linux-kernel - Re: 2.6.25-rc7: Ugh.

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Message-ID: <47F11FC6.8010701@rtr.ca>
Date:	Mon, 31 Mar 2008 13:30:46 -0400
From:	Mark Lord <lkml@....ca>
To:	Oliver Neukum <oliver@...kum.org>
Cc:	Pavel Machek <pavel@...e.cz>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Greg KH <gregkh@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>, jikos@...e.cz
Subject: Re: 2.6.25-rc7:  Ugh.

Mark Lord wrote:
> Mark Lord wrote:
>> Mark Lord wrote:
>>> Oliver Neukum wrote:
>>>> Am Montag, 31. März 2008 17:04:46 schrieb Mark Lord:
>>>>> Oliver Neukum wrote:
>>>>>> Am Montag, 31. März 2008 16:39:33 schrieb Mark Lord:
>> ..
>>>>>>> Still want USB_SUSPEND=n ?  Please explain.
>>>>>> It looks like you are hanging in the kthread for autosuspending.
>>>>>> Compiling that out should confirm it.
>>>>> ..
>>>>>
>>>>> Okay, and once we see that it works fine:  then what?
>>>>
>>>> We'll combine that information with the result of only removing usbhid
>>>> and arrive at a pretty good idea where in the kernel the hang occurs.
>>>> There are only two functions that touch autosuspend in the usbhid code.
>>>> So if it works with usbhid unloaded, either of them should be to blame.
>>> ..
>>> It does still hang with *usbhid* unloaded,
>>> but not if all USB stuff is unloaded.
>> ..
>>
>> And it does *not* hang with # CONFIG_USB_SUSPEND is not set.
>>
>>> I have figured out a way to make this much more reproducible now:
>>> When suspended, the notebook does not supply +5V over USB.
>>> But with a voltmeter, I discovered that there is sufficient capacitance
>>> on the USB +5V, that it takes many minutes for the voltage to decay
>>> from 5.1V down to near 0V.
>>>
>>> Resuming while the voltage is still relatively high, generally works.
>>> Resuming after the voltage drops to near zero, always fails (with USB 
>>> modules loaded).
>>>
>>> So I've put a 2Kohm resistor across the USB +5V lines,
>>> forcing it to decay to zero within about 5 seconds.
>>> This helps a lot for debugging here.
>>>
>>> It probably also provides a vital clue as to what is wrong.
>>> Resume seems to generally work when the USB devices maintain
>>> some amount of standby power, and always fails when they don't.
> ..
> 
> Note that it makes no difference whether I unplug all external USB devices
> prior to suspend or not.  The failure patterns remain the same.
..

I've now removed the internal USB-Bluetooth adaptor, so we can now test
without any USB devices connected.

Suspend without any USB devices plugged-in, and it *always* resumes fine
with working USB, even when the USB stuff is plugged in before hitting
the resume button.

But suspend *with* any USB device plugged-in, and it *always* fails resume,
even when the USB device is unplugged before hitting the resume button.

Conclusion:   the bug is in the usb SUSPEND code, not the RESUME code.
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