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

From: Jens Axboe
Date: Mon Aug 24 2015 - 16:15:19 EST

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
- 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

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.

Jens Axboe

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