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

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Message-ID: <CAF6AEGsK25wk28YmiwsZTenecKqCt6irx66nR-8nOFMo6Z=Dkw@mail.gmail.com>
Date:	Wed, 12 Oct 2011 09:24:56 -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 9:01 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

BR,
-R

> mmap just feels wrong in this API, which is a buffer sharing API not a
> buffer mapping API.
>
>> I guess if sharing a buffer between multiple drm devices, there is
>> nothing stopping you from having some NOT_DMABUF_MMAPABLE flag you
>> pass when the buffer is allocated, then you don't have to support
>> dmabuf->mmap(), and instead mmap via device and use some sort of
>> DRM_CPU_PREP/FINI ioctls for synchronization..
>
> Or we could make a generic CPU accessor that we don't have to worry about.
>
> Dave.
>
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