[Intel-gfx] [PATCH] drm/dp: Power cycle display if LINK_ADDRESS fails.

[Pandiyan, Dhinakaran]

On Thu, 2018-01-04 at 23:46 +0000, Pandiyan, Dhinakaran wrote:
> On Thu, 2018-01-04 at 18:21 -0500, Lyude Paul wrote:
> > Sorry for the late reply, I've been having very similar issues on my own MST hub
> > and I wanted to make sure that they were the same issue, although it seems like
> > they aren't.
> > 
> > So; I've been doing a lot of MST debugging this week and last and something I've
> > discovered needs to be taken into account sometimes with MST hubs is the actual
> > state that they're in at the point that the DRM driver detects them. I've
> > managed to on multiple occasions, get my hub into a weird state by:
> > 
> > - Plugging in MST displays into the hub
> > - Turning on the machine
> > - Unplugging MST displays from the hub (while still in the BIOS)
> > - Booting into linux
> > - Plugging MST displays into the hub
> > - Everything times out, the world explodes, the economy collapses, etc.
> > 
> > I think maybe, especially since this should be perfectly valid behavior and not
> > break well or poor behaving hubs, we should do a power cycle with the display
> > like this when the DP port initially detects an MST hub and before we start
> > doing any serious communication with it. Could you see if this fixes your issue
> > instead of the patch you've got here?
> > 
> 
> Well, my reasoning to power cycle after a failure was to not affect hubs
> that work. Besides that I also saw an odd cycle of timeouts and late
> replies when I did this.
> 
> 1. Detect hub
> 2. Toggle power state and send LINK_ADDRESS req.
> 3. LINK_ADDRESS req times out.
> 4. Toggle power state and send LINK_ADDRESS req.
> 5. Get late response for the first (and expired) LINK_ADDRESS req.
> 6. Go to step 3
> 
> It seems like toggling the power states flushes out some message buffers
> in the MST hub.
> 
> But, in the retry approach, power cycling resulted in getting the
> response for the current LINK_ADDRESS request along with the previous
> one that timed out.
> 
> I could not come up with an explanation for all the behavior. So, I
> decided to get back to this later.
> 
> 
> > As well, mind attaching your full dmesg with drm.debug=0x6?
> 
> I don't have the old logs anymore, will try to get something for you.
Here you go

Setup: Laptop -> ThinkPad dock -> Dell MST monitor -> Dell monitor
Hotplugged display to dock[passed] - https://pastebin.com/CemRGsCb
Connected boot[failed] - https://pastebin.com/jjXP6HzB

> 
> -DK
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