This is a specification for how the Vulkan loader should identify Vulkan installable client drivers (ICDs) and layers on Linux systems. This is designed for production installation of Vulkan ICDs and layers. The design is shown first for ICDs, and then the variation for layers will be discussed. 1. Installable Client Drivers: 1.1. Properly-Installed ICDs In order to find properly-installed ICDs, the Vulkan loader will scan the files in the following Linux directories: /usr/share/vulkan/icd.d /etc/vulkan/icd.d These directories will contain text information files (a.k.a. "manifest files"), that use a JSON format (NOTE: The JSON in this version of the specification is for illustration purposes, and isn't completely valid yet). The Vulkan loader will open each info file to obtain the name or pathname of an ICD shared library (".so") file. For example: { "file_format_version": "1.0.0", "ICD": { "library_path": "path to ICD library", "api_version": "1.0.0" } } The "library_path" specifies either a filename, a relative pathname, or a full pathname to an ICD shared library file. If the ICD is specified via a filename, the loader will attempt to open that file as a shared object using dlopen(), and the file must be in a directory that dlopen is configured to look in (Note: various distributions are configured differently). A distribution is free to create Vulkan-specific system directories (e.g. ".../vulkan/icd"), but is not required to do so. If the ICD is specified via a relative pathname, it is relative to the path of the info file. Relative pathnames are those that do not start with, but do contain at least one directory separator (i.e. the '/' character). For example, "lib/vendora.so" and "./vendora.so" are examples of relative pathnames. The "file_format_version" provides a major.minor.patch version number in case the format of the text information file changes in the future. If the same ICD shared library supports multiple, incompatible versions of text info file format versions, it must have multiple text info files (all of which may point to the same shared library). The “api_version” specifies the major.minor.patch version number of the Vulkan API that the shared library (referenced by "library_path") was built with. The "/usr/share/vulkan/icd.d" directory is for ICDs that are installed from Linux-distribution-provided packages. The "/etc/vulkan/icd.d" directory is for ICDs that are installed from non-Linux-distribution-provided packages. There are no rules about the name of the text files (except the .json suffix). There are no rules about the name of the ICD shared library files. For example, if the "/usr/share/vulkan/icd.d" directory contain the following files, with the specified contents: Text File Name Text File Contents -------------------------------------------------------------------------- vk_vendora.json { "ICD": { "library_path": "vendora.so" }} vendorb_vk.json { "ICD": { "library_path": "vendorb_vulkan_icd.so" }} vendorc_icd.json { "ICD": { "library_path": "/usr/lib/VENDORC/icd.so" }} then the loader will open the three files mentioned in the "Text File Contents" column, and then try to load and use the three shared libraries mentioned indicated by the ICD.library_path value. 1.2. Using Pre-Production ICDs IHV developers (and sometimes other developers) need to use special, pre-production ICDs. In some cases, a pre-production ICD may be in an installable package. In other cases, a pre-production ICD may simply be a shared library in the developer's build tree. In this latter case, we want to allow developers to point to such an ICD without modifying the properly-installed ICD(s) on their system. This need is met with the use of the "VK_ICD_FILENAMES" environment variable, which will override the mechanism used for finding properly-installed ICDs. In other words, only the ICDs listed in "VK_ICD_FILENAMES" will be used. The "VK_ICD_FILENAMES" environment variable is a colon-separated list of ICD text information files, containing the following: - A filename (e.g. "libvkicd.json") in the "/usr/share/vulkan/icd.d" or "/etc/vulkan/icd.d" system directories - A full pathname (e.g. "/my_build/my_icd.json") Typically, "VK_ICD_FILENAMES" will only contain a full pathname to one info file for a developer-built ICD. A colon is only used if more than one ICD is listed. For example, if a developer wants to refer to one ICD that they built, they could set the "VK_ICD_FILENAMES" environment variable to: /my_build/my_icd.json If a developer wants to refer to two ICDs, one of which is a properly-installed ICD, they can use the name of the text file in the system directory: vendorc_vulkan.json:/my_build/my_icd.json Notice the colon between "vendorc_vulkan.json" and "/my_build/my_icd.json". NOTE: this environment variable will be ignored for suid programs. 2. Layers: 2.1. Properly-Installed Layers In order to find properly-installed layers, the Vulkan loader will use a similar mechanism as used for ICDs. Text information files, that use a JSON format, are read in order to identify the names and attributes of layers and their extensions. The use of text info files allows the loader to avoid loading any shared library files when the application does not query nor request any extensions. Layers and extensions have additional complexity, and so their info files contain more information than ICD info files. For example, a layer shared library file may contain multiple layers/extensions (perhaps even an ICD). The Vulkan loader will scan the files in the following Linux directories: /usr/share/vulkan/explicit_layer.d /usr/share/vulkan/implicit_layer.d /etc/vulkan/explicit_layer.d /etc/vulkan/implicit_layer.d Explicit layers are those which are enabled by an application (e.g. with the vkCreateInstance function), or by an environment variable (see below). Implicit layers are those which are enabled by their existence. For example, certain application environments (e.g. Steam or an automotive infotainment system) may have layers which they always want enabled for all applications that they start. Other implicit layers may be for all applications