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  • #21
    Did anyone of the critics actually listen to the talk? Or read up the documentation of kmscon? Or at least looked up some facts before posting here? Please do that and most of your concerns will go away. Anyway, I'll try to give some insights here:

    Zeroth question: what's a kernel console for?
    It serves mainly to two purposes. To provide some booting diagnostics, especially in the earliest phases, before even the INIT has been run. Second, to provide a way to interact with the system when it has not completely booted yet. I'm sure there's more here.
    A single slide of the presentation is labeled "What is it used for?". It lists: emergency-console, lightweight-UI, session-dispatcher and more.
    KMSCON was designed to fit into these use-cases exactly but also to _optionally_ provide more.

    First question: why should we kill the kernel console? To save memory? To make the boot faster? To gain extra features? I'd like to see how these would benefit the kernel and the system administrators.
    Again, a single slide is labelled "Linux Console Problems". To name a few problems: UIs don't belong in kernel-space! It's a horrible API, horrible input handling, horrible video output handling, it's _very_ buggy, no-one knows how to fix it, ... and it is missing proper internationalization!

    Second question: Multi-seat hardware-accelerated fully-internationalized kernel console? What would it be the plus? The real one, I mean. To enter shell commands in Chinese, Arabic or with proper diacritics? To get super-fast 3D console rendering at (AFTER) the kernel boot? All those "plura" are not less then bullshit in my mind.
    As mentioned in the presentation and documentation, all of these are optional. However, lets give some use-cases here:

    Imagine you have a multi-seat environment and your xserver on one seat crashed. In this case you want an emergency-console to restart your session. Hence, you want an emergency console on that seat _only_ so you can recover that seat.
    If the console is not multi-seat aware, you would have to interrupt _all_ sessions on the system to restart a single seat. Tell that to inter-cafe-owners. Of course, you could also use ssh, admin-seat or any other idea you can come up with. But there are always more than one way to do something...

    And if you're still not convinced: If you want fast-user-switching with X you need a multi-seat aware VT dispatcher. That's just a fact. I won't explain why. Feel free to ask if you want to know more.

    Similar reasons exist why all the other features were introduced. For instance, 3D acceleration is used to render a console on multi-seat systems where the CPU is _way_ to slow to render many consoles simultaneously. That is, kmscon _can_ make use of hardware-acceleration in situations where software-rendering might be slow. But again, don't use it if you don't want it.

    If all that is "bullshit in your mind", please elaborate why...

    Third question: what if I have to troubleshoot a system which has not fully booted yet? I mean things like forced fsck, issues with the /etc directory contents and so on. No userland, no console!
    Please get your facts right. How can you run fsck if there is no userland? where do you think fsck-binary is executed? In the kernel? No!
    To make use of a console you need to run programs. Whether its bash, cp, rm, fsck or reboot, all these run in _userspace_. Hence, you need a running user-space to make use of the linux-console. So please tell me a situation where kmscon is unable to run, but the linux-console can be used to perform maintenance tasks? Please! I'm desperately looking for one. (spoiler alert: there are none!)

    If you are talking about boot/shutdown-logs or oops/panic-screens, please look at fblog/drmlog drivers which were written for this exact situation.

    Fourth question: You say that code is poorly maintained, right? So why don't you just ... maintain it?
    There is a reason that the code is poorly maintained: The code is crap. Seriously, talk to any kernel developer you want. Just two random comments I read last week on google+ from long-time kernel developers:
    Dave Airlie: "everytime I read vt.c/fbcon.c code I feel much worse."
    Benjamin Herrenschmidt: "I remember when I tried to fix some of the worst locking issues in there a while ago, I wonder if that's what gave me my stomach tumor"

    If you look into Archives you will notice that no-one likes working on it. No-one!

    And UIs don't belong in kernel-space, so why bother maintaining it? They belong in user-space!

    Fifth question: Maybe all this is for mobile smartphones running Linux, right? Please, I need some more details here as I don't get the point.
    KMSCON has nothing to do with smartphones.

    If the console is in user-space, how can we debug situations on desktop PCs (without the possibility to connect a serial console) where the kernel can't find the initrd or the root-partition, means: no userspace available?
    I think I answered that above. If you have no user-space, a serial-console is useless as well. No userspace, no shell, no use. It's that easy. (and the few things the kernel provides that work without userspace will still work with kmscon).

