Date: Mon, 30 Jun 2003 10:03:46 -0400 (EDT) From: Andrew Gallatin <gallatin@cs.duke.edu> To: Terry Lambert <tlambert2@mindspring.com> Cc: freebsd-arch@freebsd.org Subject: Re: API change for bus_dma Message-ID: <16128.17218.973911.980939@grasshopper.cs.duke.edu> In-Reply-To: <3EFD4755.49BAF150@mindspring.com> References: <3EF3C12F.9060303@btc.adaptec.com> <16124.39930.142492.356163@grasshopper.cs.duke.edu> <3EFC9F2D.6020908@btc.adaptec.com> <16124.43999.333761.397624@grasshopper.cs.duke.edu> <3EFCAC7A.6060305@btc.adaptec.com> <16124.45051.919899.414795@grasshopper.cs.duke.edu> <3EFD4755.49BAF150@mindspring.com>
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Terry Lambert writes: > Andrew Gallatin wrote: > > Most sparc's have 2 different sorts of DMA modes. One is cache > > coherent (aka DDI_DMA_CONSISTENT) -- this is what we all know and love > > from PC, alphas, macs, etc. > > "contiguous" > > > > The other mode (DDI_DMA_STREAMING) allows non cache coherent DMA. > > This requires you to call ddi_dma_sync() between your last touch of > > the data and you starting a DMA read from a device. And vice-versa > > for a DMA write. > > "scatter/gather" > > > The reason people use DDI_DMA_STREAMING is because coherent DMA > > bandwith tends to be abysmal on most sparcs. > The idea is that I want to establish a mapping that can be used many times without any driver or kernel attention. I don't want to do anything in terms of a system call, or interrupt, etc, to sync the cache with the state of the DMA'ed page before a DMA read or after a DMA write. For example, scatter gather mapping on alphas (if FreeBSD supported it for PCI devices) would be fine with me since its cache-coherent and doesn't require any ddi_dma_sync() operations. Drew
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