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VC4CL

VC4CL is an implementation of the OpenCL 1.2 standard for the VideoCore IV GPU (found in all Raspberry Pi models).

The implementation consists of:

  • The VC4CL OpenCL runtime library, running on the host CPU to compile, run and interact with OpenCL kernels.
  • The VC4C compiler, converting OpenCL kernels into machine code. This compiler also provides an implementation of the OpenCL built-in functions.
  • The VC4CLStdLib, the platform-specific implementation of the OpenCL C standard library, is linked in with the kernel by VC4C

OpenCL-Support

The VC4CL implementation supports the EMBEDDED PROFILE of the OpenCL standard version 1.2. Additionally the cl_khr_icd extension is supported, to allow VC4CL to be found by an installable client driver loader (ICD). This enables VC4CL to be used in parallel with another OpenCL implementation, e.g. pocl, which executes OpenCL code on the host CPU.

The OpenCL version 1.2 was selected as target standard version, since it is the last version of the OpenCL standard where all mandatory features can be supported.

VC4CL supports the EMBEDDED PROFILE of the OpenCL-standard, which is a trimmed version of the default FULL PROFILE. The most notable features, which are not supported by the VC4CL implementation are images, the long and double data-types, device-side printf and partitioning devices. See RuntimeLibrary for more details of (not) supported features.

VideoCore IV GPU

The VideoCore IV GPU, in the configuration as found in the Raspberry Pi models, has a theoretical maximum performance of 24 GPFLOS and is therefore very powerful in comparison to the host CPU. The GPU (which is located on the same chip as the CPU) has 12 cores, able of running independent instructions each, supports a SIMD vector-width of 16 elements natively and can access the RAM directly via DMA.

Required software

  • A C++14-capable compiler (e.g. GCC 6.3 or clang from the Raspbian repositories)
  • The VC4C compiler to compile OpenCL C-code
  • The Raspbian development files (available in the official Raspbian repository as sudo apt-get install libraspberrypi-dev)
  • The Khronos ICD Loader (available in the official Raspbian repository as sudo apt-get install ocl-icd-opencl-dev ocl-icd-dev) for building with ICD-support (e.g. allows to run several OpenCL implementations on one machine)
  • The OpenCL headers in version >= 1.2 (available in the Raspbian repositories as sudo apt-get install opencl-headers)
  • The Raspberry Pi firmware GPU-side and host-side binaries as well as the mailbox kernel module. These are already provided by default on Raspbian OS. For other Linux distributions, see here.

Build

The following configuration options are available in CMake:

  • BUILD_TESTING toggles building of test program (when configured, can be built with make TestVC4CL)
  • BUILD_DEBUG toggles building debug or release program
  • CROSS_COMPILE toggles whether to cross-compile for the Raspberry Pi. NOTE: The Raspberry Pi cross-compiler is no longer supported!
  • CROSS_COMPILER_PATH sets the root path to the Raspberry Pi cross compiler (.g. arm-linux-gnueabihf-g++-6)
  • INCLUDE_COMPILER whether to include the VC4C compiler. For the compiler to be actually included, the VC4C header and library needs to be found too
  • VC4C_HEADER_PATH sets the path to the VC4C include headers, defaults to ../VC4C/include/VC4C.h or lib/vc4c/include/VC4C.h
  • VC4CC_LIBRARY sets the path to the VC4C compiler library, defaults to ../VC4C/build/libVC4CC.xxx or lib/vc4c/build/libVC4CC.xxx
  • BUILD_ICD toggles whether to build with support for the Khronos ICD loader, requires the ICD loader to be installed system-wide
  • IMAGE_SUPPORT toggles whether to enable the very experimental image-support
  • REGISTER_POKE_KERNELS toggles the use of register-poking to start kernels (if disabled, uses the mailbox system-calls). Enabling this increases performance up to 10%, but may crash the system, if any other application accesses the GPU at the same time!
  • BUILD_DEB_PACKAGE toggles whether to create the necessary configuration to build vc4cl-xxx.deb package for installation on Raspbian. The actual packaging is started with cpack -G DEB

Khronos ICD Loader

The Khronos ICD Loaders allows multiple OpenCL implementation to be used in parallel (e.g. VC4CL and pocl), but requires a bit of manual configuration: Create a file /etc/OpenCL/vendors/VC4CL.icd with a single line containing the absolute path to the VC4CL library.

The program clinfo can be used to test, whether the ICD loader finds the VC4CL implementation. Note: the program version in the official Raspbian repository is too old and has a bug (see fix), so it must be compiled from the github repository.

Security Considerations

Because of the DMA-interface which has no MMU between the GPU and the RAM, code executed on the GPU can access any part of the main memory! This means, an OpenCL kernel could be used to read sensitive data or write into kernel memory!

Therefore, any program using the VC4CL implementation must be run as root!