Re: [RFC] drm/i915: Add sync framework support to execbuff IOCTL

John Harrison
2015-07-30T11:36:52Z

On 29/07/2015 22:19, Jesse Barnes wrote:
> On 07/07/2015 02:15 AM, Tvrtko Ursulin wrote:
>> On 07/06/2015 01:58 PM, John Harrison wrote:
>>> On 06/07/2015 10:29, Daniel Vetter wrote:
>>>> On Fri, Jul 03, 2015 at 12:17:33PM +0100, Tvrtko Ursulin wrote:
>>>>> On 07/02/2015 04:55 PM, Chris Wilson wrote:
>>>>>> It would be nice if we could reuse one seqno both for internal/external
>>>>>> fences. If you need to expose a fence ordering within a timeline
>>>>>> that is
>>>>>> based on the creation stamp rather than execution stamp, it seems like
>>>>>> we could just add such a stamp when creating the sync_pt and not worry
>>>>>> about its relationship to the execution seqno.
>>>>>>
>>>>>> Doing so does expose that requests are reordered to userspace since the
>>>>>> signalling timeline is not the same as userspace's ordered timeline.
>>>>>> Not
>>>>>> sure if that is a problem or not.
>>>>>>
>>>>>> Afaict the sync uapi is based on waiting for all of a set of fences to
>>>>>> retire. It doesn't seem to rely on fence ordering (that is knowing that
>>>>>> fence A will signal before fence B so it need only wait on fence B).
>>>>>>
>>>>>> Here's hoping that we can have both simplicity and efficiency...
>>>>> Jumping in with not even perfect understanding of everything here - but
>>>>> timeline business has always been confusing me. There is nothing in the
>>>>> uapi which needs it afaics and iirc there was some discussion at the
>>>>> time
>>>>> Jesse floated his patches that it can be removed. Based on that when I
>>>>> squashed his patches and ported them on top of John's request to fence
>>>>> conversion it ended up something like the below (manually edited a
>>>>> bit to
>>>>> be less noisy and some prep patches omitted):
>>>>>
>>>>> This implements the ioctl based uapi and indeed seqnos are not actually
>>>>> used in waits. So is this insufficient for some reason? (Other that it
>>>>> does not implement the input fence side of things.)
>>>> Yeah android syncpt on top of struct fence embedded int i915 request is
>>>> what I'd have expected.
>>> The thing I'm not happy with in that plan is that it leaves the kernel
>>> driver at the mercy of user land applications. If we return a fence
>>> object to user land via a file descriptor (or indeed any other
>>> mechanism) then that fence object must be locked until user land closes
>>> the file. If the fence object is the one embedded within our request
>>> structure then that means user land is effectively locking our request
>>> structure. Given that more and more stuff is being attached to the
>>> request, that could be a fair bit of memory tied up that we can do
>>> nothing about. E.g. if a rogue/buggy application requests a fence be
>>> returned for every batch buffer submitted but never closes them.
>>> Whereas, if we go the route of a separate fence object specifically for
>>> user land then they can leak them like a sieve and we won't really care
>>> so much.
>> I am starting to agree gradually with this view. Given all the
>> complications, referencing requests for exporting via fds feels quite
>> heavy-weight, with potentially unbound dependencies and more
>> trickiness in the future, even if we agreed on referencing and locking
>> details.
>>
>> Seqnos per context sounds like a significantly more light-weight and
>> decoupled implementation.
> I think this is the right long term direction as well; conceptually the
> per-context seqnos make the most sense in light of scheduling, and they
> let us keep things simple for sync pts as well. Only question is, who
> is signed up to make it all work?
>
> Jesse
>

That's the version I had originally. A separate fence object using a per
context per ring timeline that is safe to export to user land. However,
Daniel Vetter was very strongly convinced that using a single shared
fence object both internally and externally was the better solution.

My current implementation has a per context per ring timeline which is
used to give the fence a definitely in order and sensible seqno. It is
just not the same seqno that goes through the hardware. At least, not
yet! Although that change could be quite significant.

John.