On Fri, 14 Mar 2008 10:46:52 -0400 (EDT) Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: > On Fri, 14 Mar 2008, FUJITA Tomonori wrote: > > > > If nents doesn't change then for_each_sg() won't work right. There > > > could be an alternative macro: > > > > Oops, I thought that for_each_sg is defined like: > > > > #define for_each_sg(sglist, sg, nr, __i) \ > > for (__i = 0, sg = (sglist); __i < (nr) && sg; __i++, sg = sg_next(sg)) > > > > > > > /* > > > * Loop over each sg element, stopping at the end of the chain > > > */ > > > #define for_each_sg_all(sglist, sg, __i) \ > > > for (__i = 0, sg = (sglist); sg; __i++, sg = sg_next(sg)) > > > > > > If you added this macro to include/linux/scatterlist.h and used it > > > instead of for_each_sg() then you can get rid of nents entirely. > > > However I'm not sure whether this would be safe. Do people sometimes > > > use a subset of the entries in a scatterlist? > > > > IIRC, some drivers do that (though they might use sg_next). > > But will usb-storage ever receive a scatterlist like that? For > example, if there are three 4096-byte entries in the list, but the > transfer length is 8192 and nents is 2, then there could be a problem. If LLDs don't use the padding or drain buffer feature (USB uses neither), scsi midlayer doesn't send such (that is, the block layer doesn't create such). > (This could happen if some software layer preallocated an sg chain and > used it over and over again, each time setting nents to whatever value > was needed for a particular transfer.) > > > I don't think that we add a new macro just for this function. We could > > change for_each_sg in the above way or we could just do in > > usb_stor_access_xfer_buf > > > > for (i = 0, sg = *sgl; i < nents && sg; i++, sg = sg_next(sg)) > > This wouldn't be safe in the example I just mentioned if the > usb-storage driver tried to do three transfers, each of 4096 bytes. > All three would succeed, but in fact the third call shouldn't transfer > any data. As I explained above, it should be safe with USB. But in general, LLDs should not rely on a scatterlist about how much data they transfer. -- 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
