Linux @lemmy.ml szczuroarturo @programming.dev 7 mo. ago

why cant we connect 2 computers using USB

So i tried to connect steamdeck to pc using usb and i read its immpossible because steamdeck is a computer and some explanation on quora about strong master slave relationship. But then why is it possible for android phones to connect to pc whilist also having the ability to use USB and other usb c accesories. Also why cant it be a toggle? So that you can change whetewer this usb( or device ) is reciving data ot sending data .

54 comments

We can connect two computers over USB and we do it all the time. E.g. your phone and your PC. One or both of the computers needs to be able to act as a device. Most Android computers can switch between host and device depending on what's plugged in. It all comes down to implementation.
The USB protocol was simple by design, so it could be implemented in small dumb devices like pen drives. More specifically, it used two couples of cables, one couple was for power and the other for data (four wires in total). Having a single half-duplex data line means you need some way of arbitrating who can send data at any time. The easiest way to do it is having a single machine that decides who gets to send data (master), and the easiest way to decide the master is to not do it and have the computer always do the master. This means you couldn't connect two computers together because they would both try to be the master.

I used the past tense because you may have noticed that micro USB have 5 pins and not 4, that's because phones are computers and they use the 5th pin to decide how to behave. If it's grounded they act as a slave (the male micro to male A cable grounds it). If it has a resistor (the otg cable has it) it act as master. And if the devices are connected with a wire on that pin (on some special micro to micro) they negotiate the connection.

When they made usb 3.0 and they realized that not having the 5th wire on the usb-A was stupid, so they put it (along side some extra data lines) that's why they have an odd number of wires. So with usb 3 you can connect computers together, but you need a special cable that uses the negotiation wire. Also I don't know what software you need for it to work.

Usb-c is basically two USB 3.0 in the same cable, so you can probably connect computers with that. But often the port on the devices only uses one, so it might not be faster. Originally they put the pins for two connections so you could flip the connector, but later they realized they could use them to get double speed.
- And it was a good design - it's universal (aha) adoption proves that.
  
  Those of us old enough to remember the pain of using 9 and 25 pin serial leads and having to manually set baud rate and protocols, along with LPT and external SCSI and manufacturer specific sockets probably agree this was a problem that needed solving, and USB did do that.
- Thanks for the elaborate eli5
- Great read. Thanks
You actually can connect a Steam Deck to another PC. Look for the DeckMTP plugin on Decky Loader.

MTP is the thing Android uses when you connect your phone to a PC. It should be possible to install on any PC. Don't know about Windows but there's probably a solution for that as well.

Don't know what happens when both devices have it. I'd suspect that they would both be able to access each other's drive.
- MTP totally sucks though, it is a pain in the a**.
  
  Would rather use Syncthing or localsend
  
  Yeah, that's why I don't have it installed. Sftp is more than enough. But that's not what OP was asking.
The reason you can't is "because Intel deliberately designed it that way". Back when USB was just a notion, PDAs were a really cool thing. There was apparently concern at Intel that someday these little things might be all that someone might own. You might connect your PDA directly to the printer, rather than syncing it to your Intel Desktop and printing from there. You might connect your PDA to the modem and collect electronic mailographs directly, instead of syncing with a PC. If you could do enough without the PC middleman, you might even skip on buying an Intel computer altogether.

So, Intel baked into the protocol anything they could think of to make peer-to-peer communications impossible in USB, make life easy for the singular PC communications master, and put a timing onus on devices that forced them to be dumbed-down state machines instead of computers in their own right.
- Is there anything to support this? I couldn't find anything that really has this intend documented and Intel weren't the only on pushing for usb as the most simple protocol possible ( I recall a lot of excitement about the "u" part.. How naive at least I was back then!).
  
  I'm not knowledgeable enough to really argue against it, looking simply from an Okham point of view as "they wanted everything to connect" - the printer in the same way as that PDA.... Plus Intels de facto (IT) world domination at the time it just seems unlikely.
  
  Edit: some sentences didn't make even less sense, fixed.
I found some directions that might help.

Enabling USB-C OTG Device Mode :
Ensure the Linux device has a USB-C port that supports OTG functionality.
In the device tree, set the dr_mode property of the USB OTG controller to "peripheral" or "otg" to enable device mode.
Configure the TUSB320 USB-C controller (or equivalent) to operate in UFP (Upstream Facing Port) mode, which allows the device to act as a USB peripheral.
Configuring USB Gadget Drivers :
Load the appropriate Linux USB gadget driver for the desired functionality, such as g_ether for Ethernet over USB, g_serial for a serial device, etc.
Manually configure the USB network interface, such as assigning an IP address to usb0.
Connecting to a Host :
Use a USB-C to USB-C or USB-C to USB-A cable to connect the Linux device in OTG device mode to a host PC.
The host PC should then detect the Linux device as a USB peripheral, allowing file transfer, network connectivity, or other functionality depending on the configured gadget driver.

