On Fri, 08 Aug 2008 23:58:23 +0200 Stefan Richter <stefanr@xxxxxxxxxxxxxxxxx> wrote: > FUJITA Tomonori wrote: > > On Fri, 08 Aug 2008 23:21:28 +0200 > > Stefan Richter <stefanr@xxxxxxxxxxxxxxxxx> wrote: > >> Here is what PPC does: > >> http://lxr.linux.no/linux+v2.6.26/arch/powerpc/kernel/iommu.c#L270 > >> > >> It looks at dma_get_max_seg_size(dev); and then merges according to it. > > > > Yes, IOMMUs were not able to handle this issue but they should now. > > > > > >> That's all nice and well, but in my case (FireWire storage protocol > >> a.k.a. SBP-2, which is basically remote DMA), the max_segment_size of > >> the PCI device is different from the size limit of the protocol. We > >> currently have to deconstruct such merges in the sbp2 drivers again: > >> http://lxr.linux.no/linux+v2.6.26/drivers/firewire/fw-sbp2.c#L1384 > >> > >> Either I keep it that way, or I let the protocol driver manipulate the > >> FireWire controller's dev->dma_parms->max_segment_size (via > >> dma_set_max_seg_size() of course), which is not entirely correct. > > > > Why is it not correct? Device drivers can set > > dma_set_max_seg_size(). SCSI does that (see __scsi_alloc_queue). > > FireWire is a multi-protocol bus. SBP-2 is just one of many quite > different protocols. SBP-2 targets read or write the initiators memory > buffers via remote DMA. These buffer may be exposed as s/g lists to the > target. The protocol limits these s/g lists to up to 65535 elements of > up to 65535 bytes size each. > > FireWire controllers on the other hand get their maximum segment size > set to 65536 by the PCI subsystem. (All FireWire controllers supported > by mainline Linux are PCI or PCIe devices.) > > In case of the drivers/firewire/ stack, the SBP-2 driver is currently > the only one which uses dma_map_sg. In case of the drivers/ieee1394/ > stack, also the drivers for isochronous protocols, including userspace > drivers via raw1394, use dma_map_sg. Thanks for the explanation. > So if the SBP-2 driver manipulated the controller device's > max_segment_size, it would influence the DMA mappings of the other > protocols. It wouldn't be a big deal; the isochronous mappings could IMHO, setting the safest value to max_segment_size is fine. After all, the maximum is only 65536 bytes. > only be collapsed to chunks of at most 15 pages instead of 16 pages. > However, the mapping deconstructing code in the SBP-2 drivers is not a > big deal either. -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
