linux-kernel - RE: [RFC PATCH v2 4/8] media: videobuf2: introduce VIDEOBUF2

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Date:	Fri, 23 Dec 2011 11:38:34 +0100
From:	Marek Szyprowski <m.szyprowski@...sung.com>
To:	'Ming Lei' <ming.lei@...onical.com>
Cc:	'Mauro Carvalho Chehab' <mchehab@...radead.org>,
	'Tony Lindgren' <tony@...mide.com>,
	'Sylwester Nawrocki' <snjw23@...il.com>,
	'Alan Cox' <alan@...rguk.ukuu.org.uk>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-media@...r.kernel.org, 'Pawel Osciak' <p.osciak@...il.com>
Subject: RE: [RFC PATCH v2 4/8] media: videobuf2: introduce VIDEOBUF2_PAGE
 memops

Hello,

On Friday, December 23, 2011 10:51 AM Ming Lei wrote:

> On Fri, Dec 23, 2011 at 5:34 PM, Marek Szyprowski
> <m.szyprowski@...sung.com> wrote:
> 
> >> For example, on ARM, there is very limited kernel virtual address space reserved
> >> for DMA coherent buffer mapping, the default size is about 2M if I
> >> don't remember mistakenly.
> >
> > It can be easily increased for particular boards, there is no problem with this.
> 
> It is not easily to increase it because there is very limited space reserved for
> this purpose, see Documentation/arm/memory.txt. Also looks like it is
> not configurable.

It is really not a big issue to increase it by a few MBytes.
 
> >> > I understand that there might be some speed issues with coherent (uncached)
> >> > userspace mappings, but I would solve it in completely different way. The interface
> >>
> >> Also there is poor performance inside kernel space, see [1]
> >
> > Your driver doesn't access video data inside kernel space, so this is also not an issue.
> 
> Why not introduce it so that other drivers(include face detection) can
> benefit with it? :-)

We can get back into this once a driver which really benefits from comes.
 
> >> >
> >> > Your current implementation also abuses the design and api of videobuf2 memory
> >> > allocators. If the allocator needs to return a custom structure to the driver
> >>
> >> I think returning vaddr is enough.
> >>
> >> > you should use cookie method. vaddr is intended to provide only a pointer to
> >> > kernel virtual mapping, but you pass a struct page * there.
> >>
> >> No, __get_free_pages returns virtual address instead of 'struct page *'.
> >
> > Then you MUST use cookie for it. vaddr method should return kernel virtual address
> > to the buffer video data. Some parts of videobuf2 relies on this - it is used by file
> > io emulator (read(), write() calls) and mmap equivalent for non-mmu systems.
> >
> > Manual casting in the driver is also a bad idea, that's why there are helper functions
> 
> I don't see any casts are needed. The dma address can be got from vaddr with
> dma_map_* easily in drivers, see the usage on patch 8/8(media: video: introduce
> omap4 face detection module driver).

Sorry, but I won't accept such driver/allocator which abuses the defined API. I've already
pointed what vaddr method is used for.

> > defined for both dma_contig and dma_sg allocators: vb2_dma_contig_plane_dma_addr() and
> > vb2_dma_sg_plane_desc().
> 
> These two helpers are not needed and won't be provided by VIDEOBUF2_PAGE memops.

I gave the example. Your allocator should have something similar.

Best regards
-- 
Marek Szyprowski
Samsung Poland R&D Center



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