OpenCL on ARM (part 1)

Posted: March 29, 2013 in linaro, OpenCL

A few weeks past before Linaro Connect I had started to see what might be available for OpenCL implementations on ARM. Via a little bit of googling it seemed that the only choice would going to be for boards with a Mali 6xx GPU. This was ok since that basically boils down to the Arndale board and the Samsung Chromebook. Both good options since I happened to have a Chromebook.

I downloaded the Mali OpenCL SDK which can be found from their site.

It didn’t take long following the instructions when I realized the SDK isn’t like most SDKs. Not contained within this SDK was any form of Mali OpenCL driver. Within the Mali SDK it contains a lib directory which when you type make (and you probably have to fix their Makefile to make it work) it will yield a libOpenCL.so it’s just that it’s essentially an empty stub library. You can compile and link against what is provided but when you try and run nothing will happen. Within this library is just a long list of functions with no implementation behind it. None. Not very useful.

Via this discussion, at the very bottom we see a bit of an explanation as to why.

We (ARM) do provide a build of Linux containing the OpenCL driver to select partners under a specific license, but this is not public at this time

So they leave it to the maker of the board to at their open distribute a driver. This gives the board maker the option to not support OpenCL at all if they so choose. Ok, I respect that and it makes sense, since just because a Mali T6xx part is on a board doesn’t mean that it’s wired up universally the same way which may require some driver specific change. It’s conjecture on my part since obviously we’ve no view into the source code as it’s not Open Source.

That said, the Insignal discussion can be found on their boards here. Simply put, not yet available for Linux but supposedly available for their Android Jelly Bean.

Hrumph!

I like Android but the problem is at Connect I gave up my Arndale board to one of my coworkers. I haven’t ordered a new one since the wait times are impressively long and currently they are sold out again at HowChip.

Android to my knowledge doesn’t run on the ARM based Samsung Chromebook so I’m out of options.

Next I did a little splunking within the ChromeOS file system on my Chromebook to see if I might find something to suggest that OpenCL was there. I didn’t find libOpenCL.so in any of the usual places so it’s probably safe to say ChromeOS doesn’t make use of OpenCL. No chance of copying over any binaries for use on Linux.

So backing up what other options do I have? Well I do have an OSX option. Putting together an OpenCL HelloWorld there is quite easy. Still.

I’ve a couple Intel Linux boxes, at least it would be a place to get my feet wet in the meantime and be more in line with what OpenCL on ARM linux will be like. So on Ubuntu I proceeded. There are two options. Either Intel’s VCSource or AMD’s APP SDK both proclaiming OpenCL support.

Let’s talk about how the OpenCL infrastructure is installed. First the includes that are best placed at /usr/include/CL. Not needed of course for runtime. Next if you put the contents of each respective SDK’s lib directory into /usr/lib/OpenCL/vendor/  intel or amd then you can have both SDK’s installed at the same time. These are needed at runtime. Next you have /etc/OpenCL/vendors which will have a number of .icd files. You only need one but with multiple SDKs you’ll have more than one.  The ICD is Installable Client Driver. IE This points to the real driver. This is required at runtime. For libOpenCL.so it looks to the icd to specify which driver(s) to use making libOpenCL.so more of a traffic cop between your application that uses libOpenCL.so and the real driver. Next within /etc/ld.so.conf.d you’ll have a new file that points to where the shared libraries are. So in my case these point to /usr/lib/OpenCL/vendors/intel and /usr/lib/OpenCL/vendors/amd in separate files. Last I have symlinks for OpenCL.so OpenCL.so.1 and OpenCL.so.1.2 that all point into the libOpenCL.so implementation I’m using such as the one in /usr/lib/OpenCL/vendors/amd.

AMD’s APP wants to set the environment variable AMDAPPSDKROOT=”/opt/AMDAPP” and does so in /etc/profile.

So knowing these aspects of setup, I proceeded to try out a simple HelloWorld app that would get the list of devices, create a context and spawn off some simple work to validation things are sane.

Let’s talk about how well things work with the Intel and AMD SDKs.

Intel’s SDK for Linux indicates they only support a limited set of CPUs. GPUs are not supported. Neither is the i7 CPU which is what my laptop has. Tried to run. Fail!  Intel’s SDK does not support any of their GPUs. If you want to use their SDK with an i7 for instance you can only do so on Windows! Lame! Further why Intel would have a dep on a very limited set of CPUs is beyond me.

Ok so obviously this wasn’t going to work. Next I switched over to the AMD APP SDK. As it turns out they support OpenCL for just CPUs IE without using a GPUs or for submitting work on both CPUs and GPUs. My laptop and my main intel desktop does not have an ATI GPU so this was essential for me to use the AMD implementation since they only support ATI GPUs, and as it turns out “any” Intel based CPU. Using the AMD supplied HelloWorld OpenCL app, it ran. But.

./HelloWorld
Setting of real/effective user Id to 0/0 failed
FATAL: Module fglrx not found.
Error! Fail to load fglrx kernel module! Maybe you can switch to root user to load kernel module directly
No GPU device available.
Choose CPU as default device.
input string:
GdkknVnqkc
output string:
HelloWorld

fglrx of course is the ATI kernel module. Via OpenCL you can specify that your workload is only going to be directed at CPUs. Even tho you might do so you’ll still get this error every time. Awesome! Least as compared to the Intel offering it runs on any CPU.

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Comments
  1. Jon Bohmann says:

    The SNU-Samsung OpenCL environment does work on the Samsung ARM Chromebook.
    I managed to get it running with a few changes to the out-of-the box configuration.

  2. Jon Bohmann says:

    Here’s what I did to get the OpenCL environment on my Chromebook
    http://chromebookcomputing.blogspot.com/?view=classic

  3. tgallfoo says:

    Thanks for the pointers Jon. That’s a good library as well, tho it only supports CPU cores, not GPUs :-/ It’s most certainly better than nothing tho!

  4. D8 says:

    That error message was a bug and has been fixed in newer versions of the AMDAPP SDK.

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