[Mesa-dev] [RFC] GBM Alternate Backend Discovery & Loading

[James Jones]

I've brought this up in the past, but I have some patches implementing 
it now, so I was hoping to get some further feedback on the idea of 
supporting GBM backends external to Mesa.

https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9902

 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_<DRM driver name>.so.0 
and create a device from it if found.

-If that fails, try the built-in DRI backend.

Thanks,
-James