Fence, timeline and android sync points

Thomas Hellstrom thomas at shipmail.org
Thu Aug 14 23:54:38 PDT 2014

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.

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,


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