[Intel-gfx] drm i915 weirdness with /sys/class/drm/card0*/status

[Chris Wilson]

On Tue, Jun 16, 2015 at 01:17:29PM +0200, Lennart Poettering wrote:
> On Tue, 16.06.15 10:14, Chris Wilson (chris at chris-wilson.co.uk) wrote:
> 
> > The biggest change here is 4.1 stopped forcing the probe from sysfs
> > precisely because systemd was hitting them so often for illogical
> > reasons (being docked depends on having the lid open and an
> > external display connected!). To force the probe, you must do
> > 	$ echo detect > /sys/class/drm/*/status
> 
> The way people "dock" their laptops these days is apparently that
> they plug in an external keyboard+mouse and a display, close the lid
> and stash the laptop away somewhere. logind hence counts the displays
> connected, and if its >= 2 it doesn't suspend the machine even if the
> lid is closed. Before logind, GNOME implemented the very same
> logic. It's hence hardly "illogical", it's just how this works...

Note that a closed lid causes the internal panel to be reported as
disconnected - it always has (with the exception of a module parameter
to force otherwise to accommodate broken lid buttons). Using the number
of connected displays is thus flawed.

> > > We don't really depend on the userspace doing the reprobe, do we? On
> > > resume, we reprobe the HPD capable connectors and schedule a poll for
> > > the rest. Is it even possible the userspace sees unknown status for HPD
> > > capable connectors on resume? Makes me wonder if there's something going
> > > wrong with the hotplug irqs.
> > 
> > The unknown status is much more likely after boot, iirc. Unknown is not
> > a big problem because very, very few applications actually query the
> > current value without doing a probe (basically the new sysfs entry and
> > X during takeover). We simply have not been able to trust hotplug to
> > allow us to cache connector status at any level.
> 
> Well, logind does not probe right now. I was not aware one has to do
> this.
> 
> Is there any reason the kernel doesn't automatically probe the
> connectors when userspace asks for it and would return "unknown"
> otherwise?

Probing is very slow and will cause flicker on some machines.
 
> I can also implement a logic like that in logind, but then again,
> maybe logind should leave the probing the connectors to X or the
> kernel to figure out on their own?

Yes. logind is only using the status to count the number of connected
outputs, when it was probing I noticed because logind was using more CPU
time during X startup/shutdown than X was itself.
 
> Is the API of these files documented anywhere?

The status is the first one we added a store attribute to, probably why
we overlooked expanding the none existent documentation. We also have not
documented the meaning of the reported string.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre