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. 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"
