How to use OpenCL to write directly to linux framebuffer with zero-copy?

Question

I am using OpenCL to do some image processing and want to use it to write RGBA image directly to framebuffer. Workflow is shown below:

1) map framebuffer to user space.

2) create OpenCL buffer using clCreateBuffer with flags of "CL_MEM_ALLOC_HOST_PTR"

3) use clEnqueueMapBuffer to map the results to framebuffer.

However, it doesn't work. Nothing on the screen. Then I found that the mapped virtual address from framebuffer are not same as the virtual address mapped OpenCL. Has any body done a zero-copy move of data from GPU to framebuffer?Any help on what approach should I use for this?

Some key codes:

if ((fd_fb = open("/dev/fb0", O_RDWR, 0)) < 0) {
    printf("Unable to open /dev/fb0\n");
    return -1;
}
fb0 = (unsigned char *)mmap(0, fb0_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd_fb, 0);
...
cmDevSrc4 = clCreateBuffer(cxGPUContext, CL_MEM_READ_WRITE | CL_MEM_ALLOC_HOST_PTR, sizeof(cl_uchar) * imagesize * 4, NULL, &status);
...
fb0 = (unsigned char*)clEnqueueMapBuffer(cqCommandQueue, cmDevSrc4, CL_TRUE, CL_MAP_READ, 0, sizeof(cl_uchar) * imagesize * 4, 0, NULL, NULL, &ciErr);

Answer 1

For zero-copy with an existing buffer you need to use CL_MEM_USE_HOST_PTR flag in the clCreateBuffer() function call. In addition you need give the pointer to the existing buffer as second to last argument.

I don't know how linux framebuffer internally works but it is possible that even with the zero-copy from device to host it leads to extra copying the data to GPU for rendering. So you might want to render the OpenCL buffer directly with OpenGL. Check out cl_khr_gl_sharing extension for OpenCL.