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Implementing Vulkan

Vulkan is a low-overhead, cross-platform API for high-performance 3D graphics. Like OpenGL ES, Vulkan provides tools for creating high-quality, real-time graphics in applications. Vulkan advantages include reductions in CPU overhead and support for the SPIR-V Binary Intermediate language.

Note: This section describes Vulkan implementation; for details on Vulkan architecture, advantages, API, and other resources, see Vulkan Architecture.

To implement Vulkan, a device:

  • Must include the Vulkan Loader (provided by Android) in the build.
  • Must include a Vulkan driver (provided by SoCs such as GPU IHVs) that implements the Vulkan API. To support Vulkan functionality, the Android device needs capable GPU hardware and the associated driver. Consult your SoC vendor to request driver support.

If a Vulkan driver is available on the device, the device needs to declare FEATURE_VULKAN_HARDWARE_LEVEL and FEATURE_VULKAN_HARDWARE_VERSION system features, with versions that accurately reflect the capabilities of the device.

Vulkan Loader

The primary interface between Vulkan applications and a device's Vulkan driver is the Vulkan loader, which is part of Android Open Source Project (AOSP) (platform/frameworks/native/vulkan) and installed at /system/lib[64]/libvulkan.so. The loader provides the core Vulkan API entry points, as well as entry points of a few extensions that are required on Android and always present. In particular, Window System Integration (WSI) extensions are exported by the loader and primarily implemented in it rather than the driver. The loader also supports enumerating and loading layers that can expose additional extensions and/or intercept core API calls on their way to the driver.

The NDK includes a stub libvulkan.so library that exports the same symbols as the loader and which is used for linking. When running on a device, applications call the Vulkan functions exported from libvulkan.so (the real library, not the stub) to enter trampoline functions in the loader, which dispatch to the appropriate layer or driver based on their first argument. The vkGet*ProcAddr calls return the function pointers to which the trampolines would dispatch (i.e. it calls directly into the core API code), so calling through these function pointers (rather than the exported symbols) is slightly more efficient as it skips the trampoline and dispatch.

Driver enumeration and loading

Android expects the GPUs available to the system to be known when the system image is built. The loader uses the existing HAL mechanism (see hardware.h) for discovering and loading the driver. Preferred paths for 32-bit and 64-bit Vulkan drivers are:

/vendor/lib/hw/vulkan.<ro.hardware.vulkan>.so
/vendor/lib/hw/vulkan.<ro.product.platform>.so
/vendor/lib64/hw/vulkan.<ro.hardware.vulkan>.so
/vendor/lib64/hw/vulkan.<ro.product.platform>.so

Where <ro.hardware.vulkan> or <ro.product.platform> is replaced by the value of the system property of that name. For details and supported alternative locations, refer to libhardware/hardware.c.

In Android 7.0, the Vulkan hw_module_t derivative is trivial; only one driver is supported and the constant string HWVULKAN_DEVICE_0 is passed to open. If support for multiple drivers is added in future versions of Android, the HAL module will export a list of strings that can be passed to the module open call.

The Vulkan hw_device_t derivative corresponds to a single driver, though that driver can support multiple physical devices. The hw_device_t structure can be extended to export vkGetGlobalExtensionProperties, vkCreateInstance, and vkGetInstanceProcAddr functions. The loader can find all other VkInstance, VkPhysicalDevice, and vkGetDeviceProcAddr functions by calling the hw_device_t's vkGetInstanceProcAddr.

Layer discovery and loading

The Vulkan loader supports enumerating and loading layers that can expose additional extensions and/or intercept core API calls on their way to the driver. Android does not include layers on the system image; however, applications may include layers in their APK.

When using layers, keep in mind that Android's security model and policies differ significantly from other platforms. In particular, Android does not allow loading external code into a non-debuggable process on production (non-rooted) devices, nor does it allow external code to inspect or control the process's memory, state, etc. This includes a prohibition on saving core dumps, API traces, etc. to disk for later inspection. Only layers delivered as part of the application are enabled on production devices, and drivers must not provide functionality that violates these policies.