    How can systemd help if you either have a system without it or it gets not loaded because of the problems mentioned in the first question?
    systemd integration is optional. systemd-logind allows to spawn/manage VTs, but please don't use it if you have no use for it.

    Correct, but then you need extra hardware (a PC and/or a console server and/or a serial adapter) in order to troubleshoot a boot process! Serial consoles are quite useful, indeed. Still I don't see the point about killing the kernel console which, I think, shares code with the "other consoles".
    If your platform doesn't have a "local terminal" (aka graphics adapter w/ VGA-DVI-HDMI output), then the serial console is the only one option. And it is in kernel space!
    Again, please get your facts right. Linux-console, serial-console _and_ KMSCON share a kernel subsystem called TTY-layer. This will always be used and it's not about to be replaced! However, _only_ the linux-console uses the VT layer. Serial-console and KMSCON are totally independent of it and don't share any code with it.

    KMSCON tries to replace the VT layer. And _only_ the VT layer.

    And again, the serial-console is a kernel-space API but everything that you run in it, runs in user-space! The serial-port I/O layer runs in kernel-space. The TTY layer runs in kernel-space, but saying a console 'runs' in kernel-space is ambiguous and in most cases wrong.

    Sorry, I don't understand this. Are you talking about "USB kernel consoles"? I don't mind about console speed. I mind about it being available as soon as possible. And, going back to previous point, you need serial drivers anyway into the kernel in order to have that.
    KMSCON can be used for user-interaction in the same situations you can use the linux-console. If you don't believe me, please describe a situation where you can run a command in the linux-console but you cannot in kmscon? Please!

    I wish I could get more than guesses. Anything requires effeorts and resources. Do you think a kernel console is useless? Then fight for a complete removal. Otherwise make it as useful as possible.
    UTF-8 in the kernel console? I think I missed something. I thought that all internal kernel human messaging was in English with 7BIT-ASCII. My bad!
    Yes, the linux-console supports UTF8 indeed. But the font-rendering is limited to 512 different glyphs at a time so you cannot make use of the full Unicode set.

    Smartphones do not use X (maybe Wayland in the future) but the frame buffer. I didn't get any possible reasons for getting a userland console in Linux based smartphones, a very peculiar Linux platform...
    Some old 'regular' phones did use X (and probably still do). No modern smartphone uses fbdev (Android <=2.3.* used it but now it's obsolete). And no-one is trying to push it into smartphones. I don't know where that came up but it's not related to kmscon.

    Stop guessing. I'll try to explain myself better.
    I'm just saying that I don't see reasons to move the kernel console out of ... the kernel.
    Imagine there would be no kernel-console but only KMSCON in user-space. Could you describe reasons to move it into kernel-space? I cannot.

    So if you think all others reasons mentioned in this thread are invalid, how about this reason: It doesn't belong into the kernel. And KMSCON is an attempt to clean-up kernel code and reduce the amount of non-swappable kernel memory (by removing the linux-console).

    I say that we do need a kernel console, maybe we need a better kernel console, but that would hardly require hardware acceleration, internationalization and multiple seats.
    I say that a kernel console needs to be available as soon as possible at boot. Full stop. Anything else sounds useless to my poor mind.
    Once I get the boot done I can use normal pseudo terminals with TELNET/SSH/whatever and even X terminals on both local and network displays. That'd be it.
    I think I answered this 3 times above. If you can make use of the linux-console, then you can also make use of KMSCON. In all situations. Otherwise, please prove me wrong.

    My side of this argument is that the whole idea is crap. The VTs are only ever used as a last resort, so the simpler they are, the more likely they are to be functional. I don't want them replaced by a feature filled (bloated) userland terminal. Recovery of the system should never require internationalized letters. If you want to manage a Cyrillic filesystem through a terminal, then either fire one up after the boot, in userland, or add support for internationalization alone to the VTs, but simpler is better. This whole idea reeks of unnecessary complication.
    Why do you think CONFIG_VT is simpler than KMSCON? Please prove this with facts! If you use kmscon without any optional modules, it is mostly identical to the linux-console but in user-space.

