4 ms·

"Needless to say, open-source Linux advocates should try to use DisplayPort instead if at all possible." And at this point that's one of the several (many?) pr

by mikece 3y ago

"Needless to say, open-source Linux advocates should try to use DisplayPort instead if at all possible." And at this point that's one of the several (many?) protocols that runs over a USB-C cable, right?

conradev 3y ago
DisplayPort is packet-based and can be multiplexed with other USB-C traffic through a hub HDMI is not packet-based and so when it uses USB-C it takes over the entire cable (alt-mode)
- baby_souffle 3y ago
  > DisplayPort is packet-based and can be multiplexed with other USB-C traffic through a hub This is part of why DP is $$$ compared to HDMI. I would love to see DP start eating HDMI's lunch after this and the absolute shit show that was the HDMI 2.0 roll out but cheaper to implement is almost certainly going to be the driving factor when it comes to consumer grade TV / Displays and no console or other set top box maker is going to bother putting display port on their device if nobody's got a TV that can use it.
  - tverbeure 3y ago
    > This is part of why DP is $$$ compared to HDMI. Why is that more expensive? In terms of complexity, implementing DP vs HDMI 2.1 is not materially different. They both have fixed rate links, packets, Reed-Solomon forward error correction, DSC, etc. But IIRC DP is royalty free, and HDMI is not.
    - refulgentis 3y ago
      It sounds like it might be more about "the market will pay more for the packet-based approach, and we shall charge what the market will bear"
      - tverbeure 3y ago
        Speaking about 'packet-based approach', the way it's presented in the popular press as if it's similar to Ethernet, is a pretty gross distortion, the impact of which is not nearly as much as people seem to think it is. You can check that there in this presentation (https://www.vesa.org/wp-content/uploads/2011/01/ICCE-Presentation-on-VESA-DisplayPort.pdf https://www.vesa.org/wp-content/uploads/2011/01/ICCE-Present...), page 32 and 33. The majority of DP traffic is still brute force video data, interspersed with heavily packetized secondary data. Over the years, I've spent many hours wading through DisplayPort data debug traces, and I've always wondered what people were smoking when they called it 'packetized like Ethernet'. It's just not true. (And FWIW: even old HDMI can transport secondary data packets just the way you can with DP. It's how audio-over-HDMI is done...)
      - autoexecbat 3y ago
        Perhaps there is just more scale for HDMI making it easier to recoup r&d for new components that implement it
  - babypuncher 3y ago
    It won't happen unless VESA gets serious about adding home theater specific features to DP that HDMI has had for 20 years.
    - kiwijamo 3y ago
      For example...? My experience is that DP is excatly like HDMI from the end user's perspective but happy to learn more.
      - ThatPlayer 3y ago
        ARC and CEC come to mind. ARC probably isn't too important, but CEC is nice being able to control various boxes with the same remote. Or being able to control the TV's volume with another remote.
        babypuncher 3y ago
        ARC is actually pretty important, especially now that eARC is a thing. It finally decouples your receiver's HDMI capability requirements from your TV.
        simoncion 3y ago
        I see no __technical__ reason why you couldn't do CEC over the AUX channel... all that's required is for the software on either end to be updated to squirt and interpret the bits, and it's a low-speed protocol, so it'll fit just fine in the AUX channel. "ARC" seems to be "Audio over HDMI using CEC for discovery and control", which (if you've bothered to run CEC over the DisplayPort AUX channel) you get the rest automatically with DisplayPort. However, because both CEC and ARC are HDMI standards, you bet your biscuits that the HDMI Consortium will bar anyone who wants to use HDMI ports on their devices from shipping official firm/software that does the braindead-simple thing of running CEC over DP AUX, and having ARC-compatible firm/software. As is nearly always the case (and when it's not, it's not for long) DisplayPort is totally capable of everything HDMI is, but the HDMI Consortium stands in the way of having commercially-distributed DisplayPort "home theater" products that are compatible with the nice-to-have HDMI features.
