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

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

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.

Regards,

Tvrtko
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