OpenCL: How to properly chain kernels - opencl

Question

Ok, so I have two Kernels that both take an input and an output image and do some meaningful operation:
#pragma OPENCL EXTENSION cl_khr_3d_image_writes : enable
kernel void Kernel1(read_only image3d_t input, write_only output)
{
//read voxel and some surrounding voxels
//perform some operation
//write voxel
}
#pragma OPENCL EXTENSION cl_khr_3d_image_writes : enable
kernel void Kernel2(read_only image3d_t input, write_only output)
{
//read voxel and some surrounding voxels
//perform some other operation
//write voxel
}
#pragma OPENCL EXTENSION cl_khr_3d_image_writes : enable
kernel void KernelCombined(read_only image3d_t input, write_only output)
{
//read voxel and some surrounding voxels
//...
//perform operation of both kernels (without read, write)
//...
//write voxel
}
Now I want to chain the kernels in some cases, so what I could do is first call Kernel 1 and then Kernel2. But that means, that I have unneccesary write and reads in between. I could also write a third kernel which does both, but maintaining copy-paste code seems to be annoying. I cannot really put the content of each Kernel in a separate function as I cannot pass around the image3d_t input, to my knowledge.
Question: Is there any clever way of chaining the two kernels? Is maybe OpenCL doing something clever already that I do not know?
Edit: Added example of what I would like to achive.

Answer 1

I understand what you're asking for -- you wish to remove the image write / read cycle between kernels. With the kernels you described, this would not be efficient. In the existing kernels you "read voxel and some surrounding voxels" -- let's say that means reading 7 voxels. If you do the same read pattern in kernel 2 and 3, it's a total of 21 reads (and 3 writes). If somehow you chained these three kernels into a single kernel that wrote a single output voxel, it would need to read from many more source voxels to have the same result (because each read step was adding radius).
The scenario where kernel write/read chaining would be helpful would be for single-in/single-out kernels, like image processing where colors are modified independently of their neighbors. To do that you need a higher-level description of your kernels, and something that can generate the kernels you need based on the operations you have.

Answer 2

This is possible if you're using an opencl 2.0 capable device. enqueue_kernel allows a kernel to queue another, just like EnqueueNDRange on the host.
If you're using opencl 1.2 -- and probably all 1.x, you need to return to the host and call the next kernel (or have the next kernel already queued). You don't need to copy the buffer back to the host between kernels though, so at least you don't pay for transfer multiple times.

Answer 3

As far as I understood from your description, you shouldn't do anything special and it will work even with OpenCL 1.2 just fine.
OpenCL Command queues are IN ORDER by default and there are no need to transfer the data in between the kernel calls.
Just leave the data on the device (don't do map/unmap and Read/Write), enqueue both kernels and wait until they are finished. Here is a code snippet of how it might look:
// Enqueue first kernel
clSetKernelArg(kernel1, 0, sizeof(cl_mem), in);
clSetKernelArg(kernel1, 1, sizeof(cl_mem), out);
clEnqueueNDRange(..., kernel1, ...);
// Enqueue second kernel
clSetKernelArg(kernel2, 0, sizeof(cl_mem), in);
clSetKernelArg(kernel2, 1, sizeof(cl_mem), out);
clEnqueueNDRange(..., kernel2, ...);
// Flush the queue and wait for the results
clFlush(...); // Start the execution
clWait(...); // Wait until all operations in the queue are done
When using OOO (OUT OF ORDER) queues one can use Events (see last 3 params in clEnqueueNDRangeKernel) to specify the dependencies between the kernels and do clWaitForEvents at the end of your pipeline.