From owner-freebsd-hackers@FreeBSD.ORG  Sun Apr 22 05:44:42 2007
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Date: Sat, 21 Apr 2007 23:42:53 -0600 (MDT)
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To: alfred@freebsd.org
From: "M. Warner Losh" <imp@bsdimp.com>
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	Sat, 21 Apr 2007 23:42:46 -0600 (MDT)
Cc: hackers@freebsd.org, bde@freebsd.org
Subject: Re: serial help ?
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In message: <20070419211021.GH69188@elvis.mu.org>
            Alfred Perlstein <alfred@freebsd.org> writes:
: 
: I'm working on some custom hardware and I'm getting garbled console
: output.
: 
: I noticed that siocntxwait looks like this:
: 
: static void
: siocntxwait(iobase)
: 	Port_t	iobase;
: {
: 	int	timo;
: 
: 	/*
: 	 * Wait for any pending transmission to finish.  Required to avoid
: 	 * the UART lockup bug when the speed is changed, and for normal
: 	 * transmits.
: 	 */
: 	timo = 100000;
: 	while ((inb(iobase + com_lsr) & (LSR_TSRE | LSR_TXRDY))
: 	       != (LSR_TSRE | LSR_TXRDY) && --timo != 0)
: 		;
: }
: 
: Shouldn't there be some sort of DELAY in there?
: 
: My platform has an emulated serial device in hardware, so it
: may be that the loop could run a LOT faster than transmit can
: happen...
: 
: any ideas of what the DELAY should be?

The inb() is assumed to provide a delay of 1us, so this has a timeout
of 100ms.  Even with newer PCI devices that run about 4-5 times
faster, the timeout is still on the order of 20ms, which is adequate
for most modern baudrates (9600 baud takes 1ms, 115200 takes 10us).

Warner