[Intel-gfx] [PATCH 1/2] drm/i915: Remove redundant reprobe in i915_drm_resume

[Lyude Paul]

On Thu, 2016-11-03 at 16:25 +0000, Chris Wilson wrote:
> On Thu, Nov 03, 2016 at 12:11:55PM -0400, Lyude Paul wrote:
> > 
> > On Thu, 2016-11-03 at 12:11 -0400, Lyude Paul wrote:
> > > 
> > > On Thu, 2016-11-03 at 16:02 +0000, Chris Wilson wrote:
> > > > 
> > > > 
> > > > On Thu, Nov 03, 2016 at 11:42:37AM -0400, Lyude wrote:
> > > > > 
> > > > > 
> > > > > 
> > > > > Weine's investigation on benchmarking the suspend/resume process
> > > > > pointed
> > > > > out a lot of the time in suspend/resume is being spent reprobing.
> > > > > While
> > > > > the reprobing process is a lengthy one for good reason, we don't need
> > > > > to
> > > > > hold up the entire suspend/resume process while we wait for it to
> > > > > finish. Luckily as it turns out, we already trigger a full connector
> > > > > reprobe in i915_hpd_poll_init_work(), so we can just ditch reprobing
> > > > > in
> > > > > i915_drm_resume() entirely.
> > > > > 
> > > > > This won't lead to less time spent resuming just yet since now the
> > > > > bottleneck will be waiting for the mode_config lock in
> > > > > drm_kms_helper_poll_enable(), since that will be held as long as
> > > > > i915_hpd_poll_init_work() is reprobing all of the connectors. But
> > > > > we'll
> > > > > address that in the next patch.
> > > > > 
> > > > > Signed-off-by: Lyude <lyude at redhat.com>
> > > > > Cc: David Weinehall <david.weinehall at linux.intel.com>
> > > > > ---
> > > > >  drivers/gpu/drm/i915/i915_drv.c | 2 --
> > > > >  1 file changed, 2 deletions(-)
> > > > > 
> > > > > diff --git a/drivers/gpu/drm/i915/i915_drv.c
> > > > > b/drivers/gpu/drm/i915/i915_drv.c
> > > > > index bfb2efd..532cc0f 100644
> > > > > --- a/drivers/gpu/drm/i915/i915_drv.c
> > > > > +++ b/drivers/gpu/drm/i915/i915_drv.c
> > > > > @@ -1602,8 +1602,6 @@ static int i915_drm_resume(struct drm_device
> > > > > *dev)
> > > > >  	 * notifications.
> > > > >  	 * */
> > > > >  	intel_hpd_init(dev_priv);
> > > > > -	/* Config may have changed between suspend and resume */
> > > > > -	drm_helper_hpd_irq_event(dev);
> > > > 
> > > > The comment is still apt. This code is known to be broken since it
> > > > doesn't detect a change in monitors (e.g. a change in external
> > > > connectors
> > > > from docking) between suspend and resend. We still have to send the
> > > > uevent.
> > > > 
> > > > +	drm_kms_helper_hotplug_event(dev);
> > > 
> > > I might not have explained myself very well. The way things should look
> > > with
> > > this patch is like this:
> > > 
> > > i915_drm_resume()
> > >  -> intel_hpd_init()
> > >    -> sets dev_priv->hotplug.poll_enabled to true
> > Whoops, s/true/false/
> > 
> > > 
> > >    -> schedules dev_priv->hotplug.poll_init_work
> > >  -> continue resume…
> > > 
> > > at the same time:
> > > 
> > > i915_hpd_poll_init_work() gets scheduled and starts
> > >  -> since dev_priv->hotplug.poll_enabled == false,
> > > drm_helper_hpd_irq_event()
> > > is called
> > >   -> drm_helper_hpd_irq_event() reprobes connectors
> > >    -> if anything changed, drm_kms_helper_hotplug_event() gets called.
> 
> drm_helper_hpd_irq_event() does not detect a change in monitors, for
> example, so there is no uevent in that case.

I still think you're mistaken (and I wouldn't blame you,
drm_helper_hpd_irq_event() has a rather misleading name). Contents
of drm_helper_hpd_irq_event()

bool drm_helper_hpd_irq_event(struct drm_device *dev)
{
	struct drm_connector *connector;
	enum drm_connector_status old_status;
	bool changed = false;

	if (!dev->mode_config.poll_enabled)
		return false;

	mutex_lock(&dev->mode_config.mutex);
	drm_for_each_connector(connector, dev) {

		/* Only handle HPD capable connectors. */
		if (!(connector->polled & DRM_CONNECTOR_POLL_HPD))
			continue;

		old_status = connector->status;

		connector->status = connector->funcs->detect(connector, false);
		DRM_DEBUG_KMS("[CONNECTOR:%d:%s] status updated from %s to
%s\n",
			      connector->base.id,
			      connector->name,
			      drm_get_connector_status_name(old_status),
			      drm_get_connector_status_name(connector->status));
		if (old_status != connector->status)
			changed = true;
	}

	mutex_unlock(&dev->mode_config.mutex);

	if (changed)
		drm_kms_helper_hotplug_event(dev);

	return changed;
}

Unless I made a mistake somewhere down the line of writing these: poll_enabled
will be set to true (due to the change done in the second patch where we move
the call for enabling polling up so that it gets called before
intel_hpd_init()), which causes it to go through all the connectors and call
connector->funcs->detect() (which will be intel_dp_detect(),
intel_hdmi_detect(), etc. And then send a uevent if their status changed.

This being said there's a couple of cases I know we don't handle correctly from
writing up igt tests with the chamelium: we don't simulate a disconnect +
connect when the EDID changed from what we had last, so if we suspend the system
with monitor A connected to port foo and resume it with monitor B connected to
port foo, it will still think we're connected to monitor A. However, we don't
handle this properly with the current code either.

As for connector->funcs->detect(), it does do a real connector reprobe as
opposed to just checking the status of the HPD pins, I'd look at the detect()
functions if you're still skeptical.

> -Chris
>