Date: Wed, 28 Apr 2004 23:12:18 -0600 From: Scott Long <scottl@freebsd.org> To: Andy Farkas <andyf@speednet.com.au> Cc: freebsd-scsi@freebsd.org Subject: Re: AHC_ALLOW_MEMIO 5.2.1 Message-ID: <40908EB2.4090803@freebsd.org> In-Reply-To: <20040429135222.L64510@hewey.af.speednet.com.au> References: <6.1.0.6.2.20040428074703.024548b8@localhost> <20040429135222.L64510@hewey.af.speednet.com.au>
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Andy Farkas wrote: > On Wed, 28 Apr 2004, J.D. Bronson wrote: > > >>What happened to this kernel option on 5.2.1? >> >>AHC_ALLOW_MEMIO >> >> >>It does not seem to be in any file at all anymore? >> >> -JDB > > > Its in /sys/conf/NOTES :) > > The generated file opt_aic7xxx.h will contain it. > > I sometimes enable this option in my kernels. Whether it does anything or > not (performance-wise or other), I don't know. > On x86, MEMIO is generally faster and causes the CPU to spin less than IOPORT cycles do. It can result in a minor but measurable speed difference, though the effects of HyperThreading, when in use, could offset the benefits. In any case, the reason that this option exists is to work around motherboards that incorrectly do write-combining on MEMIO registers, something that most ahc/ahd chips do not support nor handle well. We added a somewhat sophisticated runtime test for this to the driver last year, so there really isn't a reason to not enable the option. If you do and the test detects problems, it will automatically throttle back to IOPORT. We should probably just remove the option all-together, and just use the sysctl/tunable as a backup in case problems develope. Scott
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