[Mesa-dev] [RFC] gbm: wire up fence extension

[Emil Velikov]

On 18 August 2016 at 18:28, Rob Clark <robdclark at gmail.com> wrote:
> On Thu, Aug 18, 2016 at 8:45 AM, Emil Velikov <emil.l.velikov at gmail.com> wrote:
>> On 16 August 2016 at 18:01, Rob Clark <robdclark at gmail.com> wrote:
>>> I noticed that fence extension wasn't exposed for drm/gbm, which sort
>>> of defeats the purpose of using fence fd's for synchronizing rendering
>>> and atomic pageflip.
>>>
>> Afaict this is a slightly more complete version of Philipp's patch.
>>
>> While my earlier suggestion still stands, I should have mentioned that
>> it's not a show stopper for this patch.
>> Namely: we really want a way to check and bail out on ABI mismatch
>> between libEGL and libgbm.
>
> btw, I dug up Philipp's original patch/thread..  I didn't realize that
> someone had already done this (would have saved me some time ;-))
>
> As far as ABI mismatch, we don't currently ship libEGL/libgbm
> separately.
Indeed we don't ship anything - distros do ;-) And they (only
sometimes, I hope,) do strange things without seeking
information/confirmation.

>  And there doesn't appear to be any versioning scheme so
> far.  (Although if we do split out libgbm from mesa, that would be a
> good thing to add.)
>
True and true ;-)

> So I'm inclined to punt on the version check (and actually not
> entirely sure it is even possible).
>
As you pointed out above we do want it sooner or later. I do suggest
doing it earlier than when it's too late.

Here's an idea how it could be done:
 - define a couple of version/abi macros - one of EGL and another for GBM.
 - store the GBM one in struct gbm_device or gbm_dri_device (directly
or use a vfunc which returns the version)
 - upon inheriting (in dri2_initialize_drm) from gbm check against the
local EGL version.
 - bail out on mismatch.
 - stick a juicy comment on top of the respective struct(s) "do not
move because .,." or alike.

TL;DR; Considering that things work with the (updated) patch don't let
the ABI check stop you. But _please_ don't ignore/leave it until it's
too late.

Thanks
Emil