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

[Daniel Vetter]

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.

Maybe we instead need a drm helper module option to clamp unknown
uncondiotionally to disconnected to allow users to hack around silly
userspace?
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch