Date: Wed, 24 Sep 2003 23:54:48 +0200 From: Maxime Henrion <mux@freebsd.org> To: Vincent Jardin <vjardin@free.fr> Cc: hackers@freebsd.org Subject: Re: BUS DMA sync Message-ID: <20030924215448.GF8404@elvis.mu.org> In-Reply-To: <200309242342.55788.vjardin@free.fr> References: <200309242342.55788.vjardin@free.fr>
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Vincent Jardin wrote: > Hi, > > I try to understand the purpose of the PRE sync (BUS_DMASYNC_PREREAD, > BUS_DMASYNC_PREWRITE). > > I understand POST synchronization (BUS_DMASYNC_POSTREAD, > BUS_DMASYNC_POSTWRITE) when a device to memory or a memory to device > transfert needs to be synchronized. However, what does a synchronization > before a transfert mean ? With common architectures where FreeBSD runs on, only BUS_DMASYNC_PREWRITE and BUS_DMASYNC_POSTREAD are actually meaningful and needed. It's good style to use the two others where appriopriate though, because some more exotic architecture might require it some day. BUS_DMASYNC_PREWRITE is used to synchronize before there's a DMA tranfer from host memory to a device. It's needed to ensure the device will read up-to-date memory, because recent changes to memory might still sit in a cache, so it actually flushes the memory cache. BUS_DMASYNC_POSTREAD is used to synchronize after a DMA transfer from the device to host memory. It's needed to ensure the software will get up-to-date memory as written by the device, and not outdated memory sitting in a cache, so it actually invalidates the memory cache. Cheers, Maxime
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