linux-kernel - Re: [PATCH] lib: scatterlist: add sg splitting function

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Message-ID: <55DB7E42.4070803@kernel.dk>
Date:	Mon, 24 Aug 2015 14:27:46 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Robert Jarzmik <robert.jarzmik@...e.fr>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Guennadi Liakhovetski <g.liakhovetski@....de>,
	linux-kernel@...r.kernel.org,
	Mauro Carvalho Chehab <mchehab@....samsung.com>,
	linux-media@...r.kernel.org
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@...nel.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@...e.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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