PERFORCE change 116868 for review

[Marcel Moolenaar]

http://perforce.freebsd.org/chv.cgi?CH=116868

Change 116868 by marcel at marcel_jnpr on 2007/03/29 19:45:01

	In ns8250_putc() and ns8250_getc(), don't calculate the
	a delay value of about 1/10th the time it takes to send
	a character. In ns8250_putc() we only want to avoid
	waiting indefinitely for some status change and in
	ns8250_getc() we just want to back-off from the bus
	when we wait for some status change (and unlocking the
	hardware while we're at it). This really doesn't need
	exact timing. It's sufficient to assume that regardless
	of the baudrate, we should not have to wait longer than
	a second for the status to change and we can express
	everything in terms of a fixed delay (DELAY(4) in this
	case).
	
	The upshot is that we eliminate expensive calculations
	for every character we send, with a positive side-effect
	that if we lack the precision (e.g. the exact frequency
	of the baudrate clock) we don't mess up any low-level
	I/O.

Affected files ...

.. //depot/projects/uart/dev/uart/uart_dev_ns8250.c#49 edit

Differences ...

==== //depot/projects/uart/dev/uart/uart_dev_ns8250.c#49 (text+ko) ====

@@ -285,19 +285,16 @@
 static void
 ns8250_putc(struct uart_bas *bas, int c)
 {
-	int delay, limit;
+	int limit;
 
-	/* 1/10th the time to transmit 1 character (estimate). */
-	delay = ns8250_delay(bas);
-
-	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 @@
 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);
 	}