ftdi_sio.c
index 609198d9594c..1ab2a6191013 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(8 * base, 10 * baud);
 
         divisor = divisor3 >> 3;
         divisor |= (u32)divfrac[divisor3 & 0x7] << 14;