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SubjectRe: [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



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