src/devices/graphics/implement-vulkan.jd

page.title=Implementing Vulkan
@jd:body

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<p>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 <a href="https://www.khronos.org/spir">SPIR-V
Binary Intermediate</a> language.</p>

<p class="note"><strong>Note:</strong> This section describes Vulkan
implementation; for details on Vulkan architecture, advantages, API, and other
resources, see <a href="{@docRoot}devices/graphics/arch-vulkan.html">Vulkan
Architecture</a>.</p>

<p>To implement Vulkan, a device:</p>
<ul>
<li>Must include the Vulkan Loader (provided by Android) in the build.</li>
<li>Must include a Vulkan driver (provided by SoCs such as GPU IHVs) that
implements the
<a href="https://www.khronos.org/registry/vulkan/specs/1.0-wsi_extensions/xhtml/vkspec.html">Vulkan
API</a>. To support Vulkan functionality, the Android device needs capable GPU
hardware and the associated driver. Consult your SoC vendor to request driver
support.</li>
</ul>
<p>If a Vulkan driver is available on the device, the device needs to declare
<code>FEATURE_VULKAN_HARDWARE_LEVEL</code> and
<code>FEATURE_VULKAN_HARDWARE_VERSION</code> system features, with versions that
accurately reflect the capabilities of the device.</p>

<h2 id=vulkan_loader>Vulkan Loader</h2>
<p>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)
(<code>platform/frameworks/native/vulkan</code>) and installed at
<code>/system/lib[64]/libvulkan.so</code>. 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.</p>

<p>The NDK includes a stub <code>libvulkan.so</code> 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
<code>libvulkan.so</code> (the real library, not the stub) to enter trampoline
functions in the loader (which then dispatch to the appropriate layer or driver
based on their first argument). The <code>vkGetDeviceProcAddr</code> 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. However, <code>vkGetInstanceProcAddr</code>
must still call into trampoline code.</p>

<h2 id=driver_emun>Driver enumeration and loading</h2>
<p>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
<code><a href="https://android.googlesource.com/platform/hardware/libhardware/+/marshmallow-release/include/hardware/hardware.h">hardware.h</code></a>) for
discovering and loading the driver. Preferred paths for 32-bit and 64-bit Vulkan
drivers are:</p>

<p>
<pre>
/vendor/lib/hw/vulkan.&lt;ro.product.platform&gt;.so
/vendor/lib64/hw/vulkan.&lt;ro.product.platform&gt;.so
</pre>
</p>

<p>Where &lt;<code>ro.product.platform</code>&gt; is replaced by the value of
the system property of that name. For details and supported alternative
locations, refer to
<code><a href="https://android.googlesource.com/platform/hardware/libhardware/+/marshmallow-release/hardware.c">libhardware/hardware.c</code></a>.</p>

<p>In Android 7.0, the Vulkan <code>hw_module_t</code> derivative is trivial;
only one driver is supported and the constant string
<code>HWVULKAN_DEVICE_0</code> 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 <code>module open</code> call.</p>

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

<h2 id=layer_discover>Layer discovery and loading</h2>
<p>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 7.0 does not include layers on the system image; however,
applications may include layers in their APK.</p>
<p>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.</p>

<p>Use cases for layers include:</p>
<ul>
<li><strong>Development-time layers</strong>. 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.</li>
<li><strong>Utility layers</strong>. 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.</li>
<li><strong>Injected (implicit) layers</strong>. 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.</li>
</ul>

<p>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. <code>libVKLayer_foo.so</code>). 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.</p>

<p>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
<a href="https://github.com/KhronosGroup/Vulkan-LoaderAndValidationLayers/blob/master/loader/LoaderAndLayerInterface.md">Vulkan
Loader Specification and Architecture Overview</a>. 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
<a href="https://github.com/KhronosGroup/Vulkan-LoaderAndValidationLayers/tree/android_layers">KhronosGroup/Vulkan-LoaderAndValidationLayers</a>
project on GitHub.</p>

