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 It's needed to ensure the software will get up-to-date memory as written by the device, and not outdated memory sitting in a cache, so it actually invalidates the memory cache.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 _______________________________________________ [EMAIL PROTECTED] mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to "[EMAIL PROTECTED]"
