linux-kernel - Re: [RFC][PATCH 00/30] staging: Android sync driver

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Date:	Sun, 3 Mar 2013 19:42:36 +0100
From:	Daniel Vetter <daniel.vetter@...ll.ch>
To:	John Stultz <john.stultz@...aro.org>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Maarten Lankhorst <maarten.lankhorst@...onical.com>,
	Erik Gilling <konkers@...roid.com>,
	Rob Clark <robclark@...il.com>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	Greg KH <gregkh@...uxfoundation.org>,
	dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org,
	Android Kernel Team <kernel-team@...roid.com>
Subject: Re: [RFC][PATCH 00/30] staging: Android sync driver

On Fri, Mar 1, 2013 at 1:42 AM, John Stultz <john.stultz@...aro.org> wrote:
> As proposed yesterday, here's the Android sync driver patches for
> staging.
>
> I've preserved the commit history, but moved all the changes over
> to be against the staging directory (instead of drivers/base).
>
>
> The goal of submitting this driver to staging is to try to get
> more collaberation, as there are some similar efforts going on
> in the community with dmabuf-fences. My email from yesterday with
> more details for how I hope this goes is here:
>         http://comments.gmane.org/gmane.linux.kernel/1448420

I've been offline in a week of snowboarding, but I'll throw my late
Ack - I've discussed this a bit with John offline and I agree with his
general plan for integrating android sync points into mainline.

> Erik also provided a nice background on the patch set in his
> reply yesterday, which I'll quote here:
>
> "In Honeycomb where we introduced the Hardware Composer HAL.  This is a
> userspace layer that allows composition acceleration on a per platform
> basis.  Different SoC vendors have implemented this using overlays, 2d
> blitters, a combinations of both, or other clever/disgusting means.
> Along with the HWC we consolidated a lot of our camera and media
> pipeline to allow their input to be fed into the GPU or
> display(overlay.)  In order to exploit parallelism the the graphics
> pipeline, this introduced lots of implicit synchronization
> dependancies.  After a couple years of working with many different SoC
> vendors, we found that it was really difficult to communicate our
> system's expectations of the implicit contract and it was difficult
> for the SoC vendors to properly implement the implicit contract in
> each of their IP blocks (display, gpu, camera, video codecs).  It was
> also incredibly difficult to debug when problems/deadlocks arose.

dma_buf fences should be tons easier to debug thanks to integration
with lockdep. Also their design fundamentally excludes deadlock-loops
in the fences themselves. And I also think that we should be able to
hide the complexity from most drivers in e.g. drm/ttm or the v2l core.
So I'm still bullish on implicit fencing (and will keep on pushing
that for all things intel).

But I guess the simpler programming model afforded by that for
userspace isn't of much use for the google guys now that they've
pushed the effort to convert SurfaceFlinger to explicit fence handling
...

Cheers, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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