<h2 id=wsi>Window System Integration (WSI)</h2>
<p>The Window System Integration (WSI) extensions <code>VK_KHR_surface</code>,
<code>VK_KHR_android_surface</code>, and <code>VK_KHR_swapchain</code> are
implemented by the platform and live in <code>libvulkan.so</code>. The
<code>VkSurfaceKHR</code> and <code>VkSwapchainKHR</code> objects and all
interaction with <code>ANativeWindow</code> is handled by the platform and is
not exposed to drivers. The WSI implementation relies on the
<code>VK_ANDROID_native_buffer</code> 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.</p>

<h3 id=gralloc_usage_flags>Gralloc usage flags</h3>
<p>Implementations may need swapchain buffers to be allocated with
implementation-defined private gralloc usage flags. When creating a swapchain,
the platform asks the driver to translate the requested format and image usage
flags into gralloc usage flags by calling:</p>

<p>
<pre>
VkResult VKAPI vkGetSwapchainGrallocUsageANDROID(
    VkDevice            device,
    VkFormat            format,
    VkImageUsageFlags   imageUsage,
    int*                grallocUsage
);
</pre>
</p>

<p>The <code>format</code> and <code>imageUsage</code> parameters are taken from
the <code>VkSwapchainCreateInfoKHR</code> structure. The driver should fill
<code>*grallocUsage</code> with the gralloc usage flags required for the format
and usage (which are combined with the usage flags requested by the swapchain
consumer when allocating buffers).</p>

<h3 id=gralloc_usage_flags>Gralloc-backed images</h3>

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

<p><pre>
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;
} VkNativeBufferANDROID;
</pre></p>

<p>When creating a gralloc-backed image, the <code>VkImageCreateInfo</code> has
the following data:</p>

<p><pre>
 .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
</pre></p>

<h3 id=acquire_image>Aquiring images</h3>
<p><code>vkAcquireImageANDROID</code> acquires ownership of a swapchain image
and imports an externally-signalled native fence into both an existing
<code>VkSemaphore</code> object and an existing <code>VkFence</code> object:</p>

<p><pre>
VkResult VKAPI vkAcquireImageANDROID(
    VkDevice            device,
    VkImage             image,
    int                 nativeFenceFd,
    VkSemaphore         semaphore,
    VkFence             fence
);
</pre></p>

<p>This function is called during <code>vkAcquireNextImageWSI</code> to import a
native fence into the <code>VkSemaphore</code> and <code>VkFence</code> 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 <code>VkSemaphore</code> and
<code>VkFence</code> into the same pending state as
<code>vkQueueSignalSemaphore</code> and <code>vkQueueSubmit</code> respectively,
so queues can wait on the semaphore and the application can wait on the fence.</p>

<p>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
<code>vkAcquireImageANDROID</code> fails and returns an error. If fenceFd is -1,
it is as if the native fence was already signalled.</p>

<h3 id=acquire_image>Releasing images</h3>
<p><code>vkQueueSignalReleaseImageANDROID</code> prepares a swapchain image for
external use, and creates a native fence and schedules it to be signalled when
prior work on the queue has completed:</p>

<p><pre>
VkResult VKAPI vkQueueSignalReleaseImageANDROID(
    VkQueue             queue,
    VkImage             image,
    int*                pNativeFenceFd
);
</pre></p>

<p>This API is called during <code>vkQueuePresentWSI</code> on the provided
queue. Effects are similar to <code>vkQueueSignalSemaphore</code>, except with a
native fence instead of a semaphore. Unlike <code>vkQueueSignalSemaphore</code>,
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
<code>*pNativeFenceFd</code> to -1. The file descriptor returned in
*<code>pNativeFenceFd</code> is owned and will be closed by the caller.</p>

<h3 id=update_drivers>Updating drivers</h3>

<p>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
<code>VK_IMAGE_LAYOUT_PRESENT_SRC_KHR</code>.</p>

<h2 id=validation>Validation</h2>
<p>OEMs can test their Vulkan implementation using CTS, which includes
<a href="{@docRoot}devices/graphics/cts-integration.html">drawElements
Quality Program (dEQP)</a> tests that exercise the Vulkan Runtime.</p>