registry/vulkan/appendices/VK_KHR_present_wait.txt

// Copyright 2019-2021 The Khronos Group, Inc.
//
// SPDX-License-Identifier: CC-BY-4.0

include::{generated}/meta/{refprefix}VK_KHR_present_wait.txt[]

=== Other Extension Metadata

*Last Modified Date*::
    2019-05-15
*IP Status*::
    No known IP claims.
*Contributors*::
  - Keith Packard, Valve
  - Ian Elliott, Google
  - Tobias Hector, AMD
  - Daniel Stone, Collabora

=== Description

This device extension allows an application that uses the
`apiext:VK_KHR_swapchain` extension to wait for present operations to
complete.
An application can use this to monitor and control the pacing of the
application by managing the number of outstanding images yet to be
presented.

include::{generated}/interfaces/VK_KHR_present_wait.txt[]

=== Issues

1) When does the wait finish?

*RESOLVED*.
The wait will finish when the present is visible to the user.
There is no requirement for any precise timing relationship between the
presentation of the image to the user, but implementations should: signal
the wait as close as possible to the presentation of the first pixel in the
new image to the user.

2) Should this use fences or other existing synchronization mechanism.

*RESOLVED*.
Because display and rendering are often implemented in separate drivers,
this extension will provide a separate synchronization API.

3) Should this extension share present identification with other extensions?

*RESOLVED*.
Yes.
A new extension, VK_KHR_present_id, should be created to provide a shared
structure for presentation identifiers.

4) What happens when presentations complete out of order wrt calls to
   vkQueuePresent? This could happen if the semaphores for the presentations
   were ready out of order.

*OPTION A*: Require that when a PresentId is set that the driver ensure that
            images are always presented in the order of calls to
            vkQueuePresent.

*OPTION B*: Finish both waits when the earliest present completes.
            This will complete the later present wait earlier than the
            actual presentation.
            This should be the easiest to implement as the driver need only
            track the largest present ID completed.
            This is also the 'natural' consequence of interpreting the
            existing vkWaitForPresentKHR specificationn.

*OPTION C*: Finish both waits when both have completed.
            This will complete the earlier presentation later than the
            actual presentation time.
            This is allowed by the current specification as there is no
            precise timing requirement for when the presentId value is
            updated.
            This requires slightly more complexity in the driver as it will
            need to track all outstanding presentId values.

=== Examples

=== Version History

 * Revision 1, 2019-02-19 (Keith Packard)
   - Initial version