    We do need a low level access for emergencies. The simpler, the better. The earlier, the more effective.
    Agaaaaain: KMSCON works in the same situations as the linux-console. In fact, it can even work in _more_ situations (like using udlfb when your PCI/embedded GPU hangs).

    Practical question, since I am still not getting it. When on a normal desktop PC (no serial console), with the new implementation, the system fails to boot, for example due to a corrupted/missing initrd, would I see the error messages on my physical display so that I can troubleshot the system?
    yes, either provided by CONFIG_VT or by fblog/drmlog.


    And you know what the greatest thing of kmscon is? It's 100% backwards compatible. So stick with VTs if you love them, switch to KMSCON if you hate them. It's not like _any_ software depends on KMSCON so it's really optional.
    But if you ever run into a situation where you'd like to tweak your emergency-console, good luck with fixing VTs. I prefer fixing KMSCON.

    Cheers
    David

    Comment


    • #22
      The entire project FAQ, if youre gonna bash the project atleast have the developers side of the story:

      This is a list of the most commonly asked questions and misconceptions.

      The console belongs in the kernel so it can run under memory pressure and/or during system failure!

      As a matter of fact the in-kernel linux console does not run under memory-pressure or during system failure, either. Therefore, there is no disadvantage in running the console in user-space. In fact, the kernel console and kmscon only implement the rendering pipeline for the text console. Anything you do with it or any program that you run on the console (including a shell like bash) runs in user-space! And when the system fails and user-space is no longer working correctly, then your bash won't run either so there is no point in having a working console layer when there is nothing to show.

      Why replacing a working and fully functional console?

      There are many different reasons why kmscon is needed. Some are only cosmetic reasons but others are solutions to big restrictions of the linux kernel console layer. The most notably ones include:

      Full internationalization support. No-one wants (and we currently do not have) full internationalized keyboard handling in the kernel. There is also no way to print a full CJK character set or even the full Unicode character set on the linux console. Adding this to the kernel would mean having big character tables in non-swappable kernel memory. Therefore, implementing this in user-space is the only way.
      Hardware accelerated drawing. With multi-seat becoming more and more common and multiple monitors connected to a single computer, we do not want to spend too much time drawing text on the CPU. However, using the GPU pipeline from the kernel would require new DRM APIs which are currently not available. With GPU-accelerated rendering we can also add anti-aliased fonts or soft-shadows which can enhance readability a lot (although others might consider this cosmetic changes).
      Controllable Monitor/Console mapping. By using the DRM API we can have as many consoles simultaneously as we want and can map them to different monitors or clone the output. We can even span a console across multiple monitors.
      Full vt220 to vt510 support. The kernel console supports only a small subset of the DEC VT APIs. It does not even correctly emulate the VT102 API (although it's pretty close to vt102). In user-space we can extend this to even support all the xterm supported escape sequences. This also includes a better scrollback-buffer which is pretty limited in the kernel console.
      Another major point which does not have anything to do with functionality: UIs do not belong into the kernel. No one draws X11 GUIs or similar in the kernel so why should we draw text UIs in the kernel? User-space is responsible for that so lets move it there.

      What happens to my boot-log, oops-log and emergency-log?

      The in-kernel fbcon (or vgacon) driver is currently used for console handling and every other text output which includes the boot log (if enabled), the oops or kernel-panic log and other emergency logs. When disabling the built-in console the fbcon driver will not work anymore so these log-modes won't be available, either.

      However, the kmscon repository includes an "fblog" called kernel driver that provides a simple text-output for exactly these cases. It will hopefully be merged into the mainline kernel, soon, so even with kmscon running you can get kernel-panics and boot-log displayed on the screen.

      Is kmscon compatible with the kernel console?

      Yes. Although kmscon was created to replace CONFIG_VT (the config option for the kernel console), it can smoothly run simultaneously with it. However, kmscon does not emulate the same terminal type as the linux console. There are several escape sequences and modes that only the linux console supports and no other emulator. But nearly no program requires them. So we decided to follow the xterm compatibility instead. That is, if you program runs under xterm (the de-facto standard in terminal emulators under linux) it should also run under kmscon (or at least that is the idea).

      Is kmscon linux-only?

      Yes. Feel free to port it to other platforms.

      Kmscon has too many dependencies!

