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

[Jens Axboe]

On 08/24/2015 02:26 PM, Andrew Morton wrote:
> On Mon, 24 Aug 2015 14:15:08 -0600 Jens Axboe <axboe@xxxxxxxxx> 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@xxxxxxx>
> >
> > 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.

Right

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

I guess that's true, errors in this area will be found pretty quickly.

--
Jens Axboe

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