[v2] USB: serial: ftdi_sio: Use rounding instead of truncating when calculating baud rate divisors.

[v2] USB: serial: ftdi_sio: Use rounding instead of truncating when calculating baud rate divisors.

[Nikolaj Fogh]

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

      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;

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 */

[v2] USB: serial: ftdi_sio: Use rounding instead of truncating when calculating baud rate divisors.

[Johan Hovold]

On Thu, Nov 22, 2018 at 09:27:46PM +0100, Nikolaj Fogh wrote:
> 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 <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;

This version turned out to be white-space damaged by gmail (?) with tabs
replaced by spaces.

I fixed it up manually, but next time try sending the patch to yourself
and make sure can apply it (and/or run checkpatch on the result).

I also shortened the summary further and edited the commit message very
slightly. The result can be seen here (soon):

	https://git.kernel.org/pub/scm/linux/kernel/git/johan/usb-serial.git/commit/?h=usb-next&id=6abd837104a3a8e1cda64fc4d7675f6c3ece9d8b

Again, thanks for fixing this.

Johan