On Tue, 2015-04-28 at 19:15 -0700, James Bottomley wrote: > On Wed, 2015-04-29 at 09:34 +0900, Akinobu Mita wrote: > > 2015-04-29 7:16 GMT+09:00 James Bottomley > > <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>: > > > On Tue, 2015-04-28 at 14:27 -0700, Andrew Morton wrote: > > >> On Sat, 25 Apr 2015 23:56:16 +0900 Akinobu Mita <akinobu.mita@xxxxxxxxx> wrote: > > >> > > >> > Some architectures enable sg chaining option while others do not. > > >> > > > >> > The requirement to enable sg chaining is that pages must be aligned > > >> > at a 32-bit boundary in order to overload the LSB of the pointer. > > >> > Regardless of whether ARCH_HAS_SG_CHAIN is defined or not, the above > > >> > requirement is always chacked by BUG_ON() in sg_assign_page. So > > >> > all architectures can enable sg chaining. > > >> > > > >> > As you can see from the changes in drivers/target/target_core_rd.c, > > >> > enabling SG chaining for all architectures allows us to allocate > > >> > discontiguous scatterlist tables which can be traversed throughout > > >> > by sg_next() without a special handling for some architectures. > > >> > > >> Thanks, I'll grab this. If anyone has concerns, speak now or hold both > > >> pieces! > > > > > > It breaks a host of architectures doesn't it? I can specifically speak > > > for PARISC: The problem is the way our iommus are consuming > > > scatterlists. They're assuming we can dereference the scatterlist as an > > > array (like this code in ccio-dma.c): > > > > > > static int > > > ccio_map_sg(struct device *dev, struct scatterlist *sglist, int nents, > > > enum dma_data_direction direction) > > > [...] > > > for(i = 0; i < nents; i++) > > > prev_len += sglist[i].length; > > > > > > If you turn on sg chaining on our architecture, we'll run off the end of > > > that array dereference and crash. > > > > > > This can all be fixed by making our architecture dma mapping code use > > > iterators instead of array lists, but that needs more code than this > > > patch provides. I assume there are similar issues on a lot of other > > > architectures, so before you can contemplate a patch like this, surely > > > all the architecture consumers have to be converted to iterator instead > > > of array format? > > > > > > The first place to start would be a survey of who's still using the > > > array format. > > > > Agreed. I could find similar issues in arch/m68k/kernel/dma.c. > > (git grep '[^a-z]sg++' shows that there are a lot of similar issues) > > OK, so the original idea of the chained SG lists was that most of the > older architectures have fixed length lists for their IOMMUs, or simply > wouldn't see a benefit with IO lengths > 0.5MB (which was the default > before chaining) so there wasn't much point converting them to chaining > if they wouldn't see any benefit from it. > > ARCH_HAS_SG_CHAIN is supposed to be completely transparent to all driver > side consumers, so there was never thought to be much point removing it. > It looks like there's some sort of cockup going on in the target driver > but otherwise, your removal patch is pretty empty, confirming this. > > Perhaps the best thing to do is just fix target and call it quits? > So the ARCH_HAS_SG_CHAIN usage in target_core_rd.c was recently added so target DIF emulation could use standard SGL iterators and correctly handle boundaries across T10-PI metadata SGL tables in the ramdisk backend. The SGLs in question are never actually mapped to a HW IOMMU, and Akinobu's current changes in mainline do support both arch cases and make common sbc_dif_copy_prot() code a bit simpler too. That said, I'd rather to keep the hack around for now so that both ARCH_HAS_SG_CHAIN types can still work, short of a full arch conversion of course.. Thanks, --nab -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
