Luc Verhaegen (libv) wrote,
Luc Verhaegen

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FOSDEMs bespoke video hardware and software.

This is a picture of a very early test setup for the upcoming video boxes for FOSDEM.

Lime2 with test board, capturing 720p

The string of hardware in front of the display (my 4:3 projector analogue, attached over hdmi) is:
* A status LCD, showing the live capture in the background (with 40% opacity)
* An Olimex Lime2 (red board) with our test board on top (green board).
* A adafruit TFP401 hdmi to parallel RGB encoder board (blue board).

You can see the white hdmi cable running from the lime2 hdmi out to the monitor. This old monitor is my test "projector", the fact that it is 4:3 makes it a good test subject.

You can also see a black cable from the capture board to another blue board with a red led. This is a banana-pi M1 as this is the current SBC being used in the FOSDEM video boxes, and i had one lying around anyway, doing nothing. It spews out a test image.

What you are seeing here is live captured data at 1280x720@60Hz, displayed on the monitor, and in the background of the status LCD, with a 1 to 2 frame delay.

Here is a close-up of the status lcd

Video box status lcd, with live capture in the background.

The text and logos in front are just mockups, the text will of course be made dynamic, as otherwise it would not be much of a status display.

And here is a close-up of the 1280x1024 monitor:
Video box status lcd, with live capture in the background.

You will notice 5 16x16 blocks, one for each corner, and one smack-dab in the middle. They encode the position on screen in the B and G components, and the frame count in the R component.

The utility that is running on the lime2 board (fosdem-video-juggler) displays the captured frames on both the LCD and the monitor. This currently tests for all four corners for all 5 blocks.

At 1920x1080, a 24h run showed about 450 pixels being off. Those were all at the start of the frame (pixels 0,0 and 1,0), as this is where our temporary TFP401 board has known issues. This should be fixed when moving to an ADV7611 encoder.

The current setup, and the target resolution for FOSDEM, is 1280x720, (with a very specific modeline for the TFP401). Not a single pixel has been off in an almost 40h run. Not a single one in 8.6 million frames.

The starry-eyed plan

FOSDEM has 28 parallel tracks, all streamed out live. Almost 750 talks on two days, all streamed out live. Nobody does that, period.

What drives this, next to a lot of blood, sweat and tears, are 28 speaker/slides video boxes, and 28 camera video boxes, plus spares. They store the capture, and stream it over the network to be re-encoded on one refurbished laptop per track, which then streams out live. The videobox stream also hits servers, marshalled in for final encoding after devroom manager/speaker review. The livestreams are just a few minutes behind; when devrooms are full, people are watching the livestream just outside the closed off devroom. As a devroom manager, the review experience is pretty slick, with review emails being received minutes after talks are over and videos being done in the next hour or so after that.

The video boxes themselves have an ingenious mix of an hdmi scaler, an active hdmi splitter, a hardware h.264 encoder (don't ask), a banana pi m1 (don't ask), an small SSD, an ethernet switch, and an atx PSU. All contained in big, heavy, laser cut wooden box. It's full of smart choices, and it does what it should do, but it is far from ideal.

There's actual quite a few brilliant bits going on in there. Like the IR LED that controls the hdmi scalers OSD menu. That LED is connected to the headphone jack of the banana pi, and Gerry has hacked up PCM files to be able to set scaling resolutions. Playing audio to step through an OSD menu! Gerry even 3D printed special small brackets, that fit in a usb connector, to hold the IR LED in place. Insane! Brilliant! But a sign that this whole setup is far from ideal.

So last november, during the video team meetup, we mused about a better solution: If we get HDMI and audio captured straight by the Allwinner SoC, then we can drive the projector directly from the HDMI out, and use the on-board h.264 encoder to convert the captured data into a stream that can be sent out over the network. This would give us full control over all aspects of video capture, display and encoding, and the whole thing would be done in OSHW, with full open source software (and mainline support). Plus, the whole thing would be a lot cheaper, a lot smaller, and thus easier to handle, and we would be able to have a ton of hot spares around.

Hashing it out

During the event, in the rare bits of free time, we were able to further discuss this plan. We also had a chat with Tsvetan from Olimex, another big fan of FOSDEM, as to what could be done for producing the resulting hardware, what restrictions there are, how much lead time would be needed etc.

Then, on monday, Egbert Eich, Uwe bonnes and i had our usual "we've been thrown out of our hotels, but still have a few hours to kill before the ICE drives us back"-brunch. When i spoke about the plan, Uwes face lighted up and he started to ask all sorts of questions. Designing and prototyping electronics is his day job at the university of Darmstadt, and he offered to design and even prototype the necessary daughterboard. We were in business!