Gateworks.com Wiki Linux OTG
Kernel.org Driver-API USB Gadget
Collabora Blog Modern Linux USB Gadget integration with Systemd Part1

A tool : gt
Rust library : usb-gadget
C Library : libusbgx
This might help ya:

https://github.com/dafta/DeckMTP
- Not sure if that'll work for SteamDeck-to-LinuxPC connection, but I'm certain that works for SteamDeck-to-Android.
  It is using USB gadgets, so it's worth a shot at least.
  
  I've used it about 2 months ago with Linux-to-Deck, back then it worked flawlessly. I don't have my deck on hand rn, but I will test it when I get back.
Google "connecting 2 computers with USB"

If you have two PCs with USB ports, you can connect them to one another using a special type of USB cable called a “bridging” cable. You can also technically connect two Macs via USB, but you'll need to add a USB-to-Ethernet adapter and Ethernet cable to the mix.

There are also articles about it:
https://hardwaresecrets.com/connecting-two-pcs-using-a-usb-usb-cable/

We do it routinely with our phones (some of us), I admit IDK why it can't be just as easy between 2 computers?
- Ethernet had this figured out almost 30 years ago with auto-negotiation. Last crossover cable I ever used was in 2004 for a customer's old hub they didn't want to replace. Yes, "hub", not "switch".
  
  Not a fair comparison IMHO - Ethernet is designed to be a connection between two or more otherwise independent peers (on L2), while USB's goal was to allow connecting simple peripheral devices to computers. There was never meant to be a situation where it's unclear which side is the Host.
  
  Also note that the bridging "cable" is literally just two USB devices, one for each computer (although they are both on the same chip, so there's that), with some internal link to pass the data.
- Permanently Deleted
- Has this changed with USB-C?
  
  Possibly. But from my research it seems to really depend.
  
  the USB-C ports on the two PCs need to support USB OTG (On-The-Go) functionality, which allows the ports to dynamically switch between host and device modes. This is what enables the direct PC-to-PC communication over the USB-C connection.
With usb-c you should be able to load a driver that allows network connectivity regardless of otg mode. Or was it Thunderbolt?

Update: I thought of thunderbolt-net which works with Thunderbolt 3 and probably USB4
- Details: https://superuser.com/a/1784608
Quora isn’t worth its weight in bits
- Quora is the new Yahoo Answers, debate me.
  
  What's the new Quora?
Because they're both hosts, you need a host and a client iirc.
- Most modern Intel chipsets support “Dual Role Device” (DRD) where they can act as host or client as needed.
  
  And if you want to get really funky, Intel also does their JTAG over USB. They are quite secretive about it, your bios should have turned it off, but it is there.
What are you trying to achieve? If your looking to transfer data I would use a Ethernet connection.

You will need to manually set the IP and netmask on each side. USB isn't designed for what you are trying to do.
- You would also need a type A to type B ethernet cable, AKA a crossover cable.
  
  Without that you will need some sort of switch to act as an intermediary between the two devices.
  
  I'm quite sure that all gigabit+ ethernet auto-negotiates. There is no shared ether, there are no dedicated tx/rx pairs anymore. It's all point-to point and constantly negotiating to make the most of every wire it's got.
  
  Well, that is just not true anymore and hasn't been in a very long time. Probably everything made in the last 20 years has auto detection and doesn't need a crossover ethernet cable. This was introduced as an optional feature sometime in the 100Base-T era and is required for gigabit ethernet.
Because USB builds a tree, with the USB-controller as stem.

Isn't there something like that for ethernet?
- Crossover cable.
  
  I still have one around just in case but wifi makes it unnecessarily these days.
  
  Also, most modern ethernet port would cross-over automatically
Back in the days we had these things. But I doubt this would work with USB-C adapters and a Steam Deck.
- Permanently Deleted
- If they haven't changed in the last decade, it's basically just 2 USB network adaptors stuck together. So it probably would still work, but I'd also prefer the modern solutions anyway.
Universal Serial Bus.

Serial communication between two hosts requires either robust auto negotiation or a crossover cable.

Usb doesn’t have good auto negotiation between two hosts so you gotta use a crossover cable.

heres one

But… there’s no guarantee that the computers usb controllers and operating systems will support it.
- Most modern NICs will auto negotiate now so the cross over cable is no longer needed.
  
  Yeah but like if instead of usb it was a db9 you’d need the tx and rx crossed over.
  
  That was called a uh null modem back in the day
  
  I was not aware of this!
- Would be nice to have a date on that statement in the product description. Seems to be outdated?
  
  Oh no, they say right there that it will not let two computers connect to each other lol.
In general, because what everyone thinks of when they say USB is a host-device protocol, and you’re trying to connect two hosts together.

But USB is a blanket term for a number of protocols, and there are protocols that allow a host to switch roles and become a device, depending on what it’s connected to.

If you see a PC being advertised with “dual role USB ports” or support for “USB-OTG”, then you can connect it to another PC. Otherwise, you almost certainly can’t.

If all you want to do is transfer files, you can use something like QuickDAV to transfer files over your local network.
You can't connect a steam deck to your PC?
- You can connect your Steam Deck to your PC with USB, but all it's going to do is charge it.
  
  I think that the Deck is able to connect as a device (MTP or CDC?), but there has been trouble with that so the current OS disables it.

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