[PATCH mesa v4] wayland: Add support for eglSwapInterval

[Tomeu Vizoso]

On 30 October 2013 09:44, Neil Roberts <neil at linux.intel.com> wrote:
> Tomeu Vizoso <tomeu at tomeuvizoso.net> writes:
>
>> What I fail to see is why a single sync should be enough, as we don't
>> know when the GPU will signal that it's done with the buffer that we
>> are waiting to be released.
>
> You are right that we don't know when the GPU will release the buffer.
> However we are not waiting for that.

Oh, sure, was saying that in the context of the RPi, there we don't
want the client to have to create more than 2 buffers per surface.

In that EGL, SwapBuffers blocks waiting for the GPU to signal that
it's done rendering with the buffer that is to become the back buffer.
With your patch for not queueing if no frame callbacks are installed,
we don't need to send any sync requests. Without it, we would need to
keep syncing until we get the release.

I know that you have dealt with it in Mesa already but wanted to reply
to Jason's comment, sorry for hijacking your thread :)

This is the relevant part of the code:

https://github.com/tomeuv/userland/commit/a4a7cb233af5e5b167a1cb6f928b7a6381380a56#diff-b1115e30ce4f2940df2cd98a85009ccaR2339

Regards,

Tomeu

> We are assuming that the GPU is
> only going to hold on to at most 2 buffers. In the DRM / Mesa case it
> needs to hold on to these because one will be used for scanout and one
> will be queued for the page flip. If we attach a third buffer then it
> will be held by the compositor as a queue for the next flip. We are
> assuming it won't have given this buffer to the GPU. Therefore if we
> attach a fourth buffer it is easy for the compositor to immediately
> release its lock on the third buffer and replace it with the fourth. So
> we can assume that the fourth attach will always generate an immediate
> release event. Sending a sync request will ensure that we always get
> this release event. So if we have four buffer slots we can assume that
> one of the attaches will always generate an immediate release event.
>
> In the non-fullscreen case where we don't really need to wait for the
> GPU, we only need 2 slots because the compositor will only lock one
> buffer.
>
> Regards,
> - Neil