Don't use a time-limiting loop that's defined in terms of the baudrate

in the putc() method.  Likewise, in the getc() method, don't check for
received characters with an interval defined in terms of the baudrate.
In both cases it works equally well to implement a fixed delay.  More
importantly, it avoids calculating a delay that's roughly 1/10th the
time it takes to send/receive a character. The calculation is costly
and happens for every character sent or received, affecting low-level
console or debug port performance significantly. Secondly, when the
RCLK is not available or unreliable, the delays could disrupt normal
operation.

The fixed delay is 1/10th the time it takes to send a character at
230400 bps.

1 files changed, 7 insertions, 13 deletions

diff --git a/sys/dev/uart/uart_dev_ns8250.c b/sys/dev/uart/uart_dev_ns8250.c
index 4e51417..abe141b 100644
--- a/sys/dev/uart/uart_dev_ns8250.c
+++ b/sys/dev/uart/uart_dev_ns8250.c
@@ -285,19 +285,16 @@ ns8250_term(struct uart_bas *bas)
 static void
 ns8250_putc(struct uart_bas *bas, int c)
 {
-	int delay, limit;
-
-	/* 1/10th the time to transmit 1 character (estimate). */
-	delay = ns8250_delay(bas);
+	int limit;
 
-	limit = 20;
+	limit = 250000;
 	while ((uart_getreg(bas, REG_LSR) & LSR_THRE) == 0 && --limit)
-		DELAY(delay);
+		DELAY(4);
 	uart_setreg(bas, REG_DATA, c);
 	uart_barrier(bas);
-	limit = 40;
+	limit = 250000;
 	while ((uart_getreg(bas, REG_LSR) & LSR_TEMT) == 0 && --limit)
-		DELAY(delay);
+		DELAY(4);
 }
 
 static int
@@ -310,16 +307,13 @@ ns8250_rxready(struct uart_bas *bas)
 static int
 ns8250_getc(struct uart_bas *bas, struct mtx *hwmtx)
 {
-	int c, delay;
+	int c;
 
 	uart_lock(hwmtx);
 
-	/* 1/10th the time to transmit 1 character (estimate). */
-	delay = ns8250_delay(bas);
-
 	while ((uart_getreg(bas, REG_LSR) & LSR_RXRDY) == 0) {
 		uart_unlock(hwmtx);
-		DELAY(delay);
+		DELAY(4);
 		uart_lock(hwmtx);
 	}