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

Jesse Barnes
2015-07-29T21:26:17Z

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