We decided to stick with the known and trusted Allwinner A20, with its extensive mainline support thanks to the large linux-sunxi community.

For the SBC, we quickly settled on the Olimex Lime2, as it just happened to expose all the pins we needed. Namely, LCD0, CSI1 (Camera Sensor Interface, and most certainly not MIPI-CSI, thanks Allwinner), I2S input and some I2C and interrupt lines. As are most things Olimex, it's fully OSHW. A quick email to Tsvetan from Olimex, and we had ourselves a small pile of Lime2 boards and supporting materials.

After a bit of back and forth, we chose the Analog Devices ADV7611 for a decoder chip, an actual HDMI to parallel RGB + I2S decoder. This is a much more intelligent and powerful converter than the rather dumb (no runtime configuration), and much older, TFP401, which actually is a DVI decoder. As an added bonus, Analog Devices is also a big open source backer, and the ADV7611 has fully open documentation.

Next to FOSDEM, Uwe also makes a point of visiting Embedded World here in Nuernberg. So just 2 weeks after FOSDEM, Uwe and I were able to do our first bit of napkin engineering in a local pub here :)

Then, another stroke of luck... While googling for more information about the ADV7611, i stumbled over the Videobrick project. Back in 2014/2015, when i was not paying too much attention to linux-sunxi, Georg Ottinger and Georg Lippitsch had the same idea, and reached the same conclusions (olimex lime1, adv7611). We got in contact, and Georg (Ottinger ;)) even sent us his test board. They had a working test board, but could not get the timing right, and tried to work around it with a hw hack. Real life also kicked in, and the project ended up losing steam. We will re-use their schematic as the basis for our work, as they also made that OSHW, so their work will definitely live on :)

In late April, FOSDEM core organizers Gerry and Mark, and Uwe and I met up in Frankfurt, and we hashed out some further details. Now the plan included an intermediate board, which connects to the rudimentary TFP401 module, so we could get some software and driver work started, and verify the functionality of some hw blocks.

Back then, i was still worried about the bandwidth limitations of the Allwinner CSI1 engine and the parallel RGB bus. If we could not reliably get 1280x720@60Hz, with a reasonable colour space, then the A20 and probably the whole project was not viable.

720p only?

Mentioning 1280x720@60Hz probably got you worried now. Why not full HD? Well, 1280x720 is plenty for our use-case.

Speakers are liable to make very poor slide design choices. Sometimes slides are totally not suited for display on projectors, and text often can not be read across the room. FOSDEM also does not have the budget to rent top of the line projection equipment for every track, we have to use what is present at the university. So 720p already gives speakers far too many pixels to play with.

Then, the sony cameras that FOSDEM rents (FOSDEM owns one itself), for filming the speaker from across the room, also do 1280x720, but at 30/25Hz. So 720p is what we need.

Having said that, we have in the meantime confirmed that the A20 can reliably handle full HD at 60Hz at full 24bit colour. With one gotcha... This is starting to eat into our available bus and dram bandwidth, and the bursty nature of both capture and the scaler on each display pipe make display glitchy with the current memory/bus/engine timing. So while slides will be 720p, we can easily support FullHD for the camera as there is no projector attached there, or if we are not scaling.

Intermediate setup

Shortly after our Frankfurt meetup, Uwe designed and made the test boards. As said, this test board and the TFP401 module is just temporary hardware to enable us to get the capture and KMS drivers in order, and to get started on h.264 encoding. The h.264 encoding was reverse engineered as part of the cedrus project (Jens Kuske, Manuel Braga, Andreas Baierl) but no driver is present for this yet, so there's quite a bit of work there still.

The TFP401 is pretty picky when it comes to supported modes. It often messes up the start of a frame; a fact that i first noticed on a Olimex 10" LCD that i connected to it to figure out signal routing. The fact that our capture engine does not know about Display Enable also does not make things any better. You can deduce from the heatsink in the picture that that TFP401 also does become warm. All that does not matter for the final solution, as we will use the ADV7611 anyway.

Current status

We currently have the banana pi preloading a specific EDID block with a TFP401 specific modeline. And then we have our CSI1 driver preset its values to work with the TFP401 correctly (we will get that info from the ADV7611 in future). With those settings, we do have CSI capture running reliably, and we are showing it reliably on both the status LCD and the "projector". Not a single pixel has been lost in a ~40h run, with about 21.6TB of raw pixel data transferred, or 8.6 million frames (i then lost 3 frames, as i logged into to bpi and apt-get upgraded, wait and see how we work around load issues and KMS in future).

