October 2, 2013

FPGA is for Freedom

by Andrey Filippov

In this post I write about our current development, my first experience with Xilinx Zynq, and also try to summarize the 10+ years experience with Xilinx FPGA devices. It is a mixture of the admiration for their state of the art silicon devices and frustration caused by the software. Please excuse my sometimes harsh words and analogies – I really would like to see Xilinx prosper and acquire software vision that matches the freedom that Ross Freeman brought to developers of the electronic devices when he invented FPGA and started Xilinx.

Before the new camera design started

We planned to update our current line of cameras for some time – Elphel current model NC353 is in production for almost 7 years. Thanks to the Xilinx FPGA, it is possible to upgrade it long after it was built. In 2009 we developed the new system board, built a first unit and started working with it. This board was designed around new (in 2009) Xilinx Spartan 6 and Texas Instruments DaVinci processor. Memory and the CPU performance were increased, the board could support two sensors simultaneously (instead of just one in the older models), but in the 10373 camera system board I was not satisfied with the bandwidth of the datapath between the FPGA and the processor – it was enough for current sensors but in my opinion it did not have enough margin for the future sensor upgrades and we decided to put this project on hold and look for the better match between the CPU and FPGA.

Later we heard the news about the coming Xilinx Zynq devices, but initial rumors indicated that it is very unlikely these chips will be supported by freeware development software. Luckily, that proved to be wrong and Xilinx announced that most of the devices (excluding only the largest XC7Z045) will be supported by the free for download WebPack. Zynq combines dual core ARM CPU (with a rich set of standard peripherals) and high performance FPGA on the same chip, so it should be an exact match for our purposes and intrinsically high bandwidth between CPU and FPGA – parameter that killed our NC373 camera before it was born.

Impressed by Zynq when working on the board design

The news was really exciting, and I was waiting impatiently for the new devices to become available and the free for download status of the required software to be confirmed – many of Elphel customers are developers and we can not force them to acquire software that would be more expensive than the hardware they purchase from us. By June 2013, when I was able to designate time for the full time work on the new project, both conditions were met and I started working on the circuit and PCB design. Zynq features looked very nice and documentation was quite sufficient to work on the design, it turned out to have some little but very convenient bonuses like decoupling capacitors embedded in the package – we use memory mounted on the opposite to the CPU side of the board so it is difficult to have short decoupling connections for both of them. High speed serializer/deserializer capability of virtually all of the I/O pins made it possible to have the dual-function sensor port connectors compatible with our current sensor front ends (SFE) with 12-16 bit parallel interface and capable of running serial links (such as multi-lane MIPI). Backward compatibility will reduce time before we’ll be able to start shipping NC393 cameras (and replace system boards in our Eyesis line of products), high-speed serial capability will allow cameras to keep up with new emerging high-performance sensors.

Initially, I planned to have only 3 sensor ports: one GTX to implement SATA interface, some GPIOs for inter-camera synchronization and interfacing daughter-boards (similar to what we had on our 10369 interface board for the NC353 camera) and dedicated DDR3 memory. Yes, Zynq has really nice access from the PL (programmable logic – FPGA part of the chip) to the system memory, but it is still beneficial to have memory that is not shared with the CPU and has a specialized controller fine-tuned for image processing applications. And I thought I’d need 676-ball package to fit all external devices. But by carefully going through the documentation, I realized that with the flexible I/O banking of Zynq it is possible to fit everything needed in a significantly smaller 484-ball package and to have four (instead of just three) sensor ports.

 A small cloud on the horizon

When working on the circuit design, I needed to make sure that the pins I designate for the DDR3 memory interface are valid – such interface implementation is rather challenging and there are multiple rules that have to be satisfied simultaneously. Even as we do not plan to use Xilinx stock memory controller in the camera, I thought that the software “wizard” that instantiates it in the design may be a good tool to verify the selected pinout – that’s all that I needed at this stage of the design. So I went ahead to install the software. During this process, I learned that to use freeware software (and I already explained why it is the only kind of the non-free software we can use for our products), I have to install mandatory component that transmits data from my computer to Xilinx. It is funny – being a free software/open hardware company, we post all our development files on Sourceforge, but they still prefer to dig in our “dirty laundry”. This was very unpleasant news, and the license agreement stated that, because of the nature of the Internet, they have no responsibility if any of the information they get from my computer will accidentally get to where it was not supposed to get to. OK, I decided, I’ll deal with it later when I’ll really need it to work on the FPGA design; for now, I just need to install it and try the memory controller generator, then after; uninstall the software (hopefully together with the spy agent).

