How to write a new user-space device driver for PoCL

The first required steps are adding the CMake infrastructure, and making the driver known to device initialization. Required CMake changes are: a boolean option to enable the driver (optionally autodetected at CMake time), adding the bool to, finding all required libraries, linking & compilation options, and correctly linking the driver depending on ENABLE_LOADABLE_DRIVERS (these can be copied from existing drivers). To make the driver known to device initialization in devices.c, add necessary #includes, and the new driver name to pocl_device_types and pocl_devices_init_ops.

Setting device properties

In the driver’s init callback, the driver should discover and set up each device’s properties. These are stored as members of struct _cl_device_id, e.g. max_work_item_dimensions or max_compute_units. Some of them are internal to PoCL, e.g. device_side_printf is a bool that changes the processing of the ‘printf’ function in PoCL’s LLVM passes. There are some helpers for setting up properties; for setting up versioned (OpenCL 3.0) properties, see lib/CL/devices/common.c; the same file also has pocl_init_default_device_infos which sets up some reasonable defaults. By default the entire struct is memset to zero, so all properties are zero / false.

PoCL’s internal driver API

Next step is to implement PoCL’s internal driver API operations (callbacks) for the device. They’re described in lib/CL/pocl_cl.h, in struct pocl_device_ops. The declarations for all operations can be obtained by including and using the GEN_PROTOTYPES macro. The declarations will be named pocl_<DRIVER_NAME>_<OP_NAME>, e.g. pocl_level0_map_mem. Many of these callbacks are optional; there’s no need to implement all to get a useful driver. Any callback that is implemented, needs to be assigned in the ops struct of the struct _cl_device_id in the pocl_<DRIVER>_init_device_ops function. For an example, see pocl_basic_init_device_ops in lib/CL/devices/basic/basic.c. If the callback is optional and will not be implemented, it can be left as NULL in ops.

The callbacks can be divided into several groups:

  • probe & initialization (required)

  • memory mapping, allocation & freeing (required)

  • event handling (required)

  • compilation callbacks (required)

  • command execution callbacks (optional)

  • SVM callbacks (optional)

  • Image callbacks (optional)

  • Object create / free callbacks (optional)

  • D2D migration callbacks (optional)

  1. probe & initialization - probe, init, reinit, uninit

    you’ll need to implement at least probe and init; probe is only called once, when libpocl is loaded, and its job is to find out how many devices are available. init is then called for each device in succession. Implementing reinit & uninit is optional - these release any resources after the last context is released. This is useful mainly for debugging memory leaks; since there is no clReleasePlatform in OpenCL, calling uninit should release everything - anything left over is a leak.

  2. memory allocation - alloc_mem_obj, free

    these allocate & free device-side memory. See documentation on struct _cl_mem->device_ptrs and pocl_mem_identifier. If the device supports images, these must also allocate/free images. Buffers (cl_mem objects) in OpenCL are per-context; the allocation of the backing storage (device memory) in PoCL is done lazily. IOW a buffer is allocated only on devices which actually run commands using those buffers. This happens at clEnqueueXYZ time (clEnqueue -> pocl_create_command() -> can_run_command() -> device->ops->alloc_mem_obj()).

  3. memory mapping - get_mapping_ptr, free_mapping_ptr

    these are required to support clEnqueueMapBuffer; they don’t execute the actual mapping, only return the host pointer where the mapping will exist.

  4. event handling - submit, join, flush, notify, broadcast, wait_event, update_event

    these are required to properly react to and propagate event status changes. With a few exceptions (basic, cuda, almaif), the implementations for these are almost identical. Implementing custom handling is possible, but must be done extremely carefully, as it can easily lead to issues with locking and data races.

  5. compilation callbacks - build_source, build_binary, link_program etc

    the implementation of these depends on what the device intends to support.

    1. if the device will only support offline compilation, this requires only a minimal implementation of build_binary; PoCL will automatically unpack pocl-binaries into the cache directory and setup kernel metadata.

    2. if the device wants to support online (OpenCL C) compilation using LLVM infrastructure, it can use the generic pocl_driver_XYZ callback implementations from common_driver.c. These will invoke Clang/LLVM compilation, optimization & linking steps. However, the device needs to setup its metadata like LLVM triple, whether it’s SPMD, and many others. It will also need to provide its builtin library. The existing implementations for per-device builtin libraries are in lib/kernel; there are many builtins that are device-agnostic, however some are device-specific.