The allwinner A20 capture engine has parallel RGB (full 24bit mind you), a pixelclock, and two sync signals. It does not have a display enable line, and needs to be told how many clocks behind sync the data starts. This is the source of confusion for the videobrick guys, and forces us to work around the TFP401s limitations.

Another quirk of the design of the capture engine is that it uses one FIFO per channel, and each FIFO outputs to a separate memory address, with no interleaving possible. This makes our full 24bit capture a very unique planar RGB888, or R8G8B8 as i have been calling it. This is not as big an issue as you would think. Any colour conversion engine (we have 2 per display pipe, and at least one in the 2d engine) can do a planar to packed conversion for all of the planar YUV formats out there. The trick is to use an identity matrix for the actual colour conversion, so that the subpixels values go through unchanged.

But it was no surprise that neither V4L2 nor DRM/KMS currently know about R8B8G8 though :)

What you see working above already took all sorts of fixes all over the sunxi mainline code. I have collected about 80 patches against the 5.2 mainline kernel. Some are very board specfic, some are new functionality (CSI1, sprites as kms planes), but there are also some fixes to DRM/KMS, and some rather fundamental improvements to dual display support in the sun4i kms driver (like not driving our poor LCD at 154Hz). A handful of those are ready for immediate submission to mainline, and the rest will be sent upstream in time.

I've also learned that "atomic KMS" is not really atomic, and instead is "batched" (oh, if only i had been allowed to follow my nokia team to intel back in 2011). This forces our userspace application to be a highly threaded affair.

The fundamental premise of our userspace application is that we want to keep all buffers in flight. At no point should raw buffers be duplicated in memory, let alone hit storage. Only the h.264 encoded stream should be sent to disk and network. This explains the name of this userspace application; "juggler". We also never want to touch pixel data with a CPU core, or even the GPU, we want the existing specialized hardware blocks to take care of all that (display, 2d engine, h.264 encoder), as they can do so most efficiently.

This juggler currently captures a frame through V4l2, then display that buffer on the "projector" and on the status LCD, as overlays or "planes", at the earliest possible time. It then also tests the captured image for those 5 marker blocks, which is how we know that we are pixel perfect.

There is a one to two frame delay between hdmi-input and hdmi-output. One frame as we need to wait for the capture engine to complete the capture of a frame, and up to one frame until the target display is done scanning out the previous frame. While we could start guessing where the capture engine is in a frame, and could tie that to the line counter in the display engine, it is just not worth it. 1-2 frames, or a 16.67-33.33ms delay is more than good enough for our purposes. The delay on the h.264 stream is only constrained by the amount of RAM we have, as we do want to release buffers back to the capture engine at one point ;)

Current TODO list.

Since we have full r8g8b8 input, i will have to push said buffer through the 2d engine (2d engines are easy, just like overlays), so that we can feed NV12 to the h.264 encoder. This work will be started now.

I also still need to go write up hdmi hotplug, both on the input (there's an interrupt for that) as on the output side.

The status text on the status LCD is currently a static png, as it is just a mockup. This too will become dynamic. The logo will become configurable. This, and other non-driver work, can hopefully be done by some of the others involved with FOSDEM video.

Once Uwe is back from vacation, he will start work on the first version of the full ADV7611 daughterboard. Once we have that prototyped, we need to go tie the ADV7611 into the CSI1 driver, and provide a solid pair of EDID blocks, and we need to tie I2S captured audio into the h.264 stream as well.

Lots of highly important work to be done in the next 4 months.

Where is this going?

We should actually not get ahead of ourselves too much. We still need to get close to not losing a single pixel at FOSDEM 2020. That's already a pretty heady goal.

Then, for FOSDEM2021, we will work towards XLR audio in and output, and on only needing a single ethernet cable routed across a room (syncing up audio between camera and speaker boxes, instead of centralizing audio at the camera). We will add a 2.54mm pitch connector on top of the current hdmi capture daughterboard to support that audio daughterboard.

Beyond that, we might want to rework this multi-board affair to make an all-in-one board with the gigabit ethernet phy replaced with a switch chip, for a complete, single board OSHW solution. If we then tie in a small li-ion battery, we might be able to live without power for an hour or so. One could even imagine overflow rooms, where the stream of one video box is shown on another video box, all using the same hw and infrastructure.

All this for just FOSDEM?

There seems to be a lot of interest from several open source and hacker conferences. The core FOSDEM organizers are in contact with CCC and debconf over voctomix, and they also went to FOSSAsia and Froscon. Two other crucial volunteers, Vasil and Marian, are core organizers of Sofia's openFest and Plovdivs tuxcon, and they use the existing video boxes already.