Unfortunately, as it often happens, the “wizard” turned out not to be smart enough, and it told me that the 16-bit wide DDR3 interface I needed will not fit. I did verify the rules stated in the documentation again, searched online information on questions and answers about similar cases – all confirmed that the capable Zynq silicon could handle the job, but the software “wizard” prohibited it. It is quite understandable that software programs have their limitations, but when the software pretending to be “smart” is inflexible, when it (as most of the non-free code) does not allow user to comment out (to disable/bypass) specific checks, it causes frustration. So this software tried to make Zynq look less capable than it actually is, and also tried to convince me that instead of the 484-ball package, I should use larger 676-ball one, leaving less room for other components. Larger package would be more expensive for our customers too, of course.

So I just decided to move on with the circuit/PCB design regardless of my disagreement with the software – this development was described in the several previous blog posts.

By the early August, the PCB design of the Zynq-based camera system board (together with the two companion boards) was finished. I went through all the design again trying to weed out as many design errors as I could, and later that month we released the files into production. While waiting for all the components to come and the PCB to be manufactured, I started to look at the first steps in the software development I will need to be able to verify the board design. I was expecting to take the U-boot files developed for existent Zynq-based evaluation boards and tweak them to match our hardware – a rather straightforward process I did before when breathing in life in other systems. So first make U-boot work, then – proceed with the Linux kernel – both “Linux” and “U-boot” were mentioned in the documentation so I was sure I understand the overall process. I was wrong.

FSBL – a piece of proprietary code generated by the proprietary tools

Of course I understand that it may take another ten years before Xilinx will realize that the combination of the “blank tape” idea of the FPGA that they pioneered with the “totalitarian” style of development tools software is not very efficient – I’ll get to this topic later in the post. At the moment I was just looking for the Open Embedded – based distribution for existent boards that I can modify for our hardware. Internet search revealed that I still have to use proprietary tools to generate the first stage boot loader (FSBL) – piece of code responsible for the hardware initialization. This code is launched by the RBL – embedded in the chip ROM boot loader and in its turn the FSBL (starting from the Zynq OCM – internal on-chip memory) initializes external DRAM, loads and launches U-boot. Then it is the U-boot’s responsibility to take it from there and load and pass control to GNU/Linux (in the sequence that interests us). Starting with U-boot, all the code is Free Software (under mandatory for this software GNU GPL license), but not the FSBL. OK, I thought – I’ll use the tools to generate a binary blob and we’ll distribute it with our cameras. Elphel users will be able to use just the free software to re-build the camera flash image as they want. Binary blobs are nasty, and Richard Stallman would likely refuse to deal with our cameras, but we are living in the real world and so need something to start with – we can try to replace that piece of code later.

What I was not sure about was the legal status of such distribution, at least all the text files generated had Xilinx copyright and “all rights reserved” notices in the header. Funny thing is that they also have “this file is automatically generated” in the same header. To me “generated” sounds more like “created” than “copied” or “compiled” and I did not know that robots are already recognized as authors of the original works covered by the Copyright Law. So I asked this question on Xilinx forum but I was not able to get a clear answer to that question – can we redistribute FSBL custom-generated by Xilinx tools for our hardware?

We did try to generate FSBL with the tools – I failed to install the software on my computer – probably because it had too old of a version of Kubuntu and there was a conflict between the libc6 on my system and the licensing software (this funny make-pretend licensing of freebies). Oleg was luckier than me – he has a current Kubuntu version, but his operating system was still not perfect and did not completely match the development tools. When he tried to re-assign MIO pins in the tools GUI – nothing seemed to happen. Later he discovered that it actually did change; it just did not show the changes. So when he pressed “Save” and opened the same page again, there were the new (modified) values there. A little trick, but it made possible to proceed with the tools.

