[kwlug-disc] HDMI over USB 3.0 story (may be useful to you)

[Mikalai Birukou]

>> Eurika moment came with actual reading about usb 3.0 -- it can be 
>> used for video.
>
> Uh-oh, you've triggered a rant!

This is enlightening. Thank you for sharing. Wow. I guess, up to this 
moment I've had this impression, quote, "if it fits, it works". OK. Post 
2010 USB: beware! :)

>
> Video over USB-C is a minefield of complexity. When I bought my 
> current laptop I had to research this because I just wanted a dock 
> like I had on my old laptop. I don't know how a non-technical person 
> is supposed to figure out what kind of dock or cable they're supposed 
> to buy for their laptop.
>
> There's multiple options:
>
> DisplayLink:
>
>   This is video over usb.
>
>   You can use USB-A, or USB-C, and there are devices that are USB 2.0 
>   and 3.0+. They're basically software displays. They work by using a 
>   driver to make "software" second screen, and sending that 
> framebuffer   over USB to the dongle that is actually driving the 
> monitor.
>
>   The older ones could introduce latency, and resolution/refresh rate 
>   limitations. Certain types of video may suffer from compression. You 
>   probably won't notice for most desktop use, but may cause problems   
> with games, for example. That said, I'm out of touch with them, and I 
>   think they've improved these since I last looked at them years ago.
>
>   They should work pretty much the same on any hardware, assuming the 
>   software supports it (has drivers, etc). I know a few people that 
> use   these regularly with no complaints.
>
>   Years ago they had issues with Linux compatibility, but I think 
> things   are settled down now.
>   FYI: I've never used one
>
> DP-Alt Mode (usb-c):
>
>   This is video over a usb-c port, but it is *not* video over usb.
>
>   The usb-c spec divides a usb-c cable into lanes, and lets some or 
> all   lanes be configured to an "alternate" signal, such as 
> displayport.   Benefits over the above is that this is a "real" 
> displayport output   driven directly by your GPU, just like a physical 
> displayport port   would be -- no additional drivers or framebuffer 
> copies.
>
> https://en.wikipedia.org/wiki/USB-C#Alternate_Mode_partner_specifications
>
>   That also means this requires your laptop manufacturer to have 
> actually plumbed your GPU to your USB-C ports, and a lot of devices   
> don't. My Lenovo Thinkpad T14 does, but my wife's Lenovo Flex with   
> similar cpu/etc doesn't (it also doesn't charge over usb-c, because   
> Lenovo hates people that buy their consumer hardware, apparently)
>
>   Resolution/refresh rate options are difficult to understand. Is your 
>   laptop displayport 1.2? 1.4? 2.0? Which displayport version is your 
>   dongle? Are you using a quality cable? Are you using all lanes for   
> displayport, or are you splitting the cable between DP-alt mode and   
> normal usb data?
>
>   Sometimes you need to dive into spec sheets to determine if a laptop 
>   even supports this.
>
>   Also, all the specs only talk about resolutions -- I have a 
> 3440x1440   monitor, but it supports 144Hz. Will a dongle that 
> supports "4K"   3840x2160 at 60 Hz support that? Time to get the 
> calculator out...
>
>   FYI: I use this method every day. (It supports 3440x1440 at 85Hz, btw.  
>   But not 144)
>
> HDMI-Alt Mode (usb-c):
>
>   It doesn't exist. Sure, the wiki says it exists, but you'll never 
> see   it.
>
>   USB-C-to-HDMI adapters are actually DP-Alt Mode dongles with an HDMI 
>   converter chip.
>
> Thunderbolt:
>
>   This is also video over a usb-c port, but it is also *not* video 
> over   usb.
>
>   Thunderbolt is another "Alt" mode (that then can *also* carry 
> displayport), but with higher bandwidth and some increased 
> capabilities, and most importantly: branding and a logo on the port so 
>   you know you have it! And laptops advertise that it exists! You 
> don't   need to find a spec sheet!
