On Mon, 2006-02-06 at 16:45 +0000, Hugh Dickins wrote: > But I'd also want James or someone to clarify the paragraph > "Please note that the sg cannot be mapped again if it has been mapped once. > The mapping process is allowed to destroy information in the sg." > which I took as explicitly allowing what x86_64 does in gart_map_sg. > I thought James had a scenario in mind which demands this wholesale > destruction, but it seems not; and I now read that first sentence as > saying the sg must be unmapped before it can be mapped a second time, > not that it can only be mapped once. > > And add a paragraph explaining that really the one array of scatterlist > entries should be regarded as two arrays of possibly different lengths, > the page,offset,length array and the dma_address,dma_length array: > because entries of the latter may be coalesced, so that in the end > the dma_address in a scatterlength entry may bear no relation to the > page pointer in that same entry, but to the page pointer in a later entry. > Though it gets hard to explain given that not all architectures coalesce, > so may not even have a separate dma_length field; or use different naming. > If you can express this better than I, please do! Yes, I added that piece after the x86_64 problem. The original x86_64 bug was that you couldn't do dma_map_sg() then dma_unmap_sg() then dma_map_sg() on the same scatterlist (the map was destroying information which wasn't restored on the unmap, so the second map produced an incorrect scatterlist), which was causing a bug in all SCSI drivers (because that's the way SCSI requeueing works). James - : 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
