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. -- Marcel Moolenaar xcl...@mac.com _______________________________________________ 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"
