On Fri, 26 Oct 2007, Paul Mackerras wrote:
quoted text
>
> 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.

quoted text
> 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
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