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drivers.md

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Drivers

How drivers work on Linux

To use hardware acceleration, you must do two thing:

  1. insmod the .ko kernel module provided by the vendor.

    TODO: how are those inserted by default at system startup?

  2. use the header and .so provided by the vendor.

    That .so will know how to talk to the kernel module.

    E.g. on Ubuntu 16.04:

    /usr/lib/nvidia-361/libGL.so
/usr/include/nvidia-361/GL/gl.h

    If you don't, most Linux distros default to the Mesa software implementation, e.g.:

    /usr/include/GL/gl.h
/usr/lib/x86_64-linux-gnu/libGL.so

    You can observe what your executables will load with:

    ldd ./executable

    or:

    TODO how Linux distros select between both? NVIDIA ones are not in my path, only mesa.

    But the NVIDIA driver is used by default when I compile. Looks like mesa does dload on NVIDIA libraries? Looks like dlopen is used, as strace shows several lines like;

    open("/usr/lib/nvidia-361/libGL.so.1", O_RDONLY|O_CLOEXEC) = 3

    And ldd says that NVIDIA is used... so the loaded library is different form that compiled against. TODO why?!

    cat /etc/ld.so.conf.d/x86_64-linux-gnu_GL.conf

    says:

    /usr/lib/nvidia-361
/usr/lib32/nvidia-361

    which explain how that is configured.

All leading hardware implementations are currently closed source, although some open source just the kernel module (and not the .so) for fear of the GPL (even though Linux said closed source kernel modules are fine).

Switch to software renderer

You have NVIDIA installed, and want to compare it to Mesa CPU renderer.

Once you have NVIDIA installed and running, to compile with software use:

gcc a.c -lGL -Wl,-rpath=/usr/lib/x86_64-linux-gnu/mesa

or run with:

LD_LIBRARY_PATH=/usr/lib/x86_64-linux-gnu/mesa ./a

TODO: but this fails once NVIDIA is running:

libGL error: No matching fbConfigs or visuals found
libGL error: failed to load driver: swrast
X Error of failed request:  GLXBadContext
  Major opcode of failed request:  154 (GLX)
  Minor opcode of failed request:  6 (X_GLXIsDirect)
  Serial number of failed request:  42
  Current serial number in output stream:  41
X Error of failed request:  BadValue (integer parameter out of range for operation)
  Major opcode of failed request:  154 (GLX)
  Minor opcode of failed request:  24 (X_GLXCreateNewContext)
  Value in failed request:  0x0
  Serial number of failed request:  41
  Current serial number in output stream:  42