I have an rPi 400 running Raspbian (Debian GNU/Linux 11.)
If I run clinfo on it, I see 0 platforms.
I have no /etc/vendor/ directory either.
How do I run OpenCL on my rPi400?
There is no OpenCL available for the Raspberry Pi 4.
But there is CLVK which translates the CL kernel into a Vulkan kernel.
The Vulkan driver for rPi4 is called v3dv which can be used for this.
You need an up-to-date version of v3dv, so either build it from source, or make sure your distribution uses one that is not too old. At the time of writing: Raspberry Pi OS (Debian 11) has a Vulkan driver that is too old (missing the VK_KHR_storage_buffer_storage_class extension.) But Ubuntu OS 23.04 has one that will work with v3dv.
Once you have built a libOpenCL.so with CLVK, you can launch a CL application like this:
$ LD_PRELOAD=./libOpenCL.so clinfo
Lastly, I must warn that currently, the v3dv Vulkan Driver lacks a lot of extensions. For instance, it cannot do fp16, nor can it do 8bit integer operations. So not all CL kernels will run on it.