Date: Fri, 26 Dec 2008 19:33:38 -0700 From: Scott Long <scottl@samsco.org> To: Alfred Perlstein <alfred@freebsd.org> Cc: "svn-src-head@freebsd.org" <svn-src-head@freebsd.org>, "mav@freebsd.org" <mav@freebsd.org>, "src-committers@freebsd.org" <src-committers@freebsd.org>, "svn-src-all@freebsd.org" <svn-src-all@freebsd.org>, "M. Warner Losh" <imp@bsdimp.com> Subject: Re: svn commit: r186182 - head/sys/dev/ata Message-ID: <72A0CF67-F521-4C4B-955D-A80A77CC1165@samsco.org> In-Reply-To: <20081227003826.GV18389@elvis.mu.org> References: <4947D474.9040802@samsco.org> <20081216.101038.1172765453.imp@bsdimp.com> <4947F363.4010909@samsco.org> <20081216.120412.1346820326.imp@bsdimp.com> <20081227003826.GV18389@elvis.mu.org>
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On Dec 26, 2008, at 5:38 PM, Alfred Perlstein <alfred@freebsd.org> wrote: > * M. Warner Losh <imp@bsdimp.com> [081216 11:06] wrote: >> In message: <4947F363.4010909@samsco.org> >> Scott Long <scottl@samsco.org> writes: >> : M. Warner Losh wrote: >> : > In message: <4947D474.9040802@samsco.org> >> : > Scott Long <scottl@samsco.org> writes: >> : > : Alexander Motin wrote: >> : > : > Author: mav >> : > : > Date: Tue Dec 16 16:04:40 2008 >> : > : > New Revision: 186182 >> : > : > URL: http://svn.freebsd.org/changeset/base/186182 >> : > : > >> : > : > Log: >> : > : > Call ata_legacy() only once on attach and save it's >> result. Scanning PCI >> : > : > configuration registers (which are not going to change) >> on every interrupt >> : > : > looks expensive, especially when interrupt is shared. >> Profiling shows me 3% >> : > : > of time spent by atapci0 on pure network load due to IRQ >> sharing with em0. >> : > : > >> : > : >> : > : Nice change. PCI Config registers are exceptionally slow to >> access on >> : > : most systems. >> : > >> : > And we've been recommending to people for years that they avoid >> config >> : > space access in interrupt handlers. Maybe it is time for >> something >> : > that checks and prints a warning? >> : > >> : >> : With the move to memory-mapped pci config registers, there was an >> : intention to allow low-end devices to put their registers into >> config >> : space. I think I recall some legacy ultra-low end devices that >> also >> : put a few required registers into config space. So while it's >> not ideal >> : to access it from an interrupt handler, I can't think of why it >> should >> : be expressly forbidden. >> >> True. I wasn't planning on banning it, just warning about it so we >> could be purposeful in our use of it. Likely unworkable though... > > Easy enough to stash a "once" varible in the generic device struct > and warn when returning from an isr when INVARIANTS or something is > turned on. > > Then you'd only get one warning per device once it happens. > > I prefer to do nothing. It's not unsafe or erroneous to access cfg registers. But if some kind of message does get added, I insist that it drop all pretenses and say, "you're too poor to run freebsd, come back when you can afford better hardware.". :) Scott
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