There are other things that I did not like in the recommended way of the FSBL generation. One is that though I usually prefer a nice GUI to the “black screen” of the command line interface, there are some definite limitations. I like GUI when it saves me from remembering a lot of commands and command options – it could be OK if I had to do my job in a relatively small area. But in a small company, we have to often switch from mechanical design to web development, Verilog code debugging, kernel drivers or image processing – all these activities have their specific tools. But GUI for new board configuration is not that useful according to my personal experience. A standard configuration file with many properly commented settings is more convenient. Configuring a new Zynq-based board for most developers is something they do not need to perform a dozen times a day – once a year is a more reasonable estimate. When you develop a new board you have to go through many manual steps: studying documentation, looking for the board components, and developing a circuit diagram and PCB layout. Going through a long list of settings, reading comments and optionally modifying some values is a very useful process for the new board, as it can help to avoid design errors that would be left unnoticed if you just clicked on several GUI buttons. Adding more configuration parameters to GUI is usually more expensive than just defining more configuration values, so more parameters are likely to be hard-coded in the software and so out of user control. Another problem of the GUI approach – I was concerned I would eventually hit a similar problem I already hit with the smart Memory Interface Generator I described above, the problem that was always a nightmare for me when I had to upgrade the FPGA development tools – new version often refused to compile the code that worked with the old version, changed the rules that are impossible to bypass. And as the code is closed, you do not have many options to tell the software that you are the boss, not it.

Configuring Zynq hardware for a commercial evaluation board with GUI – it may look cool, but the configuration is mostly already defined by the board design, so each board can come with the board-specific long and boring (but nicely commented) configuration file.

 The Ezynq project

Considering all these shortcomings of the use of the FSBL I decided to evaluate feasibility of bypassing this proprietary code completely. According to Xilinx documentation, it seemed possible, and we did not need all of the functionality of the FSBL and the FSBL generation software. We definitely do not need booting of the secret code (Zynq has elaborate hardware and software support for such feature); we also do not need to configure the FPGA portion (PL) until the system is running operating system (FSBL allows early configuration). Our plan was to add extra functionality (previously handled by FSBL) to U-boot itself so all the board configuration is done with #define CONFIG_* statements in the appropriate header files. To prevent conflict between the new parameters and already existent Zynq-related ones in U-boot name scope, we added ‘E’, starting all the parameters with “CONFIG_EZYNQ_” – this is where the project name came from. The project is available in Elphel Git repository at Sourceforge.

For this unexpected project, we purchased a nice small MicroZed evaluation board (it is the first evaluation board I ever used in my career) so we had an official software that boots and runs on this board. Even implementation of the subset of the FSBL functionality, with configuration files ready for only one board, having several known (and probably plenty of unknown) bugs, took me a whole month of programming. In that process I had to go through the documentation on many of the Zynq peripherals and their control registers, DDR3 memory interface – that will likely help me when developing the software for the actual camera. While working on the reimplementation, I was comparing the generated FSBL output against documetation and noticed several mismatches between the two, but none seem to be critical. Our code will need some cleanup – at the beginning I did not know the exact details of what will be needed, and this is my first program in Python, but the program proved to work and we’ll maintain it and use it with future Elphel camera software distributions. I also believe that there are other developers who share my view that the best FPGA silicon on the planet deserves different software, software made for the developers – not just for the cool looking presentations. And we would like other developers to try this code, creating configuration files for the Zynq-based boards they have. There are more technical details in the README file in the git repository and we are always willing to answer questions about this program.

 Why I believe Xilinx will turn towards Free Software

When Ross Freeman, FPGA inventor and one of the Xilinx founders, compared the new device with a “blank tape,” he defined the future of the new class of the devices; devices where the user, and not the chip manufacturer, is in full control. It would be like it was with the magnetic tapes where people could record whatever they liked, and not just what the record companies did. It was especially important in the USSR, where I was born – the most famous and loved by the Soviet people Russian singer, Vladimir Vysotsky, “lived” mostly on the magnetic tapes recorded by people against the will of the Soviet government. Magnetic tapes were the medium that brought us the Beatles – we loved them and perceived them as a “Band of Freedom.”

Freedom is the intrinsic feature of the FPGA. I think it is better than “Field” for the first letter in the acronym. Unfortunately, the analogy with the “blank tape” does not go much farther – in the non-free country, we were free to use any brand of the tape recorder (domestic or brought from abroad) with the same tape. If the Soviet government had the same level of control over the recorders as the FPGA manufacturers have now over the required development tools, we would never be able to listen to Vysotsky or the Beatles.

Some ten years ago, Wim Roelandts, then CEO of Xilinx, had a presentation in Salt Lake City that I attended. When answering questions, he said that more than 98 percent of the company revenue comes from the FPGA (“blank tape”) sales, and less than two percent from the software. Maybe the numbers have changed by now, but I do not think the difference is radical.