>
>   But this requires special hardware for thunderbolt, and it's mostly 
>   Intel-specific with very few exceptions (at least until USB4 
> actually   makes it to our desks).
>
>   Thunderbolt does offer significantly broader capabilities -- you can 
>   attach a whole external GPU via thunderbolt if you want, and run   
> monitors off that.
>
>   Generally thunderbolt docks have been less fuss from what I understand.
>
>   (FYI: I've used thunderbolt before, but don't own a 
> thunderbolt-capable machine)
>
> So there's multiple ways to get video "over USB", but only one of 
> those is actually "over USB".
>
> Here's why this all really sucks: I'm on Lenovo's site and want to buy 
> a dock for my Lenovo laptop. This should be simple, right? Which of 
> these visually identical and similarly named docks do I buy:
>
>  * ThinkPad Thunderbolt 3 Workstation Dock Gen 2
>
> https://www.lenovo.com/us/en/p/accessories-and-software/docking/docking_thunderbolt-docks-(universal-cable-docks)/40any230us
>
>  * ThinkPad USB-C Dock Gen 2
>
> https://www.lenovo.com/us/en/p/accessories-and-software/docking/docking_usb-docks-(universal-cable-docks)/40as0090us
>
>  * ThinkPad Hybrid USB-C
>
> https://www.lenovo.com/us/en/p/accessories-and-software/docking/docking_usb-docks-(universal-cable-docks)/40af0135us
>
>
> If I have a thunderbolt laptop, I buy a thunderbolt dock, and all 
> should be good. However, I don't have thunderbolt. So...
>
> If I have a laptop that supports DP-alt-mode (like my Lenovo T14), the 
> latter two will both work, but the "USB-C Dock Gen 2" is a better dock 
> considering my hardware capabilities. It will have a native output 
> driven by my AMD GPU. (This is also the dock I use).
>
> If my laptop doesn't support alt-mode (like my wife's Lenovo Flex), 
> the first two docks will never give me video out (but everything else 
> will probably work).
>
> As a consumer, am I supposed to know that my T14 has alt-mode? Am I 
> supposed to know my Flex 14 doesn't? They both have the same Ryzen CPU 
> & GPU. I only found out after researching why my dock didn't work with 
> my wife's laptop.
>
> Last year my parents were looking at a USB-C dock in a store, and I 
> was on the phone trying to figure out what specific laptop they have, 
> and what specific dock they're looking at, to figure out if this dock 
> will work with it. At least Costco has hassle-free returns (yay e-waste).
>
> Part of the problem is "usb-c" is a port that can be used for many 
> things -- power, displayport, audio, thunderbolt, usb... Only one of 
> those is *actually* usb, despite the name of the port. The rest of 
> those are port-specific features that have nothing to do with usb, and 
> vary from device to device. You can't use a usb-a adapter and have 
> them work.  You can't assume a USB-C charger will charge a laptop with 
> USB-C ports.  You can't even assume that the usb-c headphone adapter 
> for my old phone will work with my new one.
>
> This is totally counter to old usb "if it fits, it works", even if I'm 
> plugging a usb-3.0 HDD into a usb-2.0 computer, or a 1.1 keyboard into 
> a 3.0 port. But I'm going to try to limit myself to one novel-length 
> rant per day... :)
>
>> Attempt 1 was using usb-A to usb-C adapter and trying hdmi thing that 
>> works with PinePhone (DisplayPort over usb something). But system 
>> didn't recognize it.
>
> Pinephone specs state it supports Displayport Alt Mode, and the dongle 
> uses that. So you can't adapt that to USB-A because it's actually a 
> usb-c port feature, not a usb feature.
>
> I just grabbed my Pinephone dongle to confirm, and it works on my 
> laptop, using DP-alt mode, because my laptop is plumbed for that in 
> hardware.
>
> And to touch the point above about HDMI alt-mode dongles, the 
> Pinephone dongle uses DP-Alt mode, and has a Chrontel CH7210 to 
> convert that to HDMI. Because HDMI alt-mode doesn't exist, even in a 
> device that ships with an HDMI-only dongle.