At 09:26 AM 12/12/00 -0600, Brian Ford wrote: >On Tue, 12 Dec 2000, Graham Stoney wrote: > > > Also, doesn't the 8260 have seperate memory subsystems to help get > > around this? > > >I assume you are referring to the local bus? Well yes, but there are >large >tradeoffs. > >If you use the local bus for the receive buffers then you can have >simultaneous CPM to local bus and CPU to 60x bus transactions. The catch >is that the local bus can not be cached. So, you trade off bus contention >for caching/bursting. The CPU must go across the 60x to local bus bridge >for those transactions. The DMA engine can burst between the 60x and >local busses. > >If the data has to end up in user space, it ends up being about a >wash, given the checksum and user space copy. I need more testing to >confirm this, though. If the user space copy was not needed, like for >routing, then it might help some. > >-- >Brian Ford >Software Engineer >Vital Visual Simulation Systems >FlightSafety International >Phone: 314-551-8460 >Fax: 314-551-8444
I've been known to be wrong in the past, and I could be missing an assumption, but local bus memory is cachable, it just isn't snoopable. If you need snooping as a prerequisite for enabling cache, that would make the local bus effectively uncachable. It also is 32 bits wide (max) rather than 64 which will affect your bus bandwidth. gvb ** Sent via the linuxppc-embedded mail list. See http://lists.linuxppc.org/