DisplayPort: Under The Hood

July 17, 2023 by Arya Voronova 14 Comments

Last time, we looked at all the things that make DisplayPort unique for its users. What about the things that make it unique for hackers? Let’s get into all the ways that DisplayPort can serve you on your modern tech wrangling adventures.

You Are Watching The AUX Channel

With DisplayPort, the I2C bus we’ve always seen come bundled with VGA, DVI and HDMI, is no more – it’s been replaced by the AUX bus. AUX is a 1 MHz bidirectional diffpair – just a bit too complex for a cheap logic analyzer, though, possibly, something you could wrangle with the RP2040’s PIOs. Hacking thoughts aside, it’s a transparent replacement for I2C, so that software doesn’t have to be rewritten – for instance, it usually does I2C device passthrough over AUX, so that EDID data can still be stored in a separate EEPROM chip on the monitor or eDP LCD panel.

AUX isn’t just a differential bus, it’s more pseudodifferential, like USB2 – for instance, AUX_P and AUX_N are used separately, with a combination of 1 MΩ and 100 kΩ pullups and pulldowns signaling different states of the physical connection – for instance, a pullup on AUX+ and a pulldown on AUX- means that an external device has been connected. If you’d like to learn which combination of resistors means what, you can find in the DisplayPort specification, which isn’t distributed openly but isn’t hard to come by, either.

Also, DisplayPort link training happens over AUX, and in order to facilitate that, a piece of DisplayPort controller’s external memory is usually exposed over the AUX channel, through a mechanism that’s called DPCD. If you dig a bit, using “DPCD” as the keyword, you can easily reach into the lower-level details of your DisplayPort connection. Some of the DPCD memory map is static, and some parts are FIFOs you can funnel data into, or out of. You can find a wide variety of documents online which describe the DPCD structure – for now, here’s a piece of Bash that works on Linux graphics drivers for AMD and Intel, and will show you you the first 16 bytes of DPCD:

# sudo dd if=/dev/drm_dp_aux0 bs=1 skip=256 count=16 |xxd 00000000: 0084 0000 0000 0000 0108 0000 0000 0000 ................ [...]

In particular, the 4th nibble (digit) here describes the amount of lanes for the DisplayPort link established – as you can see, my laptop uses a four-lane link. Also, the /dev/drm_dp_aux0 path might need to be adjusted for your device. In case you ever want to debug your DP link, having direct access to the DPCD memory space like this might help you quite a bit! For now, let’s move onto other practical aspects. Continue reading “DisplayPort: Under The Hood” →

DisplayPort: A Better Video Interface

July 11, 2023 by Arya Voronova 75 Comments

Over the years, we’ve seen a good number of interfaces used for computer monitors, TVs, LCD panels and other all-things-display purposes. We’ve lived through VGA and the large variety of analog interfaces that preceded it, then DVI, HDMI, and at some point, we’ve started getting devices with DisplayPort support. So you might think it’s more of the same. However, I’d like to tell you that you probably should pay more attention to DisplayPort – it’s an interface powerful in a way that we haven’t seen before.

The DisplayPort (shortened as DP) interface was explicitly designed to be a successor to VGA and DVI, originating from the VESA group – an organization created by multiple computer-display-related players in technology space, which has previously brought us a number of smaller-scale computer display standards like EDID, DDC and the well-known VESA mount. Nevertheless, despite the smaller scale of previous standards, DisplayPort has since become a hit in computer display space for a number of reasons, and is more ubiquitous than you might realize.

You could put it this way: DisplayPort has all the capabilities of interfaces like HDMI, but implemented in a better way, without legacy cruft, and with a number of features that take advantage of the DisplayPort’s sturdier architecture. As a result of this, DisplayPort isn’t just in external monitors, but also laptop internal displays, USB-C port display support, docking stations, and Thunderbolt of all flavors. If you own a display-capable docking station for your laptop, be it classic style multi-pin dock or USB-C, DisplayPort is highly likely to be involved, and even your smartphone might just support DisplayPort over USB-C these days. Continue reading “DisplayPort: A Better Video Interface” →

Customized IPad LCD Screen Clips Onto Macbook As A Slick Second Screen

June 7, 2014 by Rick Osgood 20 Comments

Macbook with iPad second screen

Last year, [Ben] found a good deal on iPad 3 LCD screens. He couldn’t resist buying a couple to play around with. It didn’t take him long to figure out that it’s actually quite simple to use these LCD screens with any computer. This is because the LCD panels have built-in Apple Display port interfaces. This means that you can add your own Display Port connector to the end of the LCD’s ribbon connector and just plug it into a computer. You’ll also need to hook up a back light driver, which [Ben] was able to find pre-made for around $35.

The hack doesn’t stop there, though. [Ben] wanted to have a nice, finished product. He laser cut an acrylic bezel for the LCD screen that was a perfect fit. He then milled out a space for the LCD to fit into. The acrylic was thick enough to accommodate the screen and all of the cables. To cover up the back, [Ben] chose to use the side panel of a PowerMac G5 computer case. He chose this mainly for aesthetics. He just couldn’t resist the nice brushed aluminum look with the giant Apple logo. It would be a perfect match to his Macbook.

Once the LCD panel was looking nice, [Ben] still needed a way to securely fasten it in the right place. He knew he’d want it next to his Macbook, so why not attach it directly to the Macbook? [Ben] got to work with his 3D printer and printed up some small plastic clips. The clips are glued to the iPad screen’s acrylic bezel and can be easily clipped on and off of the Macbook screen in seconds. This way his laptop is still portable, but he has the extra screen real estate when he needs it. [Ben] also printed up a plastic clip that turns the iPad’s USB power connector and the Display Port connector into one single connector. While this is obviously not required, it does effectively turn two separate plugs into one and makes the whole project that much more slick.