Date: Mon, 06 Dec 1999 00:13:09 -0800 From: Mike Smith <msmith@freebsd.org> To: Mike Smith <msmith@freebsd.org> Cc: kvandel@cs.duke.edu, freebsd-hackers@freebsd.org Subject: Re: fxp, xl driver question .. (routing) Message-ID: <199912060813.AAA01499@mass.cdrom.com> In-Reply-To: Your message of "Mon, 06 Dec 1999 00:05:53 PST." <199912060805.AAA01467@mass.cdrom.com>
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> > On Sun, 05 Dec 1999, you wrote: > Conversely, you can achieve the same latency reduction by setting the > latency timer to 16, without increasing your overheads there. (This isn't > actually entirely true, as you may run into busmaster ping-pong with more > than one in the system, but you'll get this with reduced DMA bursts as > well.) I should have summarised this by saying: Correct use of the latency timer will shorten your DMA bursts for you when necessary, giving you the best of both worlds. When it's safe to run a long burst, you will. When you need to push a device off the bus, that will happen too. And the obvious extension to the "worst case" calculation is that if you have N master devices each with a latency timer of X, your worst-case timing for CPU access to a device is (N * X) + (N * arb overhead), just in case that wasn't clear. -- \\ Give a man a fish, and you feed him for a day. \\ Mike Smith \\ Tell him he should learn how to fish himself, \\ msmith@freebsd.org \\ and he'll hate you for a lifetime. \\ msmith@cdrom.com To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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