  - kimixa 3y ago
    And the HDMI is pretty much a direct digital translation of analogue CRT signalling, blanking and timing weirdness included. Which is also not how modern LCDs implement their internal screen driving, so needs a similar level of complexity to decode and convert. I think the biggest difference in DP vs HDMI cost is simply scale - there's probably orders of magnitude more HDMI chips sold than DP.
- wolfhumble 3y ago
  > HDMI is not packet-based and so when it uses USB-C it takes over the entire cable (alt-mode) What does this exactly mean? I have an USB-C 4x1 Hub with HDMI that works at the same time as using 2 x USB-A 3.0, 1 x USB-C. Thanks!
  - izacus 3y ago
    It means that your hub uses DisplayPort to talk to the computer and then has a converter chip to convert to HDMI. Pretty much noone used HDMI Alt Mode on USB-C and is now deprecated IIRC.
    - wolfhumble 3y ago
      Okay, thanks!
    - justsomehnguy 3y ago
      Reminds me of Ethernet over HDMI.
- cesarb 3y ago
  > HDMI is not packet-based and so when it uses USB-C it takes over the entire cable (alt-mode) AFAIK, when HDMI is used over USB-C, it's actually DisplayPort over USB-C; while a HDMI alt mode was specified, nobody actually implemented it, everyone instead implemented the DisplayPort alt mode and then used a DP-to-HDMI converter chip whenever an HDMI output was required. > DisplayPort is packet-based and can be multiplexed with other USB-C traffic through a hub That's the case only for USB4 (and AFAIK earlier Thunderbolt 3). Other USB-C ports with DisplayPort alt mode simply use some of the USB 3.x pairs for raw DisplayPort, and whenever the use of all four USB 3.x pairs is required for DisplayPort due to the target resolution, then only USB 2.x can be available (USB 2.x has its own dedicated pair in the cable).
  - altairprime 3y ago
    Actual example of this: Back in the Thunderbolt 3 era, it was up to each motherboard manufacturer to decide how many pairs they routed to USB alt-mode, so it's not necessarily a safe bet to depend on it working. So, in one specific example, 4k60 over USB alt-mode DisplayPort is not supported on Apple's $4999 iMac Pro, as USB alt-mode is only assigned two pairs; however, 4k60 over Thunderbolt DisplayPort is supported, as Thunderbolt is assigned four pairs. The only way to get 4k60 out of that device is to use a Thunderbolt-only DisplayPort adapter, that has no USB mode at all. This was resolved by USB 4 and Thunderbolt 4 both incorporating a more modern DSC (display stream compression), among other things as described above, but your mileage will vary much more wildly with USB 3.
Cu3PO42 3y ago
Yes. You can run "DisplayPort Alt Mode" over USB-C. You can also transport DP over Thunderbolt over USB-C. HDMI Alt Mode also was a thing, but is now deprecated.
whalesalad 3y ago
I am using a displayport to usb-c to connect an amd gpu to an apple display and it works well.
TedDoesntTalk 3y ago
You can run HDMI over ethernet CAT-6 cables, too.
gamepsys 3y ago
You can run DP over USB-C. It's a use case they specifically support. However, if you are the cutting edge you will be using the actual DP cords. DP2.1 uses 80Gbit/s and Thunderbolt 3 USB-C is only 40 GBit/s. Once you start pushing high resolutions (at least 4K) and high framrates the entire system will be bottle necked by the cable. Top end displays are always designed to take advantage of the latest DP port, and use the entire bandwidth. USB4 will push 120GBit/s. This will be enough for 8k 120hz/4k 240hz but not much more. This will probably be enough for high end consumer displays and GPUs for a decade out or so.
fuzzy2 3y ago
Actually, it is the only (display) protocol. HMDI Alt Mode, while specified, never really manifested and is effectively dead. DisplayPort is the only alternative and is more flexible anyway. Its packet-based nature is not relevant for DisplayPort Alt Mode, by the way, because it gets dedicated pins. Sometimes enough for two lanes, sometimes enough for four. It's hilariously consumer-unfriendly. Only Thunderbolt leverages the packet stuff and can transport up to eight lanes worth of DisplayPort. macOS does not support MST. Who is supposed to get this? I don't know.

