[Mesa-dev] [PATCH 2/4] atomic: Use CPU-side KMS fence synchronisation

[Daniel Stone]

Hi,

On 2 May 2017 at 12:31, Chris Wilson <chris at chris-wilson.co.uk> wrote:
> On Tue, May 02, 2017 at 11:52:07AM +0100, Daniel Stone wrote:
>> +                     /* Wait on the CPU side for the _previous_ commit to
>> +                      * complete before we post the flip through KMS, as
>> +                      * atomic will reject the commit if we post a new one
>> +                      * whilst the previous one is still pending.
>> +                      */
>> +                     do {
>> +                             status = egl->eglClientWaitSyncKHR(egl->display,
>> +                                                                kms_fence,
>> +                                                                0,
>> +                                                                EGL_FOREVER_KHR);
>> +                     } while (status != EGL_CONDITION_SATISFIED_KHR);
>> +
>> +                     egl->eglDestroySyncKHR(egl->display, kms_fence);
>
> Would it be an interesting exercise to use that as the in-fence for the GPU?
> Not sure which is more in the spirit of kmscube.

Er, it already does so, in the first hunks this patch touches.

They are solving different issues though. We need to block on the GPU,
to ensure the GPU does not start rendering to the buffer whilst it is
still in use. But we also need to block on the CPU to ensure that we
do not try to submit an atomic commit before the previous atomic
complete has committed, since here we are not using pageflip events at
all, but solely relying on fences as our synchronisation mechanism.

Cheers,
Daniel