Re: Fence, timeline and android sync points

[Thomas Hellstrom <thomas@shipmail.org>]

On 08/15/2014 04:52 PM, Jerome Glisse wrote:
> 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@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.

But wouldn't the search-for-max scheme break if blocks complete out of
order?

/Thomas