      Kmscon has nearly no mandatory dependencies. Almost all dependencies are optional. However, without them you will lose functionality. But feel free to disable them if you do not require them.

      libudev: Udev is used for hotplugging. It is a central part of almost every linux distribution and without it hotplugged devices will not be added to kmscon.
      libsystemd-logind: Only used for multi-seat capable linux systems. If you do not use multiple seats, you can safely disable it.
      mesa: Mesa is the OpenGL implementation for linux. If you don't want hardware accelerated drawing, you can disable it and use plain libdrm for modesetting. You can even disable libdrm and just use linux fbdev.
      libxkbcommon: If you need keyboards with layouts other than US-ASCII, libxkbcommon is your friend. It handles everything which is keyboard related.
      pango: Pango is used for text-rendering. Disable it and the built-in static 8x16 font will be used.
      Last edited by dvdhrm,
      All opinions are my own not those of my employer if you know who they are.

      Comment


      • #23
        Real scenario:
        Laptop won't boot. Don't have any recovery media available. No serial ports.
        Real solution with VT:
        Read messages with VT driver (something about not finding init)
        Edit commandline with break=premount to get an initramfs shell
        fsck the root device.
        Reboot.
        Works.

        And your answer is...Let me guess:
        Add pixman and xkb data or a 3+ MB static binary plus 5 MB data files to initrd?
        Initrd space isn't free. It also means more time spent on updating the initrd and more frequent need for updates.

        Real world problem #2:
        Graphics driver won't load or won't work. You need to get to a shell in VGA text mode to debug.
        I don't see any solution for this. And don't tell me it doesn't happen.

        Some other scenarios/questions:
        What happens if/when libpixman breaks?
        When has the Linux VT driver ended up so broken it's unusable (I'm looking for the most recent date, estimated frequency, or any such number)?

        Comment


        • #24
          I like his improved console stuff but I'd love to see that in kernel and preferably with more optional dependencies. Though all this accelerated things wouldn't work on my ancient boxes anyway I guess. But these are where I need speed. Just making a cat to a logfile an it takes 5 min to scroll down on the fbcon... while over ssh in an xterm on a different computer it scrolls in less than a second. I mean, this is helpful if you need the very end or just a bit scrollback from there from a logfile.
          Userland things tend to break earlier, e.g. update a dependency lib and forget to (revdep)rebuild and bam, that thing won't work anymore or produce errors.
          Stop TCPA, stupid software patents and corrupt politicians!

          Comment


          • #25
            Originally posted by Ibidem View Post
            Real scenario:
            Laptop won't boot. Don't have any recovery media available. No serial ports.
            Real solution with VT:
            Read messages with VT driver (something about not finding init)
            Edit commandline with break=premount to get an initramfs shell
            fsck the root device.
            Reboot.
            Works.
            You get the kernel-log messages still via fblog/drmlog so you don't need VTs to get them.

            And why can't you use kmscon in initrd? Where is the problem with using kmscon instead of the linux-console here?

            This scenario works perfectly well with kmscon, too.

            And your answer is...Let me guess:
            Add pixman and xkb data or a 3+ MB static binary plus 5 MB data files to initrd?
            Initrd space isn't free. It also means more time spent on updating the initrd and more frequent need for updates.
            Did you try linking kmscon statically? If not, your point is invalid. Please provide some real data of your kmscon binary size (even statically linked).

            Real world problem #2:
            Graphics driver won't load or won't work. You need to get to a shell in VGA text mode to debug.
            I don't see any solution for this. And don't tell me it doesn't happen.
            If VGA-text mode works, then VBE mode works, too. Use dvbe+kmscon then.

            If you really find a machine with VGA-text mode but non-working VBE (I highly doubt that, but ok), then write a VGA-text mode video backend for kmscon. Where is the problem?

            Some other scenarios/questions:
            What happens if/when libpixman breaks?
            When has the Linux VT driver ended up so broken it's unusable (I'm looking for the most recent date, estimated frequency, or any such number)?
            The linux kernel is about 15million lines of code, pixman is 43 thousand lines of code (and that includes _all_ the different hardware optimizations for all the compositing functions that aren't even used!).

            You want to argue that 99,999% of the linux-kernel-code doesn't affect VTs? Then please take into account, that only the static pixman_blt() function is important to kmscon and 99,999% of pixman doesn't affect kmscon.


            Still waiting for a use-case where I'd prefer the linux-console over kmscon...
            Regards
            David

            Comment


            • #26
              Originally posted by Ibidem View Post
              Real scenario:
              Laptop won't boot. Don't have any recovery media available. No serial ports.
              Real solution with VT:
              Read messages with VT driver (something about not finding init)
              Edit commandline with break=premount to get an initramfs shell
              fsck the root device.
              Reboot.
              Works.
              Yeah (link!), cute.. Did I already mention that fsck is not in initramfs. So it must be in the kernel right? Yeah...

              Reading a library/program to render your console/fix your fs does not imply rw access to the medium where this library comes from. Moving on...

              Originally posted by Ibidem View Post
              And your answer is...Let me guess:
              Add pixman and xkb data or a 3+ MB static binary plus 5 MB data files to initrd?
              Initrd space isn't free. It also means more time spent on updating the initrd and more frequent need for updates.
              Read the FAQ EricG posted. And that would not be pixman but pango. And look! -> both optional.

              Originally posted by Ibidem View Post
              Real world problem #2:
              Graphics driver won't load or won't work. You need to get to a shell in VGA text mode to debug.
              I don't see any solution for this. And don't tell me it doesn't happen.
              Read the Phoronix article. They are working on DRM for Vesa and UEFI based systems. No GPU support -> Framebuffer!

              Originally posted by Ibidem View Post
              Some other scenarios/questions:
              What happens if/when libpixman breaks?
              X will not start. Rebuild it. Does has nothing (link!) to do with KMSCON (yet).

              Originally posted by Ibidem View Post
              When has the Linux VT driver ended up so broken it's unusable (I'm looking for the most recent date, estimated frequency, or any such number)?
              Look here (link!). This would not be needed, were VT correctly designed from the start. One more example here (link!).
              Last edited by Rexilion; 08 February 2013, 12:21 PM.

              Comment


              • #27
                Originally posted by Uqbar View Post
                Yeah! System administration is not the same as normal user activities. And when the going gets tough, the tough gets going. We need the plain old kernel consoles. Being that on local console (aka VGA/DVI/HDMI) or serial or whatever else it's irrelevant. We do need a low level access for emergencies. The simpler, the better. The earlier, the more effective.
                Well said...the kernel consoles should be kept for this reason. Perhaps the userland consoles can be made available AFTER booting is successfully completed while the system can drop to a kernel console in case of error during boot.

                Comment


                • #28
                  First, I should clarify that I don't object to KMSCON.
                  I do object to it being considered a full replacement for CONFIG_VT. It is a good thing to have, but there are problems it won't work for.
                  Originally posted by Rexilion View Post
                  Yeah (link!), cute.. Did I already mention that fsck is not in initramfs. So it must be in the kernel right? Yeah...

                  Reading a library/program to render your console/fix your fs does not imply rw access to the medium where this library comes from. Moving on...
                  Actually it's this, and it was a case where init would not mount / at all.


                  Read the FAQ EricG posted. And that would not be pixman but pango. And look! -> both optional.
                  He posted it while I was writing my post, and I made the mistake of believing Micheal's list of dependencies.

                  I really should know better than that, by now.

                  Read the Phoronix article. They are working on DRM for Vesa and UEFI based systems. No GPU support -> Framebuffer!

                  X will not start. Rebuild it. Does nothing (link!) to do with KMSCON (yet).


                  Look here (link!). This would not be needed, were VT correctly designed from the start. One more example here (link!).
                  I'm not asking whether it's buggy, but whether there are points where it failed to allow basic input. I can't tell from the commit messages whether that's the case there. Or did those break VT?
                  If I can start a shell and enter commands, that's useable, regardless how many races there are in the design.

                  You really think that VESA/framebuffer always works? I guess you've been rather limited in your reading.
                  Real-world cases I've heard of:
                  -Some gfx cards (especially SIS/S3, IIRC) don't work with vesa; it loads, but you get an unuseable display.
                  -Drivers that break after enablement
                  Real problems I've encountered with framebuffer and KMS drivers:
                  -radeonfb gets loaded by Debian because they don't ship microcode. Unuseable display.
                  -Intel black screen (2.6.32 or so, Lucid testing)
                  And then there's the whole problem with AMD and Nvidia proprietary drivers (no framebuffer/kms, only X), and some installers won't run if the kms drivers are loaded...

                  Comment


                  • #29
                    Originally posted by Ibidem View Post
                    First, I should clarify that I don't object to KMSCON.
                    I do object to it being considered a full replacement for CONFIG_VT. It is a good thing to have, but there are problems it won't work for.
                    Ok.

                    Originally posted by Ibidem View Post
                    Actually it's this, and it was a case where init would not mount / at all.
                    Ah, would have been nice you mentioned that you were not using stock configuration. But alas, point taken.

                    Originally posted by Ibidem View Post
                    He posted it while I was writing my post, and I made the mistake of believing Micheal's list of dependencies.
                    Yes, it was not mentioned they are optional. It would have been nice if it has been.

                    Originally posted by Ibidem View Post
                    I really should know better than that, by now.
                    lol

                    Originally posted by Ibidem View Post
                    I'm not asking whether it's buggy, but whether there are points where it failed to allow basic input. I can't tell from the commit messages whether that's the case there. Or did those break VT?
                    If I can start a shell and enter commands, that's useable, regardless how many races there are in the design.
                    Again, this will lower maintainance cost. This is a tradeoff between user and developer. More on this below.

                    Originally posted by Ibidem View Post
                    You really think that VESA/framebuffer always works? I guess you've been rather limited in your reading.
                    Real-world cases I've heard of:
                    -Some gfx cards (especially SIS/S3, IIRC) don't work with vesa; it loads, but you get an unuseable display.
                    -Drivers that break after enablement
                    [practical joke]Accusing me of limited reading and then mentioning things you *heard* off. Interesting...[/practical joke]

                    Yes, things break. But I take that as a part of Linux by now.

                    That is why I argue it well never be a mainstream system. I now know the concepts of building a kernel, compiling code and doing a git bisect. Things I would never know (and supposed not to know as an end user) when I sticked to using Windows. But those are the things that are part of using Linux (in my opinion).

                    As for the unsupported SiS and S3 hardware and other 'drivers that break after enablement'. Yeah, that just sucks about Linux. But can you blame them? Not really... . For me, the advantages outweight the disadvantages.

                    Originally posted by Ibidem View Post
                    Real problems I've encountered with framebuffer and KMS drivers:
                    -radeonfb gets loaded by Debian because they don't ship microcode. Unuseable display.
                    -Intel black screen (2.6.32 or so, Lucid testing)
                    And then there's the whole problem with AMD and Nvidia proprietary drivers (no framebuffer/kms, only X), and some installers won't run if the kms drivers are loaded...
                    Practical jokes aside, yes some hardware is not well supported. But that should not inhibit developers from making better decisions higher up the stack. If it's broken now, it will be broken later. I think you know that regressions are not accepted. Furthermore, it will be a long while before CONFIG_VT will be removed (and all of it's necessary code). This should not inhibit progress.

                    Aside from that, the open source drivers are making quite some progress these days (and 2.6.32 is pretty old!). It will be nice to take advantage of all this hard work.

                    I'm not against proprietary drivers, but I have no mercy to them when it comes to 'compatibility'. People are free to develop drivers outside of the tree, but end users have no right to expect that they will work with every new piece of technology that comes around. I know that not everyone has the privilege of using open source drivers. But you don't HAVE to use Linux. You could stick with Windows. I'm totally fine with that. In fact, I would not *ever* recommend Linux to anyone else as it stands now. I introduces this OS to my parents and we ran into so many little subtle things. It will probably be never worth it for the majority of people to use it!

                    Comment


                    • #30
                      UDEV

                      Originally posted by Ericg View Post
                      The entire project FAQ, if youre gonna bash the project atleast have the developers side of the story:
                      the dependency to UDEV is completely unacceptable; any distro based on busybox uses mdev instead of udev.
                      for example, sabotage linux [1]

                      in short, kmscon is a very BAD idea until there's a very simple, very portable core that has no external deps at all.
                      ideally, some statically linked fallback version would be automatically compiled while building the kernel


                      also a hardcoded dependency to glibc makes it impossible to use a modern kernel with any other C library, such as musl libc[2]

                      [1] https://github.com/rofl0r/sabotage
                      [2] http://www.musl-libc.org

                      Comment

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