On Fri, Oct 26 2007, Linus Torvalds wrote:
>
>
> 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.
Do you really allocate a fresh table for every command, or just a max
sized one at init?
> > 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.
That'd be fine with me as well, but I really don't think that a lot of
people really do need the sg count when you can just loop over the table
until it returns NULL.
--
Jens Axboe
-
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/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
