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) _______________________________________________ svn-src-head@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/svn-src-head To unsubscribe, send any mail to "svn-src-head-unsubscr...@freebsd.org"
