From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mailserv2.iuinc.com (IDENT:qmailr@mailserv2.iuinc.com [206.245.164.55]) by puffin.external.hp.com (8.9.3/8.9.3) with SMTP id XAA25155 for ; Mon, 1 Jan 2001 23:42:27 -0700 Message-Id: <200101020647.WAA26293@milano.cup.hp.com> To: Matthew Wilcox Cc: parisc-linux@thepuffingroup.com Subject: Re: [parisc-linux] ZONE_DMA In-reply-to: Your message of "Tue, 02 Jan 2001 01:21:22 PST." <20010102012122.A10153@parcelfarce.linux.theplanet.co.uk> Date: Mon, 01 Jan 2001 22:46:58 -0800 From: Grant Grundler List-ID: Matthew Wilcox wrote: > But, if possible, we want to avoid doing DAC as it cuts down the bandwidth > (and increases latency by a clock). obviously, if it's a 64-bit device > and a 64-bit controller, there's no problem, but we should try to stick > to 32-bit bus addresses where possible. That's correct. But then we can bypass the I/O MMU. SBA HW has bits to support that - but I don't fully understand all the limitations of when this "mode" can be used. Bypassing the I/O MMU would improve DMA performance and reduce work CPU has to do to "map" each page of DMA. Though it's generally a good direction, optimizing the PCI bus isn't always the right answer. Ie moving from 32- to 64-bit wide data transfers is a *much* bigger performance win than DAC. grant Grant Grundler Unix Systems Enablement Lab +1.408.447.7253