Use cases for layers include:

  • Development-time layers. These layers (validation layers, shims for tracing/profiling/debugging tools, etc.) should not be installed on the system image of production devices as they waste space for users and should be updateable without requiring a system update. Developers who want to use one of these layers during development can modify the application package (e.g. adding a file to their native libraries directory). IHV and OEM engineers who want to diagnose failures in shipping, unmodifiable apps are assumed to have access to non-production (rooted) builds of the system image.
  • Utility layers. These layers almost always expose extensions, such as a layer that implements a memory manager for device memory. Developers choose layers (and versions of those layers) to use in their application; different applications using the same layer may still use different versions. Developers choose which of these layers to ship in their application package.
  • Injected (implicit) layers. Includes layers such as framerate, social network, or game launcher overlays provided by the user or some other application without the application's knowledge or consent. These violate Android's security policies and are not supported.

In the normal state, the loader searches for layers only in the application's native library directory and attempts to load any library with a name matching a particular pattern (e.g. libVKLayer_foo.so). It does not need a separate manifest file as the developer deliberately included these layers and reasons to avoid loading libraries before enabling them don't apply.

Android allows layers to be ported with build-environment changes between Android and other platforms. For details on the interface between layers and the loader, refer to Vulkan Loader Specification and Architecture Overview. Versions of the LunarG validation layers that have been verified to build and work on Android are hosted in the android_layers branch of the KhronosGroup/Vulkan-LoaderAndValidationLayers project on GitHub.

Vulkan API Versions and Capabilities

This section describes the supported Vulkan API versions.

Vulkan API version 1.0

The Android 7.0 release added support for Vulkan API version 1.0.

Vulkan API version 1.1

The Android 9 release introduces support for the Vulkan 1.1 graphics API. For more information about the Vulkan 1.1 API, see the Vulkan 1.1 API spec.

Vulkan 1.1 support overview

Vulkan 1.1 support includes support for Vulkan 1.1 and memory/synchronization interop. This enables OEMs to support Vulkan 1.1 on devices, and developers to determine whether Vulkan 1.1 is supported on a device, and use it effectively when it is. Vulkan 1.1 does not have new hardware requirements beyond Vulkan 1.0, but most of the implementation is in the SOC-specific graphics driver, not in the framework.

The most important Vulkan 1.1 features for Android are:

  • Support for importing and exporting memory buffers and synchronization objects from outside Vulkan (for interop with camera, codecs, and GLES)
  • Support for YCbCr formats

Vulkan 1.1 also includes several smaller features and API usability enhancements.

Implementing Vulkan 1.1

Vulkan 1.1 is optional in the Android 9 release. To use it, a device must have a GPU that meets the minimum Vulkan 1.1 capabilities.

To implement Vulkan 1.1:

  1. Add a Vulkan driver that supports Vulkan 1.1 plus the additional Android 1.1 requirements (make sure to view the 9 API level documentation), or update the existing Vulkan 1.0 driver.
  2. You might need to update the kernel GPU driver, depending on your implementation.
  3. Ensure PackageManager#hasSystemFeature(PackageManager.FEATURE_VULKAN_HARDWARE_VERSION, 0x401000) returns true by adding a rule like the following to an appropriate device.mk file:

    PRODUCT_COPY_FILES += frameworks/native/data/etc/android.hardware.vulkan.version-1_1.xml: $(TARGET_COPY_OUT_VENDOR)/etc/permissions/android.hardware.vulkan.version.xml

Customizing Vulkan 1.1

Applications that depend on Vulkan 1.1 are not compatible with devices that don’t provide Vulkan 1.1 support. Some applications might not require Vulkan 1.1 but will provide additional features or performance when it is present.

Window System Integration (WSI)

The Window System Integration (WSI) extensions VK_KHR_surface, VK_KHR_android_surface, and VK_KHR_swapchain are implemented by the platform and live in libvulkan.so. The VkSurfaceKHR and VkSwapchainKHR objects and all interaction with ANativeWindow is handled by the platform and is not exposed to drivers. The WSI implementation relies on the VK_ANDROID_native_buffer extension (described below) which must be supported by the driver; this extension is only used by the WSI implementation and will not be exposed to applications.