by mikece 3y ago

conradev 3y ago

DisplayPort is packet-based and can be multiplexed with other USB-C traffic through a hub HDMI is not packet-based and so when it uses USB-C it takes over the entire cable (alt-mode)

baby_souffle 3y ago
> DisplayPort is packet-based and can be multiplexed with other USB-C traffic through a hub This is part of why DP is $$$ compared to HDMI. I would love to see DP start eating HDMI's lunch after this and the absolute shit show that was the HDMI 2.0 roll out but cheaper to implement is almost certainly going to be the driving factor when it comes to consumer grade TV / Displays and no console or other set top box maker is going to bother putting display port on their device if nobody's got a TV that can use it.
- tverbeure 3y ago
  > This is part of why DP is $$$ compared to HDMI. Why is that more expensive? In terms of complexity, implementing DP vs HDMI 2.1 is not materially different. They both have fixed rate links, packets, Reed-Solomon forward error correction, DSC, etc. But IIRC DP is royalty free, and HDMI is not.
  - refulgentis 3y ago
    It sounds like it might be more about "the market will pay more for the packet-based approach, and we shall charge what the market will bear"
    - tverbeure 3y ago
      Speaking about 'packet-based approach', the way it's presented in the popular press as if it's similar to Ethernet, is a pretty gross distortion, the impact of which is not nearly as much as people seem to think it is. You can check that there in this presentation (https://www.vesa.org/wp-content/uploads/2011/01/ICCE-Presentation-on-VESA-DisplayPort.pdf https://www.vesa.org/wp-content/uploads/2011/01/ICCE-Present...), page 32 and 33. The majority of DP traffic is still brute force video data, interspersed with heavily packetized secondary data. Over the years, I've spent many hours wading through DisplayPort data debug traces, and I've always wondered what people were smoking when they called it 'packetized like Ethernet'. It's just not true. (And FWIW: even old HDMI can transport secondary data packets just the way you can with DP. It's how audio-over-HDMI is done...)
    - autoexecbat 3y ago
      Perhaps there is just more scale for HDMI making it easier to recoup r&d for new components that implement it
- babypuncher 3y ago
  It won't happen unless VESA gets serious about adding home theater specific features to DP that HDMI has had for 20 years.
  - kiwijamo 3y ago
    For example...? My experience is that DP is excatly like HDMI from the end user's perspective but happy to learn more.
    - ThatPlayer 3y ago
      ARC and CEC come to mind. ARC probably isn't too important, but CEC is nice being able to control various boxes with the same remote. Or being able to control the TV's volume with another remote.
      - babypuncher 3y ago
        ARC is actually pretty important, especially now that eARC is a thing. It finally decouples your receiver's HDMI capability requirements from your TV.
        simoncion 3y ago
        I see no __technical__ reason why you couldn't do CEC over the AUX channel... all that's required is for the software on either end to be updated to squirt and interpret the bits, and it's a low-speed protocol, so it'll fit just fine in the AUX channel. "ARC" seems to be "Audio over HDMI using CEC for discovery and control", which (if you've bothered to run CEC over the DisplayPort AUX channel) you get the rest automatically with DisplayPort. However, because both CEC and ARC are HDMI standards, you bet your biscuits that the HDMI Consortium will bar anyone who wants to use HDMI ports on their devices from shipping official firm/software that does the braindead-simple thing of running CEC over DP AUX, and having ARC-compatible firm/software. As is nearly always the case (and when it's not, it's not for long) DisplayPort is totally capable of everything HDMI is, but the HDMI Consortium stands in the way of having commercially-distributed DisplayPort "home theater" products that are compatible with the nice-to-have HDMI features.
- kimixa 3y ago
  And the HDMI is pretty much a direct digital translation of analogue CRT signalling, blanking and timing weirdness included. Which is also not how modern LCDs implement their internal screen driving, so needs a similar level of complexity to decode and convert. I think the biggest difference in DP vs HDMI cost is simply scale - there's probably orders of magnitude more HDMI chips sold than DP.
wolfhumble 3y ago
> HDMI is not packet-based and so when it uses USB-C it takes over the entire cable (alt-mode) What does this exactly mean? I have an USB-C 4x1 Hub with HDMI that works at the same time as using 2 x USB-A 3.0, 1 x USB-C. Thanks!
- izacus 3y ago
  It means that your hub uses DisplayPort to talk to the computer and then has a converter chip to convert to HDMI. Pretty much noone used HDMI Alt Mode on USB-C and is now deprecated IIRC.
  - wolfhumble 3y ago
    Okay, thanks!
  - justsomehnguy 3y ago
    Reminds me of Ethernet over HDMI.
cesarb 3y ago
> HDMI is not packet-based and so when it uses USB-C it takes over the entire cable (alt-mode) AFAIK, when HDMI is used over USB-C, it's actually DisplayPort over USB-C; while a HDMI alt mode was specified, nobody actually implemented it, everyone instead implemented the DisplayPort alt mode and then used a DP-to-HDMI converter chip whenever an HDMI output was required. > DisplayPort is packet-based and can be multiplexed with other USB-C traffic through a hub That's the case only for USB4 (and AFAIK earlier Thunderbolt 3). Other USB-C ports with DisplayPort alt mode simply use some of the USB 3.x pairs for raw DisplayPort, and whenever the use of all four USB 3.x pairs is required for DisplayPort due to the target resolution, then only USB 2.x can be available (USB 2.x has its own dedicated pair in the cable).
- altairprime 3y ago
  Actual example of this: Back in the Thunderbolt 3 era, it was up to each motherboard manufacturer to decide how many pairs they routed to USB alt-mode, so it's not necessarily a safe bet to depend on it working. So, in one specific example, 4k60 over USB alt-mode DisplayPort is not supported on Apple's $4999 iMac Pro, as USB alt-mode is only assigned two pairs; however, 4k60 over Thunderbolt DisplayPort is supported, as Thunderbolt is assigned four pairs. The only way to get 4k60 out of that device is to use a Thunderbolt-only DisplayPort adapter, that has no USB mode at all. This was resolved by USB 4 and Thunderbolt 4 both incorporating a more modern DSC (display stream compression), among other things as described above, but your mileage will vary much more wildly with USB 3.

