linux-kernel - Re: [PATCH] videobuf2-dma-sg: Minimize the number of dma segments

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Message-ID: <CAPybu_36Pe5+RO+qmUXTQtbESnrJg4hwey1mXXRH4urc9i4GAg@mail.gmail.com>
Date:	Thu, 18 Jul 2013 09:39:14 +0200
From:	Ricardo Ribalda Delgado <ricardo.ribalda@...il.com>
To:	Marek Szyprowski <m.szyprowski@...sung.com>
Cc:	Pawel Osciak <pawel@...iak.com>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Mauro Carvalho Chehab <mchehab@...hat.com>,
	linux-media@...r.kernel.org,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] videobuf2-dma-sg: Minimize the number of dma segments

Hello again:

I have started to implemt it, but I think there is more hidden work in
this task as it seems. In order to call dma_map_sg and
max_dma_segment_size I need acess to the struct device, but (correct
me if I am wrong), vb2 is device agnostic. Adding the above
functionality will mean not only updating marvell-ccic and solo6x10,
but updating all the vb2 buffers.

Also after some readings, maybe the sg compactation should not be done
here, but in dma_map_sg. According to the doc:

"""
The implementation is free to merge several consecutive sglist entries
into one (e.g. if DMA mapping is done with PAGE_SIZE granularity, any
consecutive sglist entries can be merged into one provided the first one
ends and the second one starts on a page boundary - in fact this is a huge
advantage for cards which either cannot do scatter-gather or have very
limited number of scatter-gather entries) and returns the actual number
of sg entries it mapped them to. On failure 0 is returned.
"""

So, my proposal would be to alloc with alloc_pages to try to get
memory as coherent as possible, then split the page, set the sg in
PAGE_SIZE lenghts, and then let the dma_map_sg do its magic. if it
doesnt do compactation, fix dma_map_sg, so more driver could take
advantage of it.

I could also of course fix marvell-ccic and solo6x10 to use sg_table.

Does anything of this make sense?

Regards

On Thu, Jul 18, 2013 at 9:15 AM, Marek Szyprowski
<m.szyprowski@...sung.com> wrote:
> Hello,
>
>
> On 7/17/2013 4:20 PM, Ricardo Ribalda Delgado wrote:
>>
>> Hello again Marek
>>
>> In my system I am doing the scatter gather compaction on device
>> driver... But I agree that it would be better done on the vb2 layer.
>>
>> For the oversize sglist we could do one of this two things.
>>
>> If we want to have a simple pass processing we have to allocate an
>> structure A for the worts case, work on that structure. then allocate
>> a structure B for the exact size that we need, memcpy A to B, and
>> free(A).
>>
>> Otherwise we need two passes. One to allocate the pages, and another
>> one to allocate the pages and find out the amount of sg, and another
>> to greate the sg structure.
>>
>> What do you prefer?
>
>
> I would prefer two passes approach. In the first pass you just fill the
> buf->pages array with order 0 entries and count the total number of memory
> chunks (adding support for max dma segment size at this point should be
> quite easy). In the second pass you just allocate the scatter list and
> fill it with previously allocated pages.
>
> I have also the following changes on my TODO list for vb2-dma-sg:
> - remove custom vb2_dma_sg_desc structure and replace it with common
> sg_table structure
> - move mapping of the scatter list from device driver to vb2-dma-sg module
> to simplify driver code and unify memory management across devices (so the
> driver just gets correctly mapped scatter list and only reads dma addresses
> of each memory chunk, no longer needs to track buffer state/ownership).
> The correct flow is to call dma_map_sg() at buffer allocation,
> dma_unmap_sg() at free and dma_sync_for_{device,cpu} in prepare/finish
> callbacks. The only problem here is the need to convert all existing users
> of vb2-dma-sg (marvell-ccic and solo6x10) to the new interface.
>
> However I have completely no time atm to do any of the above changes. Would
> You like to take any of the above tasks while playing with vb2-dma-sg?
>
>
> Best regards
> --
> Marek Szyprowski
> Samsung R&D Institute Poland
>
>

-- 
Ricardo Ribalda
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