I can only guess at what the rationale behind the idea of reducing the value of the main (98 percent) product for the questionable benefit of a two percent byproduct is. They probably can not believe that freedom may be monetized, it increases the value (and the lack of it – decreases) of the underlying product by more than those tiny two percent. They may think that it is irrelevant, and as they produce the best tape in the world, they should use it to the competitive advantage of their tape recorders.

There is the other side of this. Totalitarianism is not competitive in the long run. The USSR was strong in the middle of the 20th century and was able to win against Hitler in WWII. Just 10 years before its collapse, I could not believe that any change would happen in my lifetime – but there is no more USSR now. In the end of the last century (and the beginning of this one), Microsoft was considered the most successful software company, a model for others. And I see some similarity between the two – trying to keep everybody under control – be it with the help of the KGB or EULA. Soviet people did not have private property (only so called “personal property”) – virtually everything belonged to the State. Same with the users of proprietary software – you do not own what you lend money for, you are just granted a temporary right to use it. Microsoft is far from over, of course, but it has seen better times, and few are considering it as a powerful Empire now. Yes, they still dominate on the desktops, but the same day approach failed in the modern areas of the web and mobile devices. In these days you have to give more control to the users – or risk becoming irrelevant. Initially Apple tried hard to prevent “jail-breaking” and not to let people to install their own software. Yes, they sure still have a lot of control, but even they had to yield some of it under the pressure of the users and competitors. It is even more valid for the faster growing Linux-based Android devices.

Xilinx itself is gradually migrating towards Free Software, at least for the code that runs on their devices. I believe this process is welcomed by Xilinx developers (who made a great job in coding Free software submitted to at least Linux kernel and U-boot) but is still not embraced completely by the management who (software-wise) got stuck in the 20th century, when the microsoviet type of the program was a model to follow. But this fight is an uphill battle, and they have to “surrender” more and more. Xilinx SDK is already based on Free Software Eclipse IDE and software components licensed under GNU GPL. I count on this trend and think that it will provide Xilinx with their own experience and prove to them that developing Free Software gives more value in return by expanding application areas and results in increased market share for the devices.

But this shift to Free Software does not yet apply to the main part of the software tools – tools for the FPGA or programmable logic (PL) in terms of Zynq development.

The Xilinx proprietary stronghold that still seems as stable as the USSR in early 1980-s is the FPGA development tools. They do not see much pressure to stop effectively crippling their hardware by the software because 1) Xilinx FPGAs are still the best and 2) Xilinx competitors cripple their products no less than Xilinx does itself. When I first started using reconfigurable FPGA in 2002, I was considering Altera too, but even their freebie software license had to be renewed each 3 months, so there was no guarantee that you’ll always be able to use the code you previously developed.

Competition on the FPGA market is increasing, and in addition to the traditional Xilinx+Altera duopoly, new players are emerging, such as Achronics and Tabula. It seems to me, however, that their bet to beat duopoly is based on the sheer technological advantage of the Intel 14nm process, not on the developer-friendly software that can really make a difference in this field.

Installation of the “spyware” as a mandatory component of the freeware FPGA development tools (in the paid-for versions this functionality may be disabled, but it is on by default) seems to be considered of high value – otherwise they would not risk alienating their loyal customers. Why do they do it? Probably in a desperate move to get more of the real life examples to improve their place and route and other related algorithms. I am not a specialist in these algorithms, but generally they are NP-hard and there are many approaches how to find good-enough solutions and improve. And this involuntary feedback through the spyware is needed to train the algorithms being developed. Translated to USSR analogy, it would be as utopian as to assign 3 KGB agents to every citizen to find out what each of them wants and then decide in some centralized way how to make them all feel happy. Or Apple watching on the customer use of the phones to guess what they need and designing all the apps in-house that are currently available from the independent developers. Proprietary operating systems closed to developers and fully controlled by a single company already proved their inferiority on the mobile devices where they faced a real competition.

Xilinx has a unique opportunity to change this unfortunate state. They develop, produce and sell the Real Things, and Xilinx can become as recognized in FPGA development software, as it is recognized for the FPGA devices now. They are in a position not just to invest heavily in the Free Software infrastructure as IBM and other companies do, but to do much more: jump-start and lead the new class of the FPGA development tools – tools where users are partners, not just the subjects of the surveillance. Starting and maintaining a framework of the Free (not freeware, like WebPack) tools could make a real difference and create value, like independently designed apps create value for Apple or Android gadgets. Just look around – it is the second decade of the 21st century, not the late 20th. Let users (and Xilinx users are really smart developers) get to the controls – they will innovate, and some may find solutions that would never come to the mind of Xilinx staff engineers.

