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01-device

First step: Creating a device.

Initial objects

Before we can do anything interesting with Vulkan, we need to get three objects:

  1. VkInstance which is the initial interface to the Vulkan loader library.
  2. VkPhysicalDevice which represents a GPU.
  3. VkDevice which is a connection to a physical device.

Loading the loader

The initial entry point is vkGetInstanceProcAddr, which we need to load in some OS-dependent way. Either we can link to the Vulkan library at build time, or we can load it dynamically at run time with LoadLibrary/GetProcAddress (on Windows) or dlopen/dlsym (on Linux). Loading it at run time is a bit less convenient, but it gives us an opportunity to present a nice error message to the user if they don't have any Vulkan drivers installed.

vkGetInstanceProcAddr can give us instance-independent function pointers to:

  • vkEnumerateInstanceExtensionProperties
  • vkEnumerateInstanceLayerProperties
  • vkCreateInstance

Those functions can give us a VkInstance. We can pass that VkInstance back into vkGetInstanceProcAddr to get instance-specific function pointers to:

  • vkDestroyInstance
  • vkEnumeratePhysicalDevices
  • vkGetPhysicalDeviceProperties
  • vkGetPhysicalDeviceQueueFamilyProperties
  • vkCreateDevice

Those functions can give us a VkDevice.

Once we've got a VkDevice, we have two options:

  • Continue using instance-specific functions from vkGetInstanceProcAddr for all the rest of the Vulkan API.
  • Use device-specific functions returned by vkGetDeviceProcAddr instead.

The device-specific ones are only valid for one VkDevice, but they might be a tiny bit more efficient since they can skip the code that decides which device to dispatch each call to.

vkCreateInstance

Creating an instance is mostly straightforward. We need to give it a list of the layers and extensions we want to enable, plus a VkApplicationInfo.

The VkApplicationInfo is optional but quite useful. You can use it to tell the driver about your application and engine - I assume the intention is that if you're quite popular the driver developers might add application-/engine-specific tweaks into their drivers to fix your bugs or improve your performance. With OpenGL they often did this by looking at the .exe's filename to figure out which tweaks to apply; VkApplicationInfo makes it a little bit less yucky.

VkApplicationInfo also has an apiVersion field:

applicationInfo.apiVersion = VK_MAKE_VERSION(1, 0, 3);

We use apiVersion to say which version of the API our application is written to. If the driver doesn't support that version, vkCreateInstance will fail with VK_ERROR_INCOMPATIBLE_DRIVER. For example if we set apiVersion to 1.1.0, but the driver only knows about 1.0.3 (and will therefore be missing features that are required by 1.1.0), it will fail.

The application should specify the minimum API version that includes the features it needs. In particular don't set it to VK_API_VERSION (defined by vulkan.h as the latest API version), because your application may start failing when it's rebuilt with a newer version of vulkan.h even if it doesn't use any of the new version's features.

(Vulkan's version numbering scheme is designed so that an application written for 1.0.4 will work on drivers written for 1.0.3 or for 1.0.9 - changes in the last ('patch') digit do not break compatibility in either direction, because they won't add or remove features. An application written for 1.1.x will work on drivers written for 1.2.x, but not on drivers written for 1.0.x - changes in the second ('minor') digit might add new features that old drivers won't support. And 2.x.x might be completely incompatible with 1.x.x.)

VkPhysicalDevice

vkEnumeratePhysicalDevices gives us a list of all the Vulkan-capable physical devices in the system.

If there are none, we can report an error. If there is one, we want to use it.

If there is more than one physical device, we have a non-trivial problem.

Often a user might have a fast discrete GPU, and a slow integrated GPU. If we're a high-performance game, we probably want the discrete GPU. But if we don't need top performance, maybe the integrated GPU will be fast enough and consume less power, so laptop users would appreciate us using that one. And I have one computer with a discrete GPU that's actually slower than the integrated GPU (it's only there to provide dual-monitor support).

And the user might only have a monitor connected to one GPU. We'd probably prefer to use that one, else it might be impossible or at least inefficient to copy the framebuffer across to the right GPU for display.

Or they might have monitors connected to both GPUs. And there might be multiple discrete GPUs, or any other combination.

So, how can we best handle that?

Um... I don't know. Let's just pick the first physical device in the list. Maybe later we'll add a command-line option so the user can choose a different one. (We need a better solution.)

Queue families

TODO