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

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Message-ID: <47F11D8F.4020209@rtr.ca>
Date:	Mon, 31 Mar 2008 13:21:19 -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:
>> 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.
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