On Fri, Jun 11, 2010 at 11:37:36AM -0600, Scott Long wrote: > On Jun 11, 2010, at 11:32 AM, Marcel Moolenaar wrote: > > > > On Jun 11, 2010, at 10:21 AM, Scott Long wrote: > > > >> On Jun 11, 2010, at 11:04 AM, Marcel Moolenaar wrote: > >>> > >>> On Jun 11, 2010, at 9:12 AM, Scott Long wrote: > >>> > >>>> On Jun 11, 2010, at 5:51 AM, John Baldwin wrote: > >>>>> On Thursday 10 June 2010 11:00:33 pm Marcel Moolenaar wrote: > >>>>>> Author: marcel > >>>>>> Date: Fri Jun 11 03:00:32 2010 > >>>>>> New Revision: 209026 > >>>>>> URL: http://svn.freebsd.org/changeset/base/209026 > >>>>>> > >>>>>> Log: > >>>>>> Bump MAX_BPAGES from 256 to 1024. It seems that a few drivers, bge(4) > >>>>>> in particular, do not handle deferred DMA map load operations at all. > >>>>>> Any error, and especially EINPROGRESS, is treated as a hard error and > >>>>>> typically abort the current operation. The fact that the busdma code > >>>>>> queues the load operation for when resources (i.e. bounce buffers in > >>>>>> this particular case) are available makes this especially problematic. > >>>>>> Bounce buffering, unlike what the PR synopsis would suggest, works > >>>>>> fine. > >>>>>> > >>>>>> While on the subject, properly implement swi_vm(). > >>>>> > >>>>> NIC drivers do not handle deferred load operations at all (note that > >>>>> bus_dmamap_load_mbuf() and bus_dmamap_load_mbuf_sg() enforce > >>>>> BUS_DMA_NOWAIT). > >>>>> It is common practice to just drop the packet in that case. > >>>>> > >>>> > >>>> Yes, long ago when network drivers started being converted to busdma, it > >>>> was agreed that EINPROGRESS simply doesn't make sense for them. Any > >>>> platform that winds up making extensive use of bounce buffers for > >>>> network hardware is going to perform poorly no matter what, and should > >>>> hopefully have some sort of IOMMU that can be used instead. > >>> > >>> Unfortunately things aren't as simple as is presented. > >>> > >>> For one, bge(4) wedges as soon as the platform runs out of bounce > >>> buffers when they're needed. The box needs to be reset in order to > >>> get the interface back. I pick any implementation that remains > >>> functional over a mis-optimized one that breaks. Deferred load > >>> operations are more performance optimal than failure is. > >>> > >> > >> This sounds like a bug in the bge driver. I don't see if through casual > >> inspection, but the driver should be able to either drop the mbuf > >> entirely, or requeue it on the ifq and then restart the ifq later. > >> > >>> Also: the kernel does nothing to guarantee maximum availability > >>> of DMA-able memory under load, so bounce buffers (or use of I/O > >>> MMUs for that matter) are a reality. Here too the performance > >>> argument doesn't necessarily hold because the kernel may be > >>> busy with more than just sending and receiving packets and the > >>> need to defer load operations is very appropriate. If the > >>> alternative is just dropped packets, I'm fine with that too, > >>> but I for one cannot say that *not* filling a H/W ring with > >>> buffers is not going to wedge the hardware in some cases. > >>> > >>> Plus: SGI Altix does not have any DMA-able memory for 32-bit > >>> hardware. The need for an I/O MMU is absolute and since there > >>> are typically less mapping registers than packets on a ring, > >>> the need for deferred operation seems quite acceptable if the > >>> alternative is, again, failure to operate. > >>> > >> > >> I'm not against you upping the bounce buffer limit for a particular > >> platform, but it's still unclear to me if (given bug-free drivers) it's > >> worth the effort to defer a load rather than just drop the packet and let > >> the stack retry it. One question that would be good to answer is wether > >> the failed load is happening in the RX to TX path. > > > > RX path I believe. > > > > I'm not clear why you even need bounce buffers for RX. The chip supports > 64bit addresses with no boundary or alignment restrictions. >
Some controllers have 4G boundary bug so bge(4) restricts dma address space. > Scott > > _______________________________________________ svn-src-all@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/svn-src-all To unsubscribe, send any mail to "svn-src-all-unsubscr...@freebsd.org"
