Date: Sat, 21 Apr 2007 23:42:53 -0600 (MDT) From: "M. Warner Losh" <imp@bsdimp.com> To: alfred@freebsd.org Cc: hackers@freebsd.org, bde@freebsd.org Subject: Re: serial help ? Message-ID: <20070421.234253.-937378175.imp@bsdimp.com> In-Reply-To: <20070419211021.GH69188@elvis.mu.org> References: <20070419211021.GH69188@elvis.mu.org>
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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
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