    3. if the device wants to support online (source) compilation but is NOT using LLVM, it will have to provide its own implementation of all compilation callbacks and kernel metadata extraction.

    build_source - required; but only if driver wants to support compilation from source. Note that builds from SPIR-V input are (currently) handled by the build_binary callback.

    build_binary - required; all drivers (except custom devices) are assumed to support clCreateProgramWithBinary() with at least pocl-binaries as input. If the files stored in pocl-binaries have everything required, then the implementation of this callback can be trivial; otherwise it needs to do the transformation of binaries to final device binaries.

    link_program - is required to support the clCompile/clLinkProgram combo; it’s not strictly required for a useful driver, since most OpenCL programs don’t use these.

    supports_binary - optional; is called to check whether the driver supports a specific binary. Note that pocl-binaries are always supported, so this is only called for other binaries.

    setup_metadata - required; after build, it’s called to set up the program & kernel metadata like number of arguments, argument types and so on.

    post_build_program - optional; it’s called as part of clBuild/Link/Compile after everything else has been set up (including metadata).

    build_poclbinary & compile_kernel - optional; clGetProgramInfo() called with CL_PROGRAM_BINARIES will call these, if they’re not NULL; the purpose is to do any extra steps necessary to have the program cache directory in a “useful” state, when the cache directory can be serialized into a pocl-binary.

  6. command execution callbacks - read, copy, write, map_mem etc

    These are optional because command execution can be implemented in multiple ways.

    1. there is a helper function in PoCL for executing commands in the driver, called pocl_exec_command. This helper does some preparations, then calls the driver’s callback for the command (e.g. device->ops->unmap for the EnqueUnmap type command), and then cleanups after the command. The advantage is that this is the simplest way to implement a command; the disadvantage is that pocl_exec_command is synchronous and it does not do any optimization by grouping commands.

    2. the other way to implement command execution is to not use pocl_exec_command and device->ops command callbacks, in which case you can leave those NULL and implement commands your way. The PoCL library is driven by events, and how the driver executes the commands of events, is not important to the runtime, as long as events are correctly moved through their stages (submitted->queued->running->complete) and all of the “bookkeeping” (e.g. event callbacks) is handled properly.

    A simple driver implementation using pocl_exec_command() could look like this:
    • implement the command execution callbacks (device->ops->read etc)

    • create a background thread in pocl_DRIVER_init and a simple FIFO queue;

    • when a new event arrives with a command to execute, e.g. through pocl_DRIVER_notify or pocl_DRIVER_submit, check if the event is ready to execute, if it is, push into FIFO queue;

    • in the background thread, create a loop that waits for commands to arrive in the FIFO queue, then for each command, call pocl_exec_command - this will take care of calling the correct device->ops command callback, and various bookkeeping

  7. SVM callbacks - svm_free, svm_alloc, svm_map, svm_unmap etc

    only required if the device supports SVM. To support SVM, driver will also need to set up some properties in struct _cl_device_id, at least svm_allocation_priority, svm_caps, and atomic_memory_capabilities + atomic_fence_capabilities.

  8. image support - copy_image_rect, write_image_rect, map_image etc

    only required if device supports images. To support images, driver will also need to set up some properties in struct _cl_device_id, at least image_support, num_image_formats and image_formats but many others - search for image in the struct, also look at other driver’s ops->init.

  9. create / free callbacks - free_event_data, create_kernel, init_queue, create_sampler etc

    all of these are optional. Only necessary if the driver needs some to set up / tear down some device-specific (hardware or “backend” API) resources for a cl_object. E.g. the Level0 driver uses free_kernel to release the API’s ze_kernel_handle_t handle. The “free” callbacks are called only after the refcount on the object reached zero, so it is safe to destroy the resource. The “create” callbacks are called after the OpenCL part of the cl_object has been set up.

  10. device2device migration callbacks - can_migrate_d2d, migrate_d2d

optional; they’re used to implement direct migration of buffers between two devices. Direct means avoiding copying the buffer content to host memory and then from host memory to the 2nd device.