Okay, I've completely gone down the rabbit hole of the Switch DisplayPort lanes mystery today, so I figure I may as well delve as deep as I can. For anyone who's reading this trying to make sense of my madness, please read
this post, the second part of
this post and then
this post.
In the last post, I posited that, although the TX1 SoC supports four lanes of DisplayPort 1.2a, and the
PI3USB30532 USB-C crossbar switch used in the original Switch supports sending four lanes of DisplayPort 1.2 over USB-C, I didn't know if all four lanes were actually wired up. I've subsequently realised that with an image of the Switch's motherboard and a pinout for the PI3USB30532, I can actually answer this question myself. A quick search showed me that someone had very helpfully posted
high-res photos of a desoldered Switch motherboard online, and combining this with a pinout diagram of the crossbar switch I can definitively say that the original Switch only has DisplayPort lanes DP0 and DP1 wired up between the SoC and the crossbar switch, and lanes DP2 and DP3 are unconnected.
That is, although the SoC and USB-C port on the original Switch both supported the four lanes of DisplayPort data necessary for 4K output, it's not possible for it to actually output 4K, as only two of the lanes are wired up. This is an entirely sensible decision considering Nintendo never intended to support 4K, and the original dock wouldn't have been able to output a 4K signal anyway. Wiring those lanes up would have just added needless complexity to the motherboard.
So, how about the "external_display_full_dp_lanes" configuration flag which suggests that Aula (aka the OLED Switch) is the only model which can use all four DisplayPort lanes supported by TX1/Mariko? Well, the helpful person who posted the desoldered o.g. Switch board photos is even more helpful than I thought, as they also posted
desoldered OLED model board photos! Shoutout to KreativDax if they ever see this!
We can do the exact same thing with the OLED model's motherboard, which also uses the PI3USB30532 USB-C crossbar switch, and you can quite clearly see
all four DisplayPort lanes wired up between the Mariko SoC and the USB-C crossbar Switch. That is, from a hardware point of view, everything in the Switch OLED model seems to be wired up to support output of 4K resolution video over the USB-C port. We've also known since it released that the upgraded dock that comes with the OLED model supports HDMI 2.0 output, which means the entire Switch OLED system appears to be 4K capable, seemingly just without software support.
Coming back to the Switch OS info, the "external_display_full_dp_lanes" flag indicates what I expected; that Aula (Switch OLED) is the only model physically capable of using all four DisplayPort lanes for external displays. The others either only have two lanes wired up (o.g. Switch) or have no external display support at all (Switch Lite). Having a flag in the OS for this is useful if you want to be able to output something over those lanes (ie 4K video), as software will need to know if its running on hardware that can do so or not. This flag was added in OS update 10.0, which I believe is the first update that added any references to Aula.
The other relevant Switch OS item is the "4kdp_preferred_over_usb30" that I originally posted about. From the name, we can infer that this toggles between using all four lanes of DisplayPort to send a 4K signal, or using only two DisplayPort lanes, and sending USB 3 data at the same time. This was added in OS update 12.0, which was also when the first references to CrdA (ie Cradle Aula, the Switch OLED dock) were added. This toggle makes little sense if the Switch model in question doesn't have four lanes of DisplayPort wired up to the USB-C port, which we've found out is the case for the original model. However, it makes a lot of sense for a model where you've wired up all four DP lanes (Aula), and when the OS now supports a dock which can take a 4K signal and output it to a TV (CrdA).
For quite a while I've thought that the use of HDMI 2.0-capable hardware in the OLED dock was simply a matter of it being cheaper or easier to source than the chip they were originally using, and the HDMI 2.0 support was incidental. I'm not so sure about that now, though. It was plausible that the apparent 4K support in the dock was incidental, but I don't think it's plausible that the 4K support they added to Switch OLED itself is incidental. The only plausible reason to wire up all four lanes of the SoC's DisplayPort to the USB-C port is if you intend to output resolutions higher than 2 lanes can handle. For standard TV resolutions, that means 4K. They chose not to wire these up on the original Switch, and as I mentioned above, that's a sensible decision if you don't intend on supporting 4K. Going from only wiring up two lanes on the OG model to wiring up all four lanes on the OLED model is a clear indication that they intended to use those two additional lanes for something. On top of this they added software into Switch's OS related to the use of those four lanes when adding Aula support, with one setting specifically relating to 4K output.
The more I look at it, the more I think that the OLED model
is the "4K Switch", with Nintendo dropping 4K output and higher clocks late enough in development that the motherboard had already been finalised and much of the OS support had already been added. The hardware seems to have everything necessary to output at 4K, and at least some of the software support seems to have been in place. It explains why we were hearing about developers having dev kits for a 4K capable Switch all the way back in 2020, and why as many as 11 developers were willing to talk about it in 2021, as the release would have been relatively imminent, meaning many dev kits would have been out there. It also explains why developers would report that a 4K Switch was cancelled, because functionally the "4K Switch" was cancelled from a game development point of view, even if the hardware ended up in people's hands as the OLED model. Furthermore, it explains why Nintendo, who don't usually comment on rumours, explicitly stated that no third parties had 4K Switch dev kits, because by the time they said that they'd already dropped the plans and recalled the dev kits (or even just disabled 4K output via a software update).
TLDR: The original Switch wasn't physically capable of outputting 4K via its USB-C port, but Nintendo made specific changes to the motherboard of the OLED model to support 4K output. We already know that the OLED dock was also upgraded with hardware capable of 4K output. Software changes relating to 4K output were also added to the OS alongside support for this hardware. Therefore, I believe that the Switch OLED model
is the elusive Switch Pro/Switch 4K, and that they only decided to drop 4K support and higher clocks at a late stage after the hardware had been finalised.
Edit: As a final clarifying point, although I'm talking about the technical capability to output 4K video in this post, I don't think the device was actually intended to render games at or near 4K resolution. The hardware would have been the same Mariko chip we got in the OLED model but with higher clocks, meaning noticeable but not massive performance improvements at best. I suspect that the 4K output was a secondary benefit, perhaps for a small number of games or for video streaming, like the Xbox One S. The public perception of it as a "Switch 4K" was likely due to the press focussing on the 4K output as an easy thing for non-technical audiences to latch onto, even if it was never a focus for Nintendo.