svn commit: r359899 - head/sys/dev/uart

[John Baldwin]

Author: jhb
Date: Mon Apr 13 20:43:57 2020
New Revision: 359899
URL: https://svnweb.freebsd.org/changeset/base/359899

Log:
  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.
  
  Reported by:	imp
  MFC after:	2 weeks
  Sponsored by:	DARPA
  Differential Revision:	https://reviews.freebsd.org/D24367

Modified:
  head/sys/dev/uart/uart_dev_ns8250.c

Modified: head/sys/dev/uart/uart_dev_ns8250.c
==============================================================================
--- head/sys/dev/uart/uart_dev_ns8250.c	Mon Apr 13 20:25:01 2020	(r359898)
+++ head/sys/dev/uart/uart_dev_ns8250.c	Mon Apr 13 20:43:57 2020	(r359899)
@@ -139,7 +139,7 @@ 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)