Re: [RFC v2 5/8] drm/fence: add in-fences support

[Rob Clark]

On Fri, Apr 29, 2016 at 3:48 AM, Daniel Stone <daniel@xxxxxxxxxxxxx> wrote:
> Hi,
>
> On 28 April 2016 at 23:28, Rob Clark <robdclark@xxxxxxxxx> wrote:
>> On Wed, Apr 27, 2016 at 2:39 AM, Daniel Vetter <daniel@xxxxxxxx> wrote:
>>> On Tue, Apr 26, 2016 at 01:48:02PM -0700, Greg Hackmann wrote:
>>>> A (per-CRTC?) array of fences would be more flexible.  And even in the cases
>>>> where you could make a 1-to-1 mapping between planes and fences, it's not
>>>> that much more work for userspace to assemble those fences into an array
>>>> anyway.
>>>
>>> I'm ok with an array too if that's what you folks prefer (it's meant to be
>>> used by you after all). I just don't want just 1 fence for the entire op,
>>> forcing userspace to first merge them all together. That seems silly.
>>
>> I was kinda more a fan of array too, if for no other reason that to be
>> consistent w/ how out-fences work.  (And using property just for
>> in-fence seemed slightly weird/abusive to me)
>
> I don't think it's really useful to look for much consistency between
> the two, beyond the name. I'm more concerned with consistency between
> in-fences and the implicit fences on buffers/FBs, and between
> out-fences and the page_flip_events.
>
>>> One side-effect of that is that we'd also have to rework all the internal
>>> bits and move fences around in atomic. Which means change a pile of
>>> drivers. Not sure that's worth it, but I'd be ok either way really.
>>
>> hmm, well we could keep the array per-plane (and if one layer is using
>> multiple planes, just list the same fd multiple times).. then it
>> mostly comes down to changes in the ioctl fxn itself.
>
> ... and new API in libdrm, which is going to be a serious #ifdef and
> distribution pain. The core property API has been available since
> 2.4.62 last June, but for this we'd have to write the code, wait for
> the kernel code, wait for HWC, get everything together, and then merge
> and release. That gives minimum one year of libdrm releases which have
> had atomic but not in-fence API support, if we're adding a new array.
> And I just don't really see what it buys us, apart from the need for
> the core atomic_get_property helper to statically return -1 when
> requesting FENCE_FD.

don't we have the same issue for out-fences anyway?

ofc, I suspect we could handle making fences look like properties in
userspace in libdrm (at least if there was a sane way that libdrm
could track and eventually close() old out-fence fd's).  I'm not
entirely sure this matters, I mean how do we make implicit vs explicit
fencing transparent to the compositor and the proto between
compositor<->app?

Admittedly I haven't given *too* much thought yet about the
implications to libdrm and it's users, but it seems like we need to
make a v2 API rev anyway for out-fences, and the compositor is going
to need different codepaths for explicit vs implicit (if it supports
both).  So I don't think in-fences as something other than property
really costs us anything additional?

(Unless there is some sane reason to have an intermediate state w/
in-fences but pageflip events instead of out-fences?  But that seems
odd..)

BR,
-R


> Cheers,
> Daniel