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

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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.
This commit is contained in:
Marcel Moolenaar 2007-04-03 01:21:10 +00:00
parent 7bd5b79de4
commit 35777a2a79
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=168285
1 changed files with 7 additions and 13 deletions
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20
sys/dev/uart/uart_dev_ns8250.c
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@ -285,19 +285,16 @@ ns8250_term(struct uart_bas *bas)
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 @@ 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);
}