Date: Thu, 17 Oct 2002 11:15:33 -0700 (PDT) From: Matthew Dillon <dillon@apollo.backplane.com> To: Don Bowman <don@sandvine.com> Cc: "'Kenneth P. Stox'" <stox@imagescape.com>, FreeBSD current users <current@FreeBSD.ORG> Subject: Re: RE: Dedicating an interrupt to a PC-Card slot Message-ID: <200210171815.g9HIFX3a022794@apollo.backplane.com> References: <FE045D4D9F7AED4CBFF1B3B813C8533701022CBB@mail.sandvine.com>
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:Doesn't sound like that fast an interrupt. The 16550 has a 16-byte
:send and receive fifo. Set the rx interrupt @ 14, and the tx @ 2.
:230400/8 = 28800 chars /s
:28800 / 14 = 2057 interrupts / s. This should be well within reach
:of a pentium-class machine.
Not a good idea. If you set the rx fifo at 14 you will get fewer
interrupts, sure, but you will also have less of a safety margin to
process them before the rx fifo potentially overflows and you lose
characters. This is why I changed the default fifo level from HIGH (14)
to MEDH (8) a while back.
In anycase, anything less then 10,000 interrupts a second ought to be
fine. The real problem is still going to be other things in the system
causing enough interrupt latency to result in lost characters. The most
common culprit is X windows doing big bitblits to the video card /
video memory, and I think having a non-DMA IDE interface can also cause
problems.
At 230400 baud and 10 bits per character (one start, one stop bit), a
fifo level of 14 gives you 2 character times of safety, or
approximately 86uS. At a fifo level of 8 you have 8 character times of
safety or approximately 347uS.
-Matt
Matthew Dillon
<dillon@backplane.com>
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