// Copyright 2019 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

library fuchsia.images;

using fuchsia.sysmem;

// A maximum of 16 fences is enough for the current usage of these APIs.
const int32 MAX_ACQUIRE_RELEASE_FENCE_COUNT = 16;

/// ImagePipe is a mechanism for streaming shared images between a producer
/// and a consumer which may be running in different processes.
///
/// Conceptually, the image pipe maintains a table of image resources supplied
/// by the producer into which graphical content may be stored as well as a
/// presentation queue containing a sequence of images which the producer has
/// asked the consumer to present.
///
/// The presentation queue is initially empty.
///
/// Each entry in the presentation queue consists of an image together with a
/// pair of optional synchronization fences:
/// - Acquire fence: signaled by the producer when the image is ready to be consumed
/// - Release fence: signaled by the consumer when the image is free to be freed or
///   modified by the producer
///
/// The producer performs the following sequence of steps to present content:
/// - Allocate and add some number of BufferCollections to the image pipe to allow
/// consumer to set constraints.
/// - Allocate and add some number of images (often 2 or 3) to the image pipe
///   to establish a pool using `AddImage()`.
/// - Obtain the next available image from the pool.
/// - Ask the consumer to enqueue an image for presentation and provide fences
///   using `PresentImage()`.
/// - Start rendering the image.
/// - Signal the image's acquire fence when rendering is complete.
/// - Loop to present more image, listen for signals on release fences to recycle
///   images back into the pool.
///
/// The consumer performs the following sequence of steps for each image which
/// is enqueued in the presentation queue:
/// - Await signals on the image's acquire fence.
/// - If the fence wait cannot be satisfied or if some other error is detected,
///   close the image pipe.
///   Otherwise, begin presenting the image's content.
/// - Retire the previously presented image (if any) from the presentation queue
///   and signal its release fence when no longer needed.
/// - Continue presenting the same image until the next one is ready.  Loop.
///
/// If the producer wants to close the image pipe, it should:
/// - Close its side of the connection.
/// - Wait on all release fences for buffers that it has submitted with
///   `PresentImage()`.
/// - Proceed with resource cleanup.
///
/// When the consumer detects the image pipe has closed, it should:
/// - Stop using/presenting any images from the pipe.
/// - Unmap all memory objects associated with the images in the pipe.
/// - Close all BufferCollection resources.
/// - Signal all release fences for presented and queued buffers.
/// - Close all handles to fences.
/// - Close its side of the connection.
///
/// When either party detects that a fence has been abandoned (remotely closed
/// without being signaled) it should assume that the associated image is in
/// an indeterminate state.  This will typically happen when the other party
/// (or one of its delegates) has crashed.  The safest course of action is to
/// close the image pipe, release all resources which were shared with the
/// other party, and re-establish the connection to recover.
protocol ImagePipe2 {
    /// Adds a BufferCollection resource to the image pipe.
    ///
    /// The producer is expected to set constraints on this resource for images added
    /// via `AddImage()`. The consumer can set its constraints on
    /// `buffer_collection_token` before or after. Note that the buffers won’t be
    /// allocated until all BufferCollectionToken instances are used to set
    /// constraints, on both the producer and consumer side. See collection.fidl for
    /// details.
    ///
    /// The following errors will cause the connection to be closed:
    /// - `buffer_collection_id` is already registered
    AddBufferCollection(uint32 buffer_collection_id,
                        fuchsia.sysmem.BufferCollectionToken buffer_collection_token);

    /// Adds an image resource to image pipe.
    ///
    /// `buffer_collection_id` refers to the BufferCollectionToken instance that is
    /// registered via `AddBufferCollection()`. The underlying memory objects allocated
    /// are used to address to the image data. `buffer_collection_id` refers to the
    /// index of the memory object allocated in BufferCollection.
    ///
    /// `image_format` specifiies image properties. `coded_width` and `coded_height` are
    /// used to set image dimensions.
    ///
    /// It is valid to create multiple images backed by the same memory object; they
    /// may even overlap.  Consumers must detect this and handle it accordingly.
    ///
    /// The following errors will cause the connection to be closed:
    /// - `image_id` is already registered
    /// - `buffer_collection_id` refers to an unregistered BufferCollection.
    /// - `buffer_collection_index` points to a resource index out of the initialized
    ///     BufferCollection bounds
    /// - No resource is allocated in the registered BufferCollection.
    AddImage(uint32 image_id, uint32 buffer_collection_id, uint32 buffer_collection_index,
             fuchsia.sysmem.ImageFormat_2 image_format);

    /// Removes a BufferCollection resource from the pipe.
    ///
    /// The `buffer_collection_id` resource is detached as well as all Images that are
    /// associated with that BufferCollection. Leads to the same results as calling
    /// `RemoveImage()` on all Images for `buffer_collection_id`.
    ///
    /// The producer must wait for all release fences associated with the Images to
    /// be signaled before freeing or modifying the underlying memory object since
    /// the image may still be in use in the presentation queue.
    ///
    /// The following errors will cause the connection to be closed:
    /// - `buffer_collection_id` does not reference a currently registered BufferCollection
    RemoveBufferCollection(uint32 buffer_collection_id);

    /// Removes an image resource from the pipe.
    ///
    /// The `image_id` is detached from the image resource and is free to be
    /// reused to add a new image resource.
    ///
    /// Removing an image from the image pipe does not affect the presentation
    /// queue or the currently presented image.
    ///
    /// The producer must wait for all release fences associated with the image to
    /// be signaled before freeing or modifying the underlying memory object since
    /// the image may still be in use in the presentation queue.
    ///
    /// The following errors will cause the connection to be closed:
    /// - `image_id` does not reference a currently registered image resource
    RemoveImage(uint32 image_id);

    /// Enqueues the specified image for presentation by the consumer.
    ///
    /// The `acquire_fences` are a set of fences which must all be signaled by the
    /// producer before the consumer presents the image.
    /// The `release_fences` are set of fences which must all be signaled by the
    /// consumer before it is safe for the producer to free or modify the image.
    /// `presentation_time` specifies the time on or after which the
    /// client would like the enqueued operations should take visible effect
    /// (light up pixels on the screen), expressed in nanoseconds in the
    /// `CLOCK_MONOTONIC` timebase.  Desired presentation times must be
    /// monotonically non-decreasing.
    ///
    /// `presentation_info` returns timing information about the submitted frame
    /// and future frames (see presentation_info.fidl).
    ///
    /// The producer may decide not to signal `acquire_fences` for an image.
    /// In that case, if a later image is enqueued and later image’s
    /// `presentation_time` is reached, the consumer presents the later image when
    /// later image’s `acquire_fences` are signaled. The consumer also signals
    /// earlier image’s `release_fences` and removes it from the presentation queue.
    /// This sequence works as a cancellation mechanism.
    ///
    /// The following errors will cause the connection to be closed:
    /// - `image_id` does not reference a currently registered image resource
    PresentImage(uint32 image_id, uint64 presentation_time,
                 vector<handle<event>>:MAX_ACQUIRE_RELEASE_FENCE_COUNT acquire_fences,
                 vector<handle<event>>:MAX_ACQUIRE_RELEASE_FENCE_COUNT release_fences)
        -> (PresentationInfo presentation_info);
};