Gralloc usage flags

Implementations may need swapchain buffers to be allocated with implementation-defined private gralloc usage flags. When creating a swapchain, Android 8.0 asks the driver to translate the requested format and image usage flags into gralloc usage flags by calling:

typedef enum VkSwapchainImageUsageFlagBitsANDROID {
    VK_SWAPCHAIN_IMAGE_USAGE_SHARED_BIT_ANDROID = 0x00000001,
    VK_SWAPCHAIN_IMAGE_USAGE_FLAG_BITS_MAX_ENUM = 0x7FFFFFFF
} VkSwapchainImageUsageFlagBitsANDROID;
typedef VkFlags VkSwapchainImageUsageFlagsANDROID;

VkResult VKAPI vkGetSwapchainGrallocUsage2ANDROID(
    VkDevice                          device,
    VkFormat                          format,
    VkImageUsageFlags                 imageUsage,
    VkSwapchainImageUsageFlagsANDROID swapchainUsage,
    uint64_t*                         grallocConsumerUsage,
    uint64_t*                         grallocProducerUsage
);

The format and imageUsage parameters are taken from the VkSwapchainCreateInfoKHR structure. The driver should fill *grallocConsumerUsage and *grallocProducerUsage with the gralloc usage flags required for the format and usage. The usage returned by the driver will be combined with the usage flags requested by the swapchain consumer when allocating buffers.

An earlier version of this function is called by Android 7.x. In Android 8.0 it is deprecated but will still be called if vkGetSwapchainGrallocUsage2ANDROID isn't provided by the driver:

VkResult VKAPI vkGetSwapchainGrallocUsageANDROID(
    VkDevice            device,
    VkFormat            format,
    VkImageUsageFlags   imageUsage,
    int*                grallocUsage
);

This earlier version does not support swapchain usage flags or extended gralloc usage flags.

Gralloc-backed images

VkNativeBufferANDROID is a vkCreateImage extension structure for creating an image backed by a gralloc buffer. This structure is provided to vkCreateImage in the VkImageCreateInfo structure chain. Calls to vkCreateImage with this structure happen during the first call to vkGetSwapChainInfoWSI(.. VK_SWAP_CHAIN_INFO_TYPE_IMAGES_WSI ..). The WSI implementation allocates the number of native buffers requested for the swapchain, then creates a VkImage for each one:

typedef struct {
    VkStructureType             sType; // must be VK_STRUCTURE_TYPE_NATIVE_BUFFER_ANDROID
    const void*                 pNext;

    // Buffer handle and stride returned from gralloc alloc()
    buffer_handle_t             handle;
    int                         stride;

    // Gralloc format and usage requested when the buffer was allocated.
    int                         format;
    int                         usage;
    // Beginning in Android 8.0, the usage field above is deprecated and the
    // usage2 struct below was added. The usage field is still filled in for
    // compatibility with Android 7.0 drivers. Drivers for Android 8.0
    // should prefer the usage2 struct, especially if the
    // android.hardware.graphics.allocator HAL uses the extended usage bits.
    struct {
        uint64_t                consumer;
        uint64_t                producer;
    } usage2;
} VkNativeBufferANDROID;

When creating a gralloc-backed image, the VkImageCreateInfo has the following data:

 .imageType           = VK_IMAGE_TYPE_2D
  .format              = a VkFormat matching the format requested for the gralloc buffer
  .extent              = the 2D dimensions requested for the gralloc buffer
  .mipLevels           = 1
  .arraySize           = 1
  .samples             = 1
  .tiling              = VK_IMAGE_TILING_OPTIMAL
  .usage               = VkSwapChainCreateInfoWSI::imageUsageFlags
  .flags               = 0
  .sharingMode         = VkSwapChainCreateInfoWSI::sharingMode
  .queueFamilyCount    = VkSwapChainCreateInfoWSI::queueFamilyCount
  .pQueueFamilyIndices = VkSwapChainCreateInfoWSI::pQueueFamilyIndices
In Android 8.0 and later, the platform will provide a VkSwapchainImageCreateInfo extension structure in the VkImageCreateInfo chain provided to vkCreateImage when any swapchain image usage flags are required for the swapchain. The extension structure contains the swapchain image usage flags: 
typedef struct {
    VkStructureType                        sType; // must be VK_STRUCTURE_TYPE_SWAPCHAIN_IMAGE_CREATE_INFO_ANDROID
    const void*                            pNext;

