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. (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. Alan Stern -- 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