So unless we hit a wall somewhere, this hardware, with the associated driver support, userspace application, and all the rest of the FOSDEM video infrastructure will likely hit an open source or hacker conference near you in the next few years.

Each conference is different though, and this is where complete openness, both on the hardware side, and on all of the software solutions needed for this, comes in. The hope is that the cheap hardware will just be present with the organization of each conference, and that just some configuration needs to happen for this specific conference to have reliable capture and streaming, with minimal effort.

Another possible use of this hardware, and especially the associated driver support, is, is of course a generic streaming solution, which is open source and dependable and configurable. It is much smarter to work with this hardware than to try to reverse engineer the IT9919 style devices for instance (usually known as HDMI extenders).

Then, with the openness of the ADV7611, and the more extensive debugfs support that i fear it is bound to grow soon, this hardware will also allow for automated display driver testing. Well, at least up to 1080p60Hz HDMI 1.4a. There actually was a talk in my devroom about this a while ago (which talked about one of those hdmi extenders).

So what's the budget for this?


FOSDEM is free to attend, and has a tiny budget. The biggest free/open source software conference on the planet, with their mad streaming infrastructure, spends less per visitor than said visitor will spend on public transportation in brussels over the weekend. Yes, if you take a cab from the city center to the venue, once, you will already have spent way more than the budget that FOSDEM has allocated for you.

FOSDEM is a full volunteer organization, nobody is paid to work on FOSDEM, not even the core organizers. Some of their budget comes from corporate sponsors (with a limit on what they can spend), visitors donating at the event, and part of the revenue of the beer event. This is what makes it possible for FOSDEM to exist as it does, independent of large corporate sponsors who then would want a controlling stake. This is what makes FOSDEM as grass roots and real as it is. This is why FOSDEM is that great, and also, this is why FOSDEM has endured.

The flip side of that is that crucial work like this cannot be directly supported. And neither should it be, as such a thing would quickly ruin FOSDEM, as then politics takes hold and bickering over money starts. I do get to invoice the hardware i needed to buy for this, but between Olimex' support, and the limited bits that you see there, that's not exactly breaking the bank ;)

With the brutal timeline we have though, it is near impossible for me, a self-employed consultant, to take in paid work for the next few months. Even though recruiting season is upon us again, i cannot realistically accept work at this time, not without killing this project in its tracks. If i do not put in the time now, this whole thing is just not going to happen. We've now gone deep enough into this rabbit hole that there is no way back, but there is a lot of work to be done still.

Over the past few months I have had to field a few questions about this undertaking. Statements like "why don't you turn this into a product you can sell" are very common. And my answer is always the same; Neither selling the hardware, nor selling the service, nor selling the whole solution is a viable business. Even if we did not do OSHW, the hardware is easily duplicated, so there is no chance for any real revenue let alone margin there. Selling a complete solution or trying to run it as a service, that quickly devolves into a team of people travelling the globe for two times 4 months a year (conference season). That means carting people and hardware around the globe, dealing with customs, fixing and working around endless network issues, adjusting to individual conferences needs as to what the graphics on the streams should look like, then dealing with stream post production for weeks after each event. That's not going to help anyone or anything either.

The only way for this to work is for the video capture hardware, the driver support, the "juggler", and the associated streaming infrastructure to be completely open.

We will only do a limited run of this hardware, to cover our own needs, but the hardware is and will remain OSHW, and anyone should be able to pay a company like Olimex to make another batch. The software is all GPLv2 (u-boot/kernel) and GPLv3 (juggler). This will simply become a tool that conference organizers can deploy easily and adjust as needed, but this also makes conferences responsible for their own captures.

This does mean that the time spent on writing and fixing drivers, writing the juggler tool, and designing the hw, all cannot be directly remunerated in a reasonable fashion.

Having said that, if you wish to support this work, i will happily accept paypal donations (at libv at skynet dot be). If you have a VAT number, and wish to donate a larger amount, i can provide proper invoices for my time. Just throw me an email!

This would not be a direct donation to FOSDEM though, and i am not a core FOSDEM organizer, nor do i speak for the FOSDEM organization. I am just a volunteer who has been writing graphics drivers for the past 16 years, who is, first and foremost, a devroom manager (since 2006). I am "simply" the guy who provides a solution to this problem and who makes this new hardware work. You will however be indirectly helping FOSDEM, and probably every other free/open source/hacked conference out there.

Lot's of work ahead though. I will keep you posted when we hit our next milestones, hopefully with a less humongous blog entry :)
Tags: adv7611, allwinner, capture, fosdem, h264, hdmi, lime2, olimex, sunxi, video

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