    VkSwapchainImageUsageFlagsANDROID      usage;
} VkSwapchainImageCreateInfoANDROID;

Acquiring images

vkAcquireImageANDROID acquires ownership of a swapchain image and imports an externally-signalled native fence into both an existing VkSemaphore object and an existing VkFence object:

VkResult VKAPI vkAcquireImageANDROID(
    VkDevice            device,
    VkImage             image,
    int                 nativeFenceFd,
    VkSemaphore         semaphore,
    VkFence             fence
);

This function is called during vkAcquireNextImageWSI to import a native fence into the VkSemaphore and VkFence objects provided by the application (however, both semaphore and fence objects are optional in this call). The driver may also use this opportunity to recognize and handle any external changes to the gralloc buffer state; many drivers won't need to do anything here. This call puts the VkSemaphore and VkFence into the same pending state as vkQueueSignalSemaphore and vkQueueSubmit respectively, so queues can wait on the semaphore and the application can wait on the fence.

Both objects become signalled when the underlying native fence signals; if the native fence has already signalled, then the semaphore is in the signalled state when this function returns. The driver takes ownership of the fence fd and is responsible for closing it when no longer needed. It must do so even if neither a semaphore or fence object is provided, or even if vkAcquireImageANDROID fails and returns an error. If fenceFd is -1, it is as if the native fence was already signalled.

Releasing images

vkQueueSignalReleaseImageANDROID prepares a swapchain image for external use, and creates a native fence and schedules it to be signalled after the input semaphores have signaled:

VkResult VKAPI vkQueueSignalReleaseImageANDROID(
    VkQueue             queue,
    uint32_t            waitSemaphoreCount,
    const VkSemaphore*  pWaitSemaphores,
    VkImage             image,
    int*                pNativeFenceFd
);

This API is called during vkQueuePresentKHR on the provided queue. Effects are similar to vkQueueSignalSemaphore, except with a native fence instead of a semaphore. The native fence must not signal until the waitSemaphoreCount semaphores in pWaitSemaphores have signaled. Unlike vkQueueSignalSemaphore, however, this call creates and returns the synchronization object that will be signalled rather than having it provided as input. If the queue is already idle when this function is called, it is allowed (but not required) to set *pNativeFenceFd to -1. The file descriptor returned in *pNativeFenceFd is owned and will be closed by the caller.

Many drivers can ignore the image parameter, but some may need to prepare CPU-side data structures associated with a gralloc buffer for use by external image consumers. Preparing buffer contents for use by external consumers should have been done asynchronously as part of transitioning the image to VK_IMAGE_LAYOUT_PRESENT_SRC_KHR.

If the image was created with VK_SWAPCHAIN_IMAGE_USAGE_SHARED_BIT_ANDROID, then the driver must allow vkQueueSignalReleaseImageANDROID to be called repeatedly without intervening calls to vkAcquireImageANDROID.

Shared presentable image support

Some devices can share ownership of a single image between the display pipeline and the Vulkan implementation to minimize latency. In Android 9, the loader conditionally advertises the VK_KHR_shared_presentable_image extension based on the driver's response to a call to vkGetPhysicalDeviceProperties2.

If the driver doesn't support either Vulkan 1.1 or the VK_KHR_physical_device_properties2 extension, the loader assumes shared presentable images aren't supported. Otherwise, the loader queries the driver capabilities by calling vkGetPhysicalDeviceProperties2 and including the following structure in the VkPhysicalDeviceProperties2::pNext chain:

typedef struct {
    VkStructureType sType; // must be VK_STRUCTURE_TYPE_PHYSICAL_DEVICE_PRESENTATION_PROPERTIES_ANDROID
    const void*     pNext;
    VkBool32        sharedImage;
} VkPhysicalDevicePresentationPropertiesANDROID;

If the driver is capable of sharing ownership of an image with the display system, it should set the sharedImage member to VK_TRUE.

Validation

OEMs can test their Vulkan implementation using CTS, which includes:

  • drawElements Quality Program (dEQP) tests in the CtsDeqpTestCases module, which include functional API tests for Vulkan 1.0 and 1.1.
  • The CtsGraphicsTestCases module, which tests that the device is configured correctly for Vulkan capabilities it supports.