[Intel-gfx] [PATCH] drm/i915: Avoid fluctuating to UNKNOWN connector status during forced probes

[Chris Wilson]

On Mon, Jun 15, 2015 at 05:18:40PM +0200, Daniel Vetter wrote:
> On Mon, Jun 15, 2015 at 02:37:58PM +0100, Chris Wilson wrote:
> > On Mon, Jun 15, 2015 at 03:03:39PM +0200, Daniel Vetter wrote:
> > > On Mon, Jun 15, 2015 at 01:35:21PM +0100, Chris Wilson wrote:
> > > > On Mon, Jun 15, 2015 at 02:29:28PM +0200, Daniel Vetter wrote:
> > > > > On Fri, Jun 05, 2015 at 08:46:19AM +0100, Chris Wilson wrote:
> > > > > > For old-school component TV and CRT connectors, we require a heavyweight
> > > > > > load-detection mechanism. This involves setting up a CRTC and sending a
> > > > > > signal to the output, before reading back any response. As that is quite
> > > > > > slow and CPU heavy, the process is only performed when the output
> > > > > > detection is forced by user request. As it requires a driving CRTC, we
> > > > > > often don't have the resources to complete the probe. This leaves us in
> > > > > > a quandary where the unforced path just returns the old connector
> > > > > > status, but the forced detection path elects to return UNKNOWN. If we
> > > > > > have an active connection, we likely have the resources available to
> > > > > > complete the probe - but if it is currently disconnected, then it
> > > > > > becomes unknown and triggers a hotplug event, with often quite unfortunate
> > > > > > userspace behaviour (e.g. one output is blanked and the spurious TV
> > > > > > turned on).
> > > > > > 
> > > > > > To reduce spurious hotplug events on older devices, we can prevent
> > > > > > transitions between disconnected <-> unknown.
> > > > > > 
> > > > > > v2: Convert tv_type to use proper unknown enum (Ville)
> > > > > > 
> > > > > > References: https://bugs.freedesktop.org/show_bug.cgi?id=87049
> > > > > > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > > > > > Cc: Ville Syrjälä <ville.syrjala at linux.intel.com> [v1]
> > > > > > Reviewed-by: Ville Syrjälä <ville.syrjala at linux.intel.com> [v1]
> > > > > 
> > > > > This solution is at odds with
> > > > > 
> > > > > commit b7703726251191cd9f3ef3a80b2d9667901eec95
> > > > > Author: Daniel Vetter <daniel.vetter at ffwll.ch>
> > > > > Date:   Wed Jan 21 08:45:22 2015 +0100
> > > > > 
> > > > >     drm/probe-helper: clamp unknown connector status in the poll work
> > > > > 
> > > > > We're missing this bit of logic from the hotplug handlers, but that was
> > > > > somewhat intentional since a hotplug should indicate that something has
> > > > > changed. And in the i915 hpd handler we filter by source.
> > > > > 
> > > > > Given that the report is older than me having resurrect that patch can we
> > > > > close it as fixed or do I miss something?
> > > > 
> > > > It's a different path. This also concerns the actual forced reprobe
> > > > fluctuating between two states.
> > > 
> > > In that case I still think it should be in the probe helpers. Which raises
> > > the policy decision whether we should ever hand unkown back to userspace
> > > since apparently it just can't cope sensible with it. But that call should
> > > be done consistently accross drivers.
> > 
> > Hmm, the probe helper is not the central arbiter here which is a problem
> > in moving it entirely into its domain. UNKNOWN makes a sense form the hw
> > perspective, and we still need to report that status before any probes
> > are made at all (whether that is init/resume) and the problem in this
> > case is not so much as userspace mishandling it, but the fluctuation
> > between UNKNOWN/DISCONNECTED is causing activity and reconfiguration not
> > entirely of its own fault.
> 
> Well for the poll worker part of the probe helpers I went with
> 
> 	if (new_status == unknown)
> 		connector->status = old_status;
> 
> which takes care of the init/resume time unknown->(dis)connected
> transition correctly while filtering the noise. Adding that to the
> synchronous users probe function (which is the only one that sets force ==
> true) would achieve what you want I think. But it also means that we
> almost always filter out unknown and never let userspace known that the hw
> detection is uncertain.

I thought that connected->unknown was less likely and of more
significance than disconnected<->unknown. But that was from thinking
that the output was in use whilst it was connected, and so should always
have the resources available to do the detection. On second thought, we
should always just suppress the ->unknown transition.

The best way to then report the unreliable detection would to be report
an error trying to connect to the CRT/TV (by a vblank load detect cycle)
and signal EREMOTEIO in enable -- but the modesetting code still does
not report errors, despite their continued prevalence (such as training
failures).

> Maybe we instead need a drm helper module option to clamp unknown
> uncondiotionally to disconnected to allow users to hack around silly
> userspace?

We have userspace that respects unknown, after it is what the fb_helper
was copied from. Unknown is unknown, userspace can either treat it as
potentially connector or potentially disconnected depending on best that
suites its purposes. So I don't think a global change is sensible.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre