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From my reading, the GBM core code was intended to support, or perhaps at some point in the past did support loading backends other than the built-in DRI one. This series extends the current code to enable loading of alternate backends in general, and as a straw-man proposal, specifically uses the DRM driver name of the FD passed to gbm_create_device() to construct a library name of a potential alternate backend. Other schemes could be implemented on top of the basic functionality added here, but this simple one seemed sufficient.

Issues addressed in the series:

• The ability to dynamically load backends from DSOs.
• Thread-safe tracking and refcounting of dynamically-loaded backends.
• Versioning of the GBM driver ABI to enable backwards and forwards binary compatibility between the GBM loader and backends.
• Discovery of alternate/external backend DSOs.

Here's a rundown of the backend discovery logic implemented in the series:

• If the GBM_BACKEND env var is set, attempt to load the backend library it names and create a device from it if found.
• If that fails or the env var is not set, call drmGetVersion() on the fd, and if that succeeds, attempt to load libgbm_.so.0 and create a device from it if found.
• If that fails, try the built-in DRI backend.