One may say that Xilinx already has an App Store equivalent, but the marketplace for IP cores (“vinyl records” that can be copied to the “magnetic tapes” under certain conditions) is not a substitute for the free and open FPGA development framework – users can exchange (under various free and non-free licenses, with or without compensation) their “tape records” themselves without any Xilinx involvement. In our current design, we too plan to use at least one Verilog module designed by others under GNU GPL license, and we will handle it between us and the developer directly. The other difference is that iPhone users are just phone users and the apps they download increase the functionality (and, in effect, the value) of the phone they purchase. When an FPGA developer uses a core designed by others – she just gets part of her job already done. But the increased functionality of the tools is still needed, and this functionality is usually related to much more elaborate activity than that of the casual phone app user, and FPGA developer is more likely to be able to contribute back. That does not mean, of course, that many developers will contribute new P/R algorithms, but evaluating different algorithms (including experimental ones), tweaking parameters of the goal functions – especially when the default setup can’t make it for the user – this is what many (myself included) can do. It is especially likely to happen if the users are provided with some meaningful comments on the nature of the algorithms and variable parameters.

Such development framework will make it possible for independent researchers to experiment with the new methods of (for example) timing closure, and Xilinx will have different ways to encourage (and in some cases sponsor) such development that will require less investments than when everything critical is done in-house and behind the closed doors.

When implemented, such an approach will provide multiple advantages:

  • Effectively increase the value of Xilinx silicon devices: unleash more of their power and hand it to the users. Such cases as I described above (MIG pushing me to use larger than actually needed package) will be eliminated – in my case I would just troubleshoot the MIG code for my case and submit suggested changes (I’m sure I’m not the only one who needs to use x16 DDR3 with Zynq in 484-ball package). And until the needed changes will be included in the main branch, others who need it will just be able to use my modified version.
  • Reduce the cost of the tools software development and increase its capability and quality by integrating Free Software tools (i.e. Icarus Verilog that we use ourselves for simulation of the products based on Xilinx FPGA) and user contributions. These contributions will be enabled by the open code of the software, and users will be more eager to get involved when they are treated as partners.
  • Improve customer relations. I’m sure that it’s not just me who hates the spyware planted on their computers. And Xilinx surely knows this too, so I consider the current state as a desperate measure to bring in the data that customers are reluctant to provide voluntarily. Treating users as partners (and they really should be partners as improvements of the software tools benefit both parties) is a better way to get the needed feedback (and even contributions, as users can do part of the work themselves) than the current model of interaction. Linux kernel gets on average five patches per hour from thousands of developers (Xilinx included) freely.

Is there a risk that competitors will be able to benefit from this Free Software? Sure they will; as anybody else, they will be able to use it. But they will have to play by the same rules. Even if they will be able to copy all the software and adapt it to their products, keeping the code closed (only possible if the license will be weak enough to allow it), their non-free product will have lower value for the users even if the hardware alone has the same (or even higher) performance.

I am not sure if Xilinx has another decade to stay with the old software paradigm, because as the performance and complexity of the FPGA is increasing, the quality of development software gets more important, and “quality” means the real quality for developers, not only the nice-looking interface. So if there will be some new player on the FPGA field that will be able to offer silicon lagging behind the front runners by some 3-4 years, but offering development environment based on Free Software – that company will definitely have a competitive advantage. If that will happen, I’ll go for the software, but I would definitely prefer to have the best of each – superior Xilinx FPGA devices supported by the developer-friendly, Free Software; the only software that matches the essense of the FPGA idea – its freedom.

3 responses to “FPGA is for Freedom”

  1. […] In this post I write about our current development, my first experience with Xilinx Zynq, and also try to summarize the 10+ years experience with Xilinx FPGA devices. .. from Andrey Filippov  […]

  2. Javier Serrano says:

    Very interesting, thanks! The day Xilinx decide to open up there will be no shortage of developers to make good FOSS synthesis and P&R tools: http://www.ohwr.org/attachments/821/sebastien.pdf 🙂

  3. andrey says:

    Javier, thank you for the interesting link to Sebastien’s presentation. It is not realistic for a single developer to create a complete high-performance suit of the FPGA development tools. But having the whole infrastructure open, it is very likely somebody can come up with a Free Software program on top of it, a tool that can dramatically improve performance of the whole system. It is so much more interesting to create new, than to reverse-engineer and replace missing parts, knowing that they already exist and you just need to “free” them.

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