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

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Date:	Wed, 12 Oct 2011 10:15:25 -0500
From:	Rob Clark <robdclark@...il.com>
To:	Dave Airlie <airlied@...il.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, daniel@...ll.ch
Subject: Re: [Linaro-mm-sig] [RFC 1/2] dma-buf: Introduce dma buffer sharing mechanism

On Wed, Oct 12, 2011 at 9:34 AM, Dave Airlie <airlied@...il.com> wrote:
> On Wed, Oct 12, 2011 at 3:24 PM, Rob Clark <robdclark@...il.com> wrote:
>> On Wed, Oct 12, 2011 at 9:01 AM, Dave Airlie <airlied@...il.com> wrote:
>>>> But then we'd need a different set of accessors for every different
>>>> drm/v4l/etc driver, wouldn't we?
>>>
>>> Not any more different than you need for this, you just have a new
>>> interface that you request a sw object from,
>>> then mmap that object, and underneath it knows who owns it in the kernel.
>>
>> oh, ok, so you are talking about a kernel level interface, rather than
>> userspace..
>>
>> but I guess in this case I don't quite see the difference.  It amounts
>> to which fd you call mmap (or ioctl[*]) on..  If you use the dmabuf fd
>> directly then you don't have to pass around a 2nd fd.
>>
>> [*] there is nothing stopping defining some dmabuf ioctls (such as for
>> synchronization).. although the thinking was to keep it simple for
>> first version of dmabuf
>>
>
> Yes a separate kernel level interface.

I'm not against it, but if it is a device-independent interface, it
just seems like six of one, half-dozen of the other..

Ie. how does it differ if the dmabuf fd is the fd used for ioctl/mmap,
vs if some other /dev/buffer-sharer file that you open?

But I think maybe I'm misunderstanding what you have in mind?

BR,
-R

> Well I'd like to keep it even simpler. dmabuf is a buffer sharing API,
> shoehorning in a sw mapping API isn't making it simpler.
>
> The problem I have with implementing mmap on the sharing fd, is that
> nothing says this should be purely optional and userspace shouldn't
> rely on it.
>
> In the Intel GEM space alone you have two types of mapping, one direct
> to shmem one via GTT, the GTT could be even be a linear view. The
> intel guys initially did GEM mmaps direct to the shmem pages because
> it seemed simple, up until they
> had to do step two which was do mmaps on the GTT copy and ended up
> having two separate mmap methods. I think the problem here is it seems
> deceptively simple to add this to the API now because the API is
> simple, however I think in the future it'll become a burden that we'll
> have to workaround.
>
> Dave.
>
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