On Fri, 26 Oct 2007, Paul Mackerras wrote: > > Linus Torvalds writes: > > > Nobody should *ever* walk the list to find the length. Does anybody really > > do that? Yes, we pass the thing down, but do people *need* it? > > Yes, I need it for devices that use the macintosh DBDMA > (descriptor-based DMA) hardware. The DBDMA hardware reads an array of > descriptors from system RAM, so I need to allocate an array and fill > it in with DBDMA command blocks (and then dma-map it and point the > device at it).
Yes, for allocation purposes you'd need the size ahead of time, agreed. Otherwise you have to walk the list twice. > Maybe the drivers for devices that use DBDMA are now buggy. Certainly > filling in the array of DBDMA command blocks involves walking the > list, but it would extremely useful to know how much to allocate > before we start filling them in. So we at least need an upper bound > on the number of "real" entries, even if we don't have the exact > number. Hmm. Depending on where you do this, and if this is some block-layer specific driver/code (rather than necessarily a generic SG thing), you do have the req->nr_phys_segments thing which should be that for you (ie the SG list may have _fewer_ requests in it in case some of those entries got squashed together due to be contiguous). But yeah, I don't think it would be wrong at all to have a struct scatterlist_head { unsigned int entries; unsigned int flags; /* ? */ struct scatterlist *sg; }; which would be passed down at higher levels. Linus - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/