Re: periodic wakeup from DPMS suspend

[Daniel Vetter <daniel@ffwll.ch>]

On Thu, Oct 08, 2015 at 12:44:09PM +0200, Johannes Stezenbach wrote:
> On Wed, Oct 07, 2015 at 01:26:32PM +0200, Johannes Stezenbach wrote:
> > On Tue, Oct 06, 2015 at 11:04:53PM -0400, Alex Deucher wrote:
> > > On Tue, Oct 6, 2015 at 11:10 AM, Johannes Stezenbach <js@sig21.net> wrote:
> > > >
> > > > I have a NEC EA244WMi monitor connected to an Asus P8H77-V
> > > > mainboard with Ivy Bridge Core i5-3550 via DVI.
> > > > If DPMS suspend is enabled (by xscreensaver, or for testing by
> > > > "xset dpms force off/suspend/standby"), the monitor
> > > > enters standby mode but wakes up every 10...30 seconds for
> > > > 6 seconds to display a "DVI-D: no signal" message.
> > > 
> > > Some monitors periodically scan all of their inputs if they are not
> > > actively being driven by anything to try and automatically switch to
> > > the connected input.  When the monitor scans the DVI input, it sees
> > > load on the pins and turns on, only to realize it's not getting a
> > > signal and then turns off.  If your monitor has an option to turn off
> > > input probing, that might help.
> > 
> > It's not like I didn't try to twiddle just about every
> > available menu item, and also used the monitor's
> > "Reset to Factory Defaults" to no avail.
> > However, while trying again I found a hidden menu item
> > which is only available during the short time the
> > "No Signal" message is displayed, where the left/right
> > touch buttons open a "Scan Time" setting with "Normal"
> > and "Slow" options.  After I changed it, the issue was
> > solved.  I changed it back and the issue doesn't reproduce.
> > So it looks some internal configuration of the monitor
> > was messed up and is now fixed by twiddling the hidden
> > menu item.  "Scan Time" is not documented in the manual
> > nor is there an indication in the "No Signal" popup.
> > 
> > Case closed.
> 
> And I just came back to see the monitor doing the
> wakeup cycling again.  To try something differerent
> I unplugged the charger of the laptop sitting nearby
> in suspend.  Bingo!  So it looks like an EMI issue.
> I tried a few times and could reproduce twice and
> then no more :-(  Now the monitor wakes up once
> every 5min or so...
> 
> Which brings me back to a previous question:
> 
> > maybe it is some floating signal line causing the
> > monitor to misdetect activity?
> 
> Does the Intel hardware have the capability to switch
> the DVI signal lines from high-z to ground during DPMS
> suspend, and if so, does the driver do the right thing?
> Obviously I have no clue about DVI and intel-gfx, but
> I thought to ask anyway.

We have already known fun with EMI from laptop chargers: i915 hase a
hotpug storm detection logic to combat badly-shielded boards. Could just
be another form of that problem. Afaik there's nothing we can do in the
driver about this (except trying to filter out bad noise, which we do
already).
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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