On Wednesday 28 March 2007 11:28 am, Pete Zaitcev wrote: > On Wed, 28 Mar 2007 11:25:54 -0400 (EDT), Alan Stern <[EMAIL PROTECTED]> > wrote: > > > > 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.
That is, highmem is as-always a problem. OK, that's an exception we can acknowledge. > Right. What's more, I found that the transfer_buffer may not even be > NULL in such cases. That seems error-prone. It should get fixed. > The pre-allocation of buffers and passing DMA address up and down > the stack caused a number of complications, which neither network > stack (w/ skbuff) nor block stack (w/ request) experience. For one > thing, conceptually, upper level driver has to query the DMA bits. > We hid this check inside the allocation helper, but it's still there. I don't see the issue you're alluding to. > Another complication is the special-casing of transfers to the root hub. > > Since both network and block I/O are high performance subsystems, > I always regarded all performance arguments for usb_buffer_alloc > as rubbish. They're both subsystems geared towards bulk data transfers, and the problem spots in the USB were way at the other end. Plus ISTR they don't deal with both scatterlists and normal DMA. There's a bit apples-vs-oranges component in that comment. - Dave > The poor granularity of DMA mapping is real, but it's > a different issue. > > The usbmon just brought the issues to the forefront, that's all. > > -- Pete > ------------------------------------------------------------------------- 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
