Re: [PATCH 1/3] drm/etnaviv: submit support for in-fences - Russell King

From: Russell King - ARM Linux <linux@armlinux.org.uk>
To: Lucas Stach <l.stach@pengutronix.de>
Cc: dri-devel@lists.freedesktop.org, etnaviv@lists.freedesktop.org
Subject: Re: [PATCH 1/3] drm/etnaviv: submit support for in-fences
Date: Fri, 17 Mar 2017 15:07:48 +0000	[thread overview]
Message-ID: <20170317150748.GJ21222@n2100.armlinux.org.uk> (raw)
In-Reply-To: <1489762707.16018.33.camel@pengutronix.de>

On Fri, Mar 17, 2017 at 03:58:27PM +0100, Lucas Stach wrote:
> Am Freitag, den 17.03.2017, 14:42 +0000 schrieb Russell King - ARM
> Linux:
> > On Fri, Mar 17, 2017 at 03:10:21PM +0100, Lucas Stach wrote:
> > > Am Donnerstag, den 16.03.2017, 12:05 +0100 schrieb Philipp Zabel:
> > > > Hi Gustavo,
> > > > 
> > > > On Mon, 2017-03-13 at 14:37 -0300, Gustavo Padovan wrote:
> > > > [...]
> > > > > I was thinking on some function that would iterate over all fences in
> > > > > the fence_array and check their context. The if we find our own gpu
> > > > > context in there we fail the submit.
> > > > 
> > > > Why would we have to fail if somebody feeds us our own fences? Wouldn't
> > > > it be enough to just wait if there are foreign fences in the array?
> > > 
> > > Yes, skipping the wait if all fences are from our own context is an
> > > optimization and it's certainly not an issue if someone feeds us our own
> > > fences.
> > 
> > Are you sure about that - what if we have two GPUs, a 2D and 3D GPU,
> > and we're fed an etnaviv fence for one GPU when submitting to the
> > other GPU.
> > 
> > So we do end up being fed our own fences, and we have to respect them
> > otherwise we lose inter-GPU synchronisation, and that will break
> > existing userspace.
> > 
> The etnaviv GPUs, while being on the same DRM device, have distinct
> fence contexts. So the 3D GPU will consider a fence from the 2D GPU as
> foreign and properly wait on it.
> 
> It's only when we get an in fence that has been generated as an out
> fence by one (or multiple) submits to the same GPU, that we are able to
> skip the wait and enqueue the command without waiting for the fence to
> signal.

Sounds fine then.  Thanks.

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