On Wed, Oct 31 2007 at 10:49 +0200, Jeff Garzik <jeff@xxxxxxxxxx> wrote: > I looked into killing sg_last(), but really, this is the best its gonna > get (moving sg_last to libata-core.c). > > You could maybe kill one use with caching, but in the other sg_last() > callsites there isn't another s/g loop we can stick a "last_sg = sg;" > into. > > libata is stuck because we undertake the highly unusual operation of > fiddling with the final S/G element, to enforce 32-bit alignment. > > Of course we could eliminate all that nasty fiddling/padding > completely, including sg_last(), if other areas of the kernel would > guarantee ahead of time that buffer lengths are always a multiple > of 4........ > > Jeff > OK Now I'm confused. I thought that ULD's can give you SG's that are actually longer than bufflen and that, at the end, the bufflen should govern the transfer length. Now FS_PC commands are sector aligned so you do not have problems with that. The BLOCK_PC commands have 2 main sources that I know of one is sg && bsg from user mode that can easily enforce 4 bytes alignment. The second is kernel services which 80% of these are done by scsi_execute(). All These can be found and fixed. Starting with scsi_execute(). Another place can be blk_rq_map_sg(), since all IO's are bio based. It can enforce alignment too. I would start by sticking a WARN_ON(qc->pad_len) and see if it triggers, what are the sources of that. Please note that the code already has a 4 bytes alignment assumption about the start of the transfer, other wise the first SG can also have a none aligned length, which is not checked for. Boaz - To unsubscribe from this list: send the line "unsubscribe linux-ide" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
