linux-kernel - Re: [RFC v2 5/8] drm/fence: add in-fences support

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Date:	Thu, 28 Apr 2016 20:02:05 +0200
From:	Daniel Vetter <daniel@...ll.ch>
To:	Gustavo Padovan <gustavo.padovan@...labora.co.uk>
Cc:	Ville Syrjälä <ville.syrjala@...ux.intel.com>,
	Gustavo Padovan <gustavo@...ovan.org>,
	Daniel Stone <daniel@...ishbar.org>,
	Greg Hackmann <ghackmann@...gle.com>,
	Daniel Stone <daniels@...labora.com>,
	Riley Andrews <riandrews@...roid.com>,
	dri-devel <dri-devel@...ts.freedesktop.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Arve Hjønnevåg <arve@...roid.com>,
	John Harrison <John.C.Harrison@...el.com>
Subject: Re: [RFC v2 5/8] drm/fence: add in-fences support

On Thu, Apr 28, 2016 at 7:55 PM, Gustavo Padovan
<gustavo.padovan@...labora.co.uk> wrote:
> 2016-04-28 Ville Syrjälä <ville.syrjala@...ux.intel.com>:
>
>> On Thu, Apr 28, 2016 at 07:43:16PM +0200, Daniel Vetter wrote:
>> > On Thu, Apr 28, 2016 at 6:56 PM, Ville Syrjälä
>> > <ville.syrjala@...ux.intel.com> wrote:
>> > >>  - better for tracing, can identify the buffer/fence promptly
>> > >
>> > > Can fences be reused somehow while still attached to a plane, or ever?
>> > > That might cause some oddness if you, say, leave a fence attached to one
>> > > plane and then do a modeset on another crtc perhaps which needs to turn
>> > > the first crtc off+on to reconfigure something.
>> >
>> > Fences auto-disappear of course and don't stick around when you
>> > duplicate the drm_plane_state again. I still don't really get the real
>> > concerns though ...
>>
>> Properties that magically change values shouldn't exist IMO. I guess if
>> you could have write-only properties or something it migth be sensible?
>
> We can just not return FENCE_FD on get_props, that would make it
> write-only.

We do actually return a value for get_props, but it's -1 which for fds
means "no fd". That's to make sure userspace can save&restore any prop
without causing harm.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch