linux-kernel - Re: [PATCH 09/17] drm/radeon: use common fence implementation for fences

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Date:	Tue, 22 Jul 2014 13:52:39 +0200
From:	Daniel Vetter <daniel@...ll.ch>
To:	Christian König <christian.koenig@....com>
Cc:	Dave Airlie <airlied@...il.com>,
	Maarten Lankhorst <maarten.lankhorst@...onical.com>,
	Thomas Hellstrom <thellstrom@...are.com>,
	nouveau <nouveau@...ts.freedesktop.org>,
	LKML <linux-kernel@...r.kernel.org>,
	dri-devel <dri-devel@...ts.freedesktop.org>,
	Ben Skeggs <bskeggs@...hat.com>,
	"Deucher, Alexander" <alexander.deucher@....com>
Subject: Re: [PATCH 09/17] drm/radeon: use common fence implementation for
 fences

On Tue, Jul 22, 2014 at 01:46:07PM +0200, Daniel Vetter wrote:
> On Tue, Jul 22, 2014 at 10:43:13AM +0200, Christian König wrote:
> > Am 22.07.2014 06:05, schrieb Dave Airlie:
> > >On 9 July 2014 22:29, Maarten Lankhorst <maarten.lankhorst@...onical.com> wrote:
> > >>Signed-off-by: Maarten Lankhorst <maarten.lankhorst@...onical.com>
> > >>---
> > >>  drivers/gpu/drm/radeon/radeon.h        |   15 +-
> > >>  drivers/gpu/drm/radeon/radeon_device.c |   60 ++++++++-
> > >>  drivers/gpu/drm/radeon/radeon_fence.c  |  223 ++++++++++++++++++++++++++------
> > >>  3 files changed, 248 insertions(+), 50 deletions(-)
> > >>
> > > From what I can see this is still suffering from the problem that we
> > >need to find a proper solution to,
> > >
> > >My summary of the issues after talking to Jerome and Ben and
> > >re-reading things is:
> > >
> > >We really need to work out a better interface into the drivers to be
> > >able to avoid random atomic entrypoints,
> > 
> > Which is exactly what I criticized from the very first beginning. Good to
> > know that I'm not the only one thinking that this isn't such a good idea.
> 
> I guess I've lost context a bit, but which atomic entry point are we
> talking about? Afaics the only one that's mandatory is the is
> fence->signaled callback to check whether a fence really has been
> signalled. It's used internally by the fence code to avoid spurious
> wakeups. Afaik that should be doable already on any hardware. If that's
> not the case then we can always track the signalled state in software and
> double-check in a worker thread before updating the sw state. And wrap
> this all up into a special fence class if there's more than one driver
> needing this.
> 
> There is nothing else that forces callbacks from atomic contexts upon you.
> You can use them if you see it fit, but really if it doesn't suit your
> driver you can just ignore that part and do process based waits
> everywhere.

Aside: The fence-process-callback has already been implemented by nouveau
with the struct fence_work in nouveau_fence.c. Would make loads of sense
to move that code into the driver core and adapat it to Maarten's struct
fence once this has all landed.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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