Re: [PATCH] lib: scatterlist: add sg splitting function

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 24 Aug 2015 14:15:08 -0600 Jens Axboe <axboe@kernel.dk> wrote:

> On 08/08/2015 02:44 AM, Robert Jarzmik wrote:
> > Sometimes a scatter-gather has to be split into several chunks, or sub
> > scatter lists. This happens for example if a scatter list will be
> > handled by multiple DMA channels, each one filling a part of it.
> >
> > A concrete example comes with the media V4L2 API, where the scatter list
> > is allocated from userspace to hold an image, regardless of the
> > knowledge of how many DMAs will fill it :
> >   - in a simple RGB565 case, one DMA will pump data from the camera ISP
> >     to memory
> >   - in the trickier YUV422 case, 3 DMAs will pump data from the camera
> >     ISP pipes, one for pipe Y, one for pipe U and one for pipe V
> >
> > For these cases, it is necessary to split the original scatter list into
> > multiple scatter lists, which is the purpose of this patch.
> >
> > The guarantees that are required for this patch are :
> >   - the intersection of spans of any couple of resulting scatter lists is
> >     empty.
> >   - the union of spans of all resulting scatter lists is a subrange of
> >     the span of the original scatter list.
> >   - streaming DMA API operations (mapping, unmapping) should not happen
> >     both on both the resulting and the original scatter list. It's either
> >     the first or the later ones.
> >   - the caller is reponsible to call kfree() on the resulting
> >     scatterlists.
> >
> > Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
> 
> I think this looks fine. But do we really need the Kconfig option? It's 
> not a lot of code, and it seems silly to put the onus on the driver for 
> having to enable something that is a subset of the SG api.

Blame me for that.  It's so that all kernels don't need to carry a lump
of code which only a small number of media drivers actually use.

The tradeoff is a bit of once-off build-time effort versus a permanent
runtime gain for many systems.  That's a good tradeoff.