On Mon, 14 Feb 2011 01:01:12 -0800 "Nicholas A. Bellinger" <nab@xxxxxxxxxxxxxxx> wrote: > > btw, can we kill the non scatter/gather data path? I think that we > > should always use the scatter/gather data transfer. > > Unfortuately it's not that easy. The main reason why CDB type > SCF_SCSI_CONTROL_NONSG_IO_CDB was originally added (back in 2.2/2.4 > days) was because certain LLDs had a problem with basic control CDBs > using SGLs.. > > Obviously we are way past that point with drivers/scsi today, but the > main reason today why SCF_SCSI_CONTROL_NONSG_IO_CDB still exists is > because of CDB emulation for complex stuff in target_core_cdb.c. It has > historically proven much easier to code complex CDB emulation using a > contigious buffer, than with walking SGL formatted memory. > > Converting over the more complex CDB emulation stuff to SGLs would > somewhat painful, at least without adding an extra location allocation + > copy into SGLs (not a big deal for CONTROL CDB stuff), lib/scatterlist.c provides nice helper functions to copy data between a linear buffer and an SG list. I think that at least, it's more clear than now. > or something else > to obtain a virtually contigious buffer for building the emulated > response. Hmm, I don't think that very large contiguous buffer for CONTROL CDB so why can't we allocate a physically contiguous buffer for it? We could play with vmap for a virtually contiguous buffer but it adds the complicated cache issue so I don't like that. -- 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
