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Date: Mon, 8 Jan 1996 19:12:08 +1100
From: Bruce Evans <bde@zeta.org.au>
Message-Id: <199601080812.TAA16700@godzilla.zeta.org.au>
To: hackers@FreeBSD.ORG, karl@bagpuss.demon.co.uk
Subject: Re: Changing the 16550 FIFO trigger
Sender: owner-hackers@FreeBSD.ORG
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>I have a 386/16 with 8MB, running FreeBSD-Release-2.1.  I need to use an
>internal modem (with a 16550), running at 28.8.  I understand the default
>sio trigger is FIFO_TRIGGER_14 (ie a 2 byte fifo).  Due to the slowness of
>this machine, I am thinking of decreasing this to FIFO_TRIGGER_8, this giving
>an 8 byte fifo.  Is this a good idea, and whats the best way to make the
>change?

Try it first.  It should work unless you have a bus-hogging DMA controller
or multiple high speed modems.  The 2 byte fifo reserve is agressive, but
it's twice as large as the 1 byte reserve on 8250's.  You can get another
safety factor of 2 or 3 by connecting to the modem at 57600 bps or 38400
bps instead of 115200 bps.

The easiest way to modify it is to edit the source.  This is not best.
1.1.5 attempted to choose the best trigger level dynamically but didn't
get it right.

Bruce