On Tue, 27 Mar 2007, David Brownell wrote: > > Hmm... A CPU has to access all that data, and reading a cache line > > at a time surely as better. Although on the other hand, it's something > > streaming and thus just using the cache space for no good reason. > > Right. "Potentially". Displacing hot cachelines for use-once data > will in various cases be a lose.
It would be nice to see somewhere a more explicit and detailed discussion of the tradeoffs. Under what conditions is it better to allocate DMA memory rather than just using kmalloc() and calling dma_map_single()? > > OK... I admit I must do something for usbmon working better when > > DMA is pre-allocated, especially on PPC (where it simply gives > > up immediately and returns 'D' tag). But it's somewhat difficult. > > There's no portable API which I could use. > > That is, when the application manages DMA adressing? The driver "should" > provide both the DMA and regular address, since not every underlying > controller wil use the DMA address for all transfers. That's not quite true. The driver is obliged to provide a regular address if the controller doesn't use DMA, but it isn't obliged if the controller does use DMA. And in fact it frequently happens with usb-storage; the underlying SCSI drivers provide a scatter-gather list with page addresses that are suitable for DMA but may not lie in kernel-accessible memory. Alan Stern ------------------------------------------------------------------------- Take Surveys. Earn Cash. Influence the Future of IT Join SourceForge.net's Techsay panel and you'll get the chance to share your opinions on IT & business topics through brief surveys-and earn cash http://www.techsay.com/default.php?page=join.php&p=sourceforge&CID=DEVDEV _______________________________________________ linux-usb-devel@lists.sourceforge.net To unsubscribe, use the last form field at: https://lists.sourceforge.net/lists/listinfo/linux-usb-devel
