Messages in this thread | | | Date | Mon, 24 Aug 2015 14:27:46 -0600 | From | Jens Axboe <> | Subject | Re: [PATCH] lib: scatterlist: add sg splitting function |
| |
On 08/24/2015 02:26 PM, Andrew Morton wrote: > 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.
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
| |