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

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Message-ID: <CAKMK7uFpQfAkoEqAJc8hX6k_kOsXR=u5O=fgyaNfaDM89cciSw@mail.gmail.com>
Date:	Mon, 5 Dec 2011 23:33:36 +0100
From:	Daniel Vetter <daniel@...ll.ch>
To:	Rob Clark <rob@...com>
Cc:	Daniel Vetter <daniel@...ll.ch>, t.stanislaws@...sung.com,
	linux@....linux.org.uk, Arnd Bergmann <arnd@...db.de>,
	linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	linaro-mm-sig@...ts.linaro.org, linux-mm@...ck.org,
	m.szyprowski@...sung.com, Sumit Semwal <sumit.semwal@...aro.org>,
	linux-arm-kernel@...ts.infradead.org, linux-media@...r.kernel.org
Subject: Re: [RFC v2 1/2] dma-buf: Introduce dma buffer sharing mechanism

On Mon, Dec 05, 2011 at 04:11:46PM -0600, Rob Clark wrote:
> On Mon, Dec 5, 2011 at 3:23 PM, Daniel Vetter <daniel@...ll.ch> wrote:
> > On Mon, Dec 05, 2011 at 02:46:47PM -0600, Rob Clark wrote:
> >> I sort of preferred having the DMABUF shim because that lets you pass
> >> a buffer around userspace without the receiving code knowing about a
> >> device specific API.  But the problem I eventually came around to: if
> >> your GL stack (or some other userspace component) is batching up
> >> commands before submission to kernel, the buffers you need to wait for
> >> completion might not even be submitted yet.  So from kernel
> >> perspective they are "ready" for cpu access.  Even though in fact they
> >> are not in a consistent state from rendering perspective.  I don't
> >> really know a sane way to deal with that.  Maybe the approach instead
> >> should be a userspace level API (in libkms/libdrm?) to provide
> >> abstraction for userspace access to buffers rather than dealing with
> >> this at the kernel level.
> >
> > Well, there's a reason GL has an explicit flush and extensions for sync
> > objects. It's to support such scenarios where the driver batches up gpu
> > commands before actually submitting them.
>
> Hmm.. what about other non-GL APIs..  maybe vaapi/vdpau or similar?
> (Or something that I haven't thought of.)

They generally all have a concept of when they've actually commited the
rendering to an X pixmap or egl image. Usually it's rather implicit, e.g.
the driver will submit any outstanding batches before returning from any
calls.
-Daniel
-- 
Daniel Vetter
Mail: daniel@...ll.ch
Mobile: +41 (0)79 365 57 48
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