started on a given system (e.g. layers that overlay frames-per-second). Implicit layers are enabled automatically, whereas explicit layers must be enabled explicitly. What distinguishes a layer as implicit or explicit is by which directory its layer information file exists in. The "/usr/share/vulkan/*_layer.d" directories are for ICDs that are installed from Linux-distribution-provided packages. The "/etc/vulkan/*_layer.d" directories are for ICDs that are installed from non-Linux-distribution-provided packages. The information file is in the JSON format and contains the following information: - (required) "file_format_version" – same as for ICDs, except that the format version can vary independently for ICDs and layers. - (required) "name" - layer name - (required) "type" - which layer chains should the layer be activated on. Allowable values are "INSTANCE", "DEVICE", "GLOBAL". Global means activate on both device and instance chains. - (required) "library_path" - filename / full path / relative path to the text file - (required) "api_version" – same as for ICDs. - (required) "implementation_version" – layer version, a single number increasing with backward compatible changes. - (required) "description" – informative decription of the layer. - (optional) "device_extensions" or "instance_extensions" - array of extension information as follows - (optional) extension "name" - Vulkan registered name - (optional) extension "spec_version" - extension specification version, a single number, increasing with backward compatible changes. - (optional) extension "entrypoints" - array of device extension entrypoints; not used for instance extensions - (sometimes required) "functions" - mapping list of function entry points. If multiple layers exist within the same shared library (or if a layer is in the same shared library as an ICD), this must be specified to allow each layer to have its own vkGet*ProcAddr entrypoints that can be found by the loader. At this time, only the following two functions are required: - "vkGetInstanceProcAddr" name - "vkGetDeviceProcAddr" name - (optional for implicit layers) "enable_environment" requirement(s) - environment variable and value required to enable an implicit layer. This environment variable (which should vary with each "version" of the layer, as in "ENABLE_LAYER_FOO_1") must be set to the given value or else the implicit layer is not loaded. This is for application environments (e.g. Steam) which want to enable a layer(s) only for applications that they launch, and allows for applications run outside of an application environment to not get that implicit layer(s). - (required for implicit layers) "disable_environment" requirement(s) - environment variable and value required to disable an implicit layer. Note: in rare cases of an application not working with an implicit layer, the application can set this environment variable (before calling Vulkan functions) in order to "blacklist" the layer. This environment variable (which should vary with each "version" of the layer, as in "DISABLE_LAYER_FOO_1") must be set (not particularly to any value). If both the "enable_environment" and "disable_environment" variables are set, the implicit layer is disabled. For example: { "file_format_version" : "1.0.0", "layer": { "name": "OverlayLayer", "type": "DEVICE", "library_path": "libvkOverlayLayer.so", "api_version" : "1.0.0", "implementation_version" : "2", "description" : "LunarG HUD layer", "functions": { "vkGetInstanceProcAddr": "Overlaylayer_GetInstanceProcAddr", "vkGetDeviceProcAddr": "OverlayLayer_GetDeviceProcAddr" }, "instance_extensions": [ { "name": "VK_LUNARG_DEBUG_REPORT", "spec_version": "1" }, { "name": "VK_VENDOR_DEBUG_X", "spec_version": "3" } ], "device_extensions": [ { "name": "VK_LUNARG_DEBUG_MARKER", "spec_version": "1", "entrypoints": ["vkCmdDbgMarkerBegin", "vkCmdDbgMarkerEnd"] } ], "disable_environment": { "DISABLE_LAYER_OVERLAY_1": "" } } } The "library_path" specifies either a filename, a relative pathname, or a full pathname to a layer shared library (".so") file, which the loader will attempt to load using dlopen(). If the layer is specified via a filename, the loader will attempt to open that file as a shared object using dlopen(), and the file must be in a directory that dlopen is configured to look in (Note: various distributions are configured differently). A distribution is free to create Vulkan-specific system directories (e.g. ".../vulkan/layers"), but is not required to do so. If the layer is specified via a relative pathname, it is relative to the path of the info file (e.g. for cases when an application provides a layer that is in the same directory hierarchy as the rest of the application files). There are no rules about the name of the text files (except the .json suffix). There are no rules about the name of the layer shared library files. 2.2. Using Pre-Production Layers As with ICDs, developers may need to use special, pre-production layers, without modifying the properly-installed layers. This need is met with the use of the "VK_LAYER_PATH" environment variable, which will override the mechanism using for finding properly-installed layers. Because many layers may exist on a system, this environment variable is a colon-separated list of directories that contain layer info files. Only the directories listed in "VK_LAYER_PATH" will be scanned for info files. Each colon-separated entry is: - The full pathname of a directory containing layer info files In addition to overriding the mechanism for finding layers, the following environment variables are used to select one or more layers/extensions (respectively) to explicitly enable: - "VK_INSTANCE_LAYERS" for instance/global layers/extensions, enabled at vkCreateInstance time - "VK_DEVICE_LAYERS" for device layers/extensions, enabled at vkCreateDevice time These are colon-separated lists of extension names, as listed in the "name" field of the info file. The loader will load all layers/extensions that match the given extension name(s), ignoring the "version" fields. NOTE: these environment variables will be ignored for suid programs.