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.product.platform>.so /vendor/lib64/hw/vulkan.<ro.product.platform>.so
Where <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.
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::pQueueFamilyIndicesIn 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.
VK_SWAPCHAIN_IMAGE_USAGE_SHARED_BIT_ANDROID, then the driver must
allow vkQueueSignalReleaseImageANDROID to be called repeatedly
without intervening calls to vkAcquireImageANDROID.
Validation
OEMs can test their Vulkan implementation using CTS, which includes drawElements Quality Program (dEQP) tests that exercise the Vulkan Runtime.