Date: Wed, 18 Sep 1996 09:26:50 -0700 From: David Greenman <dg@root.com> To: "Mike Durian" <durian@plutotech.com> Cc: Michael Smith <msmith@atrad.adelaide.edu.au>, freebsd-hackers@freebsd.org Subject: Re: Special Cycles on the PCI bus Message-ID: <199609181626.JAA12461@root.com> In-Reply-To: Your message of "Wed, 18 Sep 1996 10:08:53 MDT." <199609181608.KAA12961@pluto.plutotech.com>
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>>Are they causing you serious grief? > > They will. We're building custom hardware that uses 4 PCI busses and >*lots* of bandwidth. We want to minimize the PCI bus traffic as >much as possible. > Since none of the chips we are using claim to produce these >special cycles (and certainly not illegal ones), they are most likely >being produced by some side effect. Perhaps some code is accidentally >writing to an undocumented register in the Triton chipset that causes >the special cycles. In any case, the special cycles are most likely >symptoms of some software bug that might have other unwanted effects. >Even if the special cycles are themselves innocuous, we should at least >determine why they are generated. Perhaps they are caused by the inb(0x84)'s that are used as time delays in a couple places of the kernel. The inb(0x84) is supposed to be an unused port and thus should take about 1.25us to read from. This isn't always true, however, as people have pointed out in the past. We should probably replace the last few places where these are used with a calibrated wait loop. -DG David Greenman Core-team/Principal Architect, The FreeBSD Project
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