| # Loader Description |
| |
| ## Overview |
| The Loader implements the main XGL library: libXGL.so on Linux. It handles |
| layer management and driver management. Loader driver management includes |
| finding driver librairies and loading them. Aditionally, the loader dispatches |
| the API calls to the correct driver based on the GPU selected by the app. The |
| loader fully supports multi-gpu operation. |
| |
| Loader layer management includes finding layer libraries and activating them |
| as requested. Loader correctly sets up layer and its own dispatch tables to |
| support multiple layers activated. Each active layer can intercept a subset of |
| the full API entrypoints. A layer which doesn't intercept a given entrypoint |
| will be skipped for that entrypoint. The loader supports layers that operate |
| on multiple GPUs. |
| |
| ## Environment Variables |
| LIBXGL\_DRIVERS\_PATH directory for loader to search for ICD driver libraries to open |
| |
| LIBXGL\_LAYERS\_PATH directory for loader to search for layer libraries that may get activated and used at xglCreateDevice() time. |
| |
| LIBXGL\_LAYER\_NAMES colon separate list of layer names to be activated. Example, |
| LIBXGL\_LAYER\_NAMES=MemTracker:DrawState |
| |
| ## Interface to driver (ICD) |
| - xglInitAndEnumerateGpus exported |
| - xglGetProcAddr exported and returns valid function pointers for all the XGL API entrypoints |
| - all objects created by ICD can be cast to (XGL\_LAYER\_DISPATCH\_TABLE **) |
| where the loader will replace the first entry with a pointer to the dispatch table which is |
| owned by the loader. This implies two things for ICD drivers: |
| 1. the ICD must return a pointer for the opaque object handle |
| 2. this pointer points to a structure with the first entry being a pointer. |
| - the ICD may or may not implement a dispatch table |
| - ICD entrypoints can be named anything including the offcial xgl name such as xglCreateDevice(). However, beware of inter |
| posing by dynamic OS library loaders if the offical names are used. On Linux, |
| if offical names are used, the ICD library must be linked with -Bsymbolic |
| |
