Date: Sun, 13 Oct 2002 23:34:22 -0700 From: Peter Wemm <peter@wemm.org> To: Wes Peters <wes@softweyr.com> Cc: Terry Lambert <tlambert2@mindspring.com>, ticso@cicely.de, "M. Warner Losh" <imp@bsdimp.com>, hch@infradead.org, dillon@apollo.backplane.com, vova@sw.ru, nate@root.org, arch@FreeBSD.org Subject: Re: Database indexes and ram Message-ID: <20021014063422.1056D2A88D@canning.wemm.org> In-Reply-To: <3DAA2C4F.9E15CA75@softweyr.com>
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Wes Peters wrote: > Terry Lambert wrote: > > > > Bernd Walter wrote: > > > Of course they can do. > > > It's just a matter if the card and the board support 2 address cycles. > > > Or if the board can map the pci reachable space - as alphas can do. > > > > The question is whether you can say reliably that all cards that > > will be sharing cached data space can do this, or whether you > > will have to bounce the data to below 4G. > > > > If you can't *know*, then to ensure operation, you *must* bounce > > the data to proactively guarantee that the physical address will > > be in range of the card's DMA engine. > > > > Among other things, this means recognizing a 32 bit card in a 64 > > bit slot, and a 64 bit card in a 32 bit slot, and a 64 bit card > > in a 64 bit slot, but which has only 32 bits worth of electrical > > connector on the physical card. > > > > If you can guarantee that, then you can do it without bouncing. > > > > Can you do that? > > No, and that's exactly why the Linux developers took the tack they did: > all of the DMA targets are allocated in the lower 4GB of physical address > space. It was quite an intelligent decision, one that made me grin when > I "got it." And then there's the AGP remap table stuff to provide a window from anywhere in memory into the lower 4G of space that's within reach of 32 bit PCI devices... Cheers, -Peter -- Peter Wemm - peter@wemm.org; peter@FreeBSD.org; peter@yahoo-inc.com "All of this is for nothing if we don't go to the stars" - JMS/B5 To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-arch" in the body of the message
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