On Thu, 10 Jul 2008 17:56:08 -0400 (EDT) Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote: > When I thought about it more, I realized that this accounting of virtual > segments in I/O layer can't work correctly at all. If an architecture > defines symbols BIOVEC_VIRT_MERGEABLE and BIOVEC_VIRT_OVERSIZE, it > declares that it's IOMMU must merge any two regions satisfying these > conditions. But in an IOMMU, it is impossible to guarantee, because: Yeah, IOMMUs can't guarantee that. The majority of architectures set BIO_VMERGE_BOUNDARY to 0 so they don't hit this, I think. > * the bus address is allocated basiclly randomly, so we can hit > dev->dma_parms->segment_boundary_mask any time. This will prevent virtual > merging from happenning. I/O layer doesn't know the bus address at the > time it merges requests, so it can't predict when this happens. > > * the IOMMU isn't guaranteed to find a continuous space in it's bus > address space. If it skips over already mapped regions, it can't perform > virtual merging. > > * when creating the mapping, we can hit per-device limit > "dev->dma_parms->max_segment_size" --- but the I/O layer checks only > against global limit BIOVEC_VIRT_OVERSIZE. (this last issue is fixable, > the previous two are not). I think that the block layer can handle this properly via q->max_segment_size. We have the same value at two different places. Yeah, it's not good... BTW, inia100_template sets sg_tablesize to SG_ALL. If the controller has at most 32 SG entries per request, we need to fix that. -- To unsubscribe from this list: send the line "unsubscribe sparclinux" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
