Date: Thu, 03 Jan 2002 12:02:03 -0800 From: Terry Lambert <tlambert2@mindspring.com> To: Poul-Henning Kamp <phk@critter.freebsd.dk> Cc: Bruce Evans <bde@zeta.org.au>, Mike Silbersack <silby@silby.com>, freebsd-arch@FreeBSD.ORG Subject: Re: DELAY accuracy Re: cvs commit: src/sys/dev/usb uhci.c Message-ID: <3C34B8BB.BAF3DB2B@mindspring.com> References: <831.1010050137@critter.freebsd.dk>
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Poul-Henning Kamp wrote: > I have actually thought about DELAY() a fair bit. Go figure. 8-). (Poul is our timecounter guru). > I agree that code shouldn't depend too much on the accuracy of DELAY() > but on the other hand I think we can do much better than we do today. This is the number one take-away: depend *only* on the DELAY() being *AT LEAST* as long as the specified interval. > Obviously, nanosleep() will need a MD part for short delays, but long > delays can be handled MI in timecounter land, since the timecounters > have already hold of the hardware. > > On the other hand, nanosleep() would mostly be for very short intervals, > and the changes that for instance the TSC might experience are minor > compared to the interval. Actually, this misses the middle case, where you want a longer interval, but its termination point still requires very high resolution (yes, I know that "+/-3uS after 6 hours" would be weird, but "+/-3uS" after some interval a couple of orders of magnitude larger tha "1uS" does make sense -- e.g. floppy disk and other cruddy hardware). > Summary: > a) A lot more can be done to improve things. > b) Not doing so properly discourages people from using it. Both good things... 8-). -- Terry To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-arch" in the body of the message
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