Date: Mon, 31 Oct 2005 03:51:18 +0100 From: Daniel Rock <freebsd@deadcafe.de> To: "M. Warner Losh" <imp@bsdimp.com> Cc: Alexander@Leidinger.net, gurney_j@resnet.uoregon.edu, freebsd-current@freebsd.org, davidxu@freebsd.org Subject: Re: TSC instead of ACPI: powerd doesn't work anymore (to be expected?) Message-ID: <436586A6.4090907@deadcafe.de> In-Reply-To: <20051030.161606.65680605.imp@bsdimp.com> References: <20051030093718.GE39253@dragon.NUXI.org> <4364D90F.3090205@samsco.org> <20051030195936.GZ4115@funkthat.com> <20051030.161606.65680605.imp@bsdimp.com>
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M. Warner Losh schrieb: > Actually, ioport reads can be faster than the 1us that's widely > quoted. ioport reads can be as fast as ~125ns (2 cycles at 16MHz). > However, experience has shown that they are rarely this fast. I've > seen 8MB/s pio over the pci bus on some custom hardware we have, which > 2Mreads/sec which is about 500ns per read. I think that the pci > hardware that I was reading had a few extra wait states... According to "Software Optimization Guide for AMD64 Processors": http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/25112.PDF the "in" machine instructions have a latency of ~180 clock cycles. But this will be only the lower theoretical limit. Daniel
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