linux-kernel - Re: [Linaro-mm-sig] [PATCH 2/3] dma-buf: add support for kernel cpu access

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Message-ID: <CAKMK7uEcukmyyqL6N2XMwMDjGe81NkhmuE1k0QyeJYKF0ufktg@mail.gmail.com>
Date:	Mon, 5 Mar 2012 22:31:52 +0100
From:	Daniel Vetter <daniel.vetter@...ll.ch>
To:	Rob Clark <rob.clark@...aro.org>
Cc:	Chris Wilson <chris@...is-wilson.co.uk>,
	linaro-mm-sig@...ts.linaro.org,
	LKML <linux-kernel@...r.kernel.org>,
	DRI Development <dri-devel@...ts.freedesktop.org>,
	linux-media@...r.kernel.org
Subject: Re: [Linaro-mm-sig] [PATCH 2/3] dma-buf: add support for kernel cpu access

On Fri, Mar 2, 2012 at 23:53, Rob Clark <rob.clark@...aro.org> wrote:
> nitially the expectation was that userspace would not pass a buffer
> to multiple subsystems for writing (or that if it did, it would get
> the undefined results that one could expect)..  so dealing w/
> synchronization was punted.

Imo synchronization should not be part of the dma_buf core, i.e.
userspace needs to ensure that access is synchronized.
begin/end_cpu_access are the coherency brackets (like map/unmap for
device dma). And if userspace asks for a gun and some bullets, the
kernel should just deliver. Even in drm/i915 gem land we don't (and
simply can't) make any promises about concurrent reads/writes/ioctls.

> I expect, though, that one of the next steps is some sort of
> sync-object mechanism to supplement dmabuf

Imo the only reason to add sync objects as explicit things is to make
device-to-device sync more efficient by using hw semaphores and
signalling lines. Or maybe a quick irq handler in the kernel that
kicks of the next device. I don't think we should design these to make
userspace simpler.

Cheers, Daniel
-- 
Daniel Vetter
daniel.vetter@...ll.ch - +41 (0) 79 365 57 48 - http://blog.ffwll.ch
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