linux-kernel - Re: [RFC v2 1/2] dma-buf: Introduce dma buffer sharing mechanism

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Date:	Tue, 10 Jan 2012 10:34:39 +0900
From:	InKi Dae <daeinki@...il.com>
To:	Rob Clark <rob@...com>
Cc:	Sumit Semwal <sumit.semwal@...com>, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
	linaro-mm-sig@...ts.linaro.org, dri-devel@...ts.freedesktop.org,
	linux-media@...r.kernel.org, linux@....linux.org.uk, arnd@...db.de,
	jesse.barker@...aro.org, m.szyprowski@...sung.com,
	t.stanislaws@...sung.com, Sumit Semwal <sumit.semwal@...aro.org>,
	daniel@...ll.ch
Subject: Re: [RFC v2 1/2] dma-buf: Introduce dma buffer sharing mechanism

2012/1/10 Rob Clark <rob@...com>:
> On Mon, Jan 9, 2012 at 4:10 AM, InKi Dae <daeinki@...il.com> wrote:
>> note : in case of sharing a buffer between v4l2 and drm driver, the
>> memory info would be copied vb2_xx_buf to xx_gem or xx_gem to
>> vb2_xx_buf through sg table. in this case, only memory info is used to
>> share, not some objects.
>
> which v4l2/vb2 patches are you looking at?  The patches I was using,
> vb2 holds a reference to the 'struct dma_buf *' internally, not just
> keeping the sg_table
>

yes, not keeping the sg_table. I mean... see a example below please.

static void vb2_dma_contig_map_dmabuf(void *mem_priv)
{
    struct sg_table *sg;
     ...
     sg = dma_buf_map_attachment(buf->db_attach, dir);
     ...
     buf->dma_addr = sg_dma_address(sg->sgl);
     ...
}

at least with no IOMMU, the memory information(containing physical
memory address) would be copied to vb2_xx_buf object if drm gem
exported its own buffer and vb2 wants to use that buffer at this time,
sg table is used to share that buffer. and the problem I pointed out
is that this buffer(also physical memory region) could be released by
vb2 framework(as you know, vb2_xx_buf object and the memory region for
buf->dma_addr pointing) but the Exporter(drm gem) couldn't know that
so some problems would be induced once drm gem tries to release or
access that buffer. and I have tried to resolve this issue adding
get_shared_cnt() callback to dma-buf.h but I'm not sure that this is
good way. maybe there would be better way.

Thanks.

> BR,
> -R
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