Date: Sat, 22 Dec 2001 19:38:12 -0800 (PST) From: Matthew Dillon <dillon@apollo.backplane.com> To: Bruce Evans <bde@zeta.org.au> Cc: Mike Silbersack <silby@silby.com>, <cvs-committers@FreeBSD.ORG>, <cvs-all@FreeBSD.ORG> Subject: Re: cvs commit: src/sys/dev/sio sio.c Message-ID: <200112230338.fBN3cCe99476@apollo.backplane.com> References: <20011223141152.Y10385-100000@gamplex.bde.org>
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:I fixed the software problems in 386BSD-0.0. Software latency bugs
:are occasionally reintroduced. -current has several problems in this
:area. They are mostly design problems so they are hard to fix. However,
:I think the latency problems in -current only break sio on old machines.
:IIRC, spinlocks are never held for more than about "only" 2000 (or is
:it 0x2000) instructions. On a 386/20, 2000 instructions took something
:like 400 usec, but on not so modern machines (1GHz) it takes more like
:4 usec. The driver needs a latency of less than about 75 usec to work
:for all supported hardware.
:
:Most silo overflows seem to be caused by video hardware hogging the bus.
:
:Bruce
And I've written serial drivers for 8085, 8086, 68000, 68008,
68020, 68030, Perkin-Elmer (my god! Interrupt driven microcode!),
68xx, 683xx, 486's (in assembly), etc etc etc etc. I am not
insensitive to interrupt latency issues or unknowledgeable.
On a 16MHz 68000 I was able to write a serial interrupt handler
that took less then 5uS per character (inclusive of *all* interrupt
overhead). On a 486 it took a little longer due to all the ISA
accesses.
In anycase, it's moot on a modern day machine. The PCI/ISA
accesses alone eat more cpu then all the rest of the instructions
combined.
If our users are running X sessions and trying to operate a dialup
link, we have to support that. Setting the FIFO level to MEDHI is
totally and completely reasonable. Since National screwed up the
design of the 16550[A] from day 1 (and it is STILL screwed up), we
have had to live with it and that means dealing with these issues so
our users don't have to.
-Matt
Matthew Dillon
<dillon@backplane.com>
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