Fence, timeline and android sync points

Jerome Glisse j.glisse at gmail.com
Fri Aug 15 07:52:04 PDT 2014

On Fri, Aug 15, 2014 at 08:54:38AM +0200, Thomas Hellstrom wrote:
> On 08/14/2014 09:15 PM, Jerome Glisse wrote:
> > On Thu, Aug 14, 2014 at 08:47:16PM +0200, Daniel Vetter wrote:
> >> On Thu, Aug 14, 2014 at 8:18 PM, Jerome Glisse <j.glisse at gmail.com> wrote:
> >>> Sucks because you can not do weird synchronization like one i depicted in another
> >>> mail in this thread and for as long as cmdbuf_ioctl do not give you fence|syncpt
> >>> you can not such thing cleanly in non hackish way.
> >> Actually i915 can soon will do that that.
> > So you will return fence|syncpoint with each cmdbuf_ioctl ?
> >
> >>> Sucks because you have a fence object per buffer object and thus overhead grow
> >>> with the number of objects. Not even mentioning fence lifetime issue.
> >>>
> >>> Sucks because sub-buffer allocation is just one of many tricks that can not be
> >>> achieved properly and cleanly with implicit sync.
> >>>
> >>> ...
> >> Well I heard all those reasons and I'm well of aware of them. The
> >> problem is that with current hardware the kernel needs to know for
> >> each buffer how long it needs to be kept around since hw just can't do
> >> page faulting. Yeah you can pin them but for an uma design that
> >> doesn't go down well with folks.
> > I am not thinking with fancy hw in mind, on contrary i thought about all
> > this with the crappiest hw i could think of, in mind.
> >
> > Yes you can get rid of fence and not have to pin memory with current hw.
> > What matter for unpinning is to know that all hw block are done using the
> > memory. This is easily achievable with your beloved seqno. Have one seqno
> > per driver (one driver can have different block 3d, video decoding, crtc,
> > ...) each time a buffer is use as part of a command on one block inc the
> > common seqno and tag the buffer with that number. Have each hw block write
> > the lastest seqno that is done to a per block location. Now to determine
> > is buffer is done compare the buffer seqno with the max of all the signaled
> > seqno of all blocks.
> >
> > Cost 1 uint32 per buffer and simple if without locking to check status of
> > a buffer.
> Hmm?
> The trivial and first use of fence objects in the linux DRM was
> triggered by the fact that a
> 32-bit seqno wraps pretty quickly and a 32-bit solution just can't be
> made robust.
> Now a 64-bit seqno will probably be robust for forseeable future, but
> when it comes to implement that on 32-bit hardware and compare it to a
> simple fence object approach,

Using same kind of arithemic as use for jiffies would do it provided that
there is a checking that we never let someobject pass above a certain age.

> /Thomas

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