What signal does laptop power button send?

I have a HP Pavillion dv6500 laptop. When I boot ubuntu live USB, the bootup process hangs indefinitely at around the point where it's scanning hardware.

Then when I quickly press the power button, the bootup process continues. (obviously I'm not holding down the power button long enough to shut it down)

What signal would a laptop power button send that the bootup process (hardware scan) would be waiting for?
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More about what signal laptop power button send
  1. Does this happen when you boot normally from the hard drive? I just had a bit of a search as it rang a bell with me and it seems that model of laptop is a bit notorious for issues with the power button cables, nvida graphics HW and a few other things.

    There are a few guides suggesting things to do. I just googled 'HP Pavillion dv6500 power button' and found loads of suggestions on similar issues although not quite the same symptoms as most folk with them are on Windows.

    I'd start by trying another distro on a different USB stick if you can. Maybe something like Slax which is not based on Debian like Ubuntu is. If you get the same problem then it would suggest a HW problem. You might find that a new power button circuit and connectors will fix it but the impression I get from a bit of reading is that many people wish they had bought a different laptop.
  2. It also happens on backtrack 3 (slax based).

    So you're saying that a typical laptop power button should not send any kind of signal/trigger when pushed in momentarily?

    Well it happens with this laptop. I assume it's a "bonus feature" of this model, but I'm just wondering what the heck it's doing, and why the hardware detection process is impacted. After the system boots up there are no issues.
  3. I suspect it's connected to S3 sleep state but that's just a semi informed guess. As I said there are lots of people reporting issues with the power button circuit and while on a traditional PC these are all on the motherboard I suspect it's part of the Power button daughter board on the laptop you have.
  4. Thanks for the reply. Indeed when I quickly press the power button, the gnome shutdown dialogue pops up, and I've seen that on other laptops as well.

    Since it works as expected there, I do not suspect a hardware problem with the hardware, other than wondering why the bootup process is affected by the button that triggers the S3 sleep state.

    My ultimate question would be how to disable this effect (through a boot option perhaps) so that the laptop will boot smoothly.
  5. Does it boot ok from the hard drive? Does it boot Windows OK? If the answer to those two questions is yes then you might be able to fix with boot options but I really do suspect it's a HW issue, the amount of people having problems with the power button with Windows installed just points in that direction.

    That said I'd love to be proved wrong.
  6. Ok, I've had another look at this guide:


    I think it might well be worth you trying the boot options it covers under the F6 section, in particular try acpi=off and the second and third option in the list. It might yet be enough to prove me wrong on my HW issue theory...

    Let us know how you get on.
  7. Yes, almost all of those extra buttons on laptops these days will send either:
    -An ACPI event (usually the case for power, sleep, etc. buttons)
    -A (possibly non-standard, but some are, e.g. vol+/-) keycode which can be mapped to some function/key (that doesn't exist on your current keyboard), etc.

    In either case, you want to use the appropriate tool to "listen" in for the keystrokes, example:
    brad@onyx:~$ acpi_listen 
    //I press "Fn + F11"
    ibm/hotkey HKEY 00000080 0000100b

    brad@onyx:~$ xev
    //Some initial stuff scrolls by concerning the creation of the window that just popped up
    //press some keys, in this case the vol+
    KeymapNotify event, serial 22, synthetic NO, window 0x0,
        keys:  4294967228 0   0   0   16  0   0   0   0   0   0   0   0   0   0   0   
               0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   
    KeyRelease event, serial 22, synthetic NO, window 0x5400001,
        root 0x27a, subw 0x0, time 356352017, (49,476), root:(1621,1001),
        state 0x0, keycode 36 (keysym 0xff0d, Return), same_screen YES,
    "   XLookupString gives 1 bytes: (0d) "
        XFilterEvent returns: False

    This will at least show you if there is something to latch into in terms of the keys (if you wanted to for later, but yes, the issue you're seeing seems to be boot and ACPI-related, try turning off ACPI)
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