Cu3PO42 3y ago

Yes. You can run "DisplayPort Alt Mode" over USB-C. You can also transport DP over Thunderbolt over USB-C. HDMI Alt Mode also was a thing, but is now deprecated.

whalesalad 3y ago

I am using a displayport to usb-c to connect an amd gpu to an apple display and it works well.

TedDoesntTalk 3y ago

You can run HDMI over ethernet CAT-6 cables, too.

gamepsys 3y ago

You can run DP over USB-C. It's a use case they specifically support. However, if you are the cutting edge you will be using the actual DP cords. DP2.1 uses 80Gbit/s and Thunderbolt 3 USB-C is only 40 GBit/s. Once you start pushing high resolutions (at least 4K) and high framrates the entire system will be bottle necked by the cable. Top end displays are always designed to take advantage of the latest DP port, and use the entire bandwidth. USB4 will push 120GBit/s. This will be enough for 8k 120hz/4k 240hz but not much more. This will probably be enough for high end consumer displays and GPUs for a decade out or so.

fuzzy2 3y ago

Actually, it is the only (display) protocol. HMDI Alt Mode, while specified, never really manifested and is effectively dead. DisplayPort is the only alternative and is more flexible anyway. Its packet-based nature is not relevant for DisplayPort Alt Mode, by the way, because it gets dedicated pins. Sometimes enough for two lanes, sometimes enough for four. It's hilariously consumer-unfriendly. Only Thunderbolt leverages the packet stuff and can transport up to eight lanes worth of DisplayPort. macOS does not support MST. Who is supposed to get this? I don't know.