svn commit: r365476 - in stable: 11/sys/dev/uart 12/sys/dev/uart

[John Baldwin]

Author: jhb
Date: Tue Sep  8 22:19:06 2020
New Revision: 365476
URL: https://svnweb.freebsd.org/changeset/base/365476

Log:
  MFC 359899: Correct baud rate error calculation.
  
  Shifting right by 1 is not the same as dividing by 2 for signed
  values.  In particular, dividing a signed value by 2 gives the integer
  ceiling of the (e.g. -5 / 2 == -2) whereas shifting right by 1 always
  gives the floor (-5 >> 1 == -3).
  
  An embedded board with a 25 Mhz base clock results in an error of
  -30.5% when used with a baud rate of 115200.  Using division, this
  truncates to -30% and is permitted.  Using the shift, this fails and
  is rejected causing TIOCSETA requests to fail with EINVAL and breaking
  getty(8).
  
  Using division gives the same error range for both over and under baud
  rates and also makes the code match the behavior documented in the
  existing comment about supporting boards with 25 Mhz clocks.

Modified:
  stable/11/sys/dev/uart/uart_dev_ns8250.c
Directory Properties:
  stable/11/   (props changed)

Changes in other areas also in this revision:
Modified:
  stable/12/sys/dev/uart/uart_dev_ns8250.c
Directory Properties:
  stable/12/   (props changed)

Modified: stable/11/sys/dev/uart/uart_dev_ns8250.c
==============================================================================
--- stable/11/sys/dev/uart/uart_dev_ns8250.c	Tue Sep  8 21:39:34 2020	(r365475)
+++ stable/11/sys/dev/uart/uart_dev_ns8250.c	Tue Sep  8 22:19:06 2020	(r365476)
@@ -136,7 +136,7 @@ ns8250_divisor(int rclk, int baudrate)
 	actual_baud = rclk / (divisor << 4);
 
 	/* 10 times error in percent: */
-	error = ((actual_baud - baudrate) * 2000 / baudrate + 1) >> 1;
+	error = ((actual_baud - baudrate) * 2000 / baudrate + 1) / 2;
 
 	/* enforce maximum error tolerance: */
 	if (error < -UART_DEV_TOLERANCE_PCT || error > UART_DEV_TOLERANCE_PCT)