linux-kernel - Re: [Intel-gfx] [PATCH 1/2] drm/i915: Remove redundant reprobe in i915_drm

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Date:   Thu, 03 Nov 2016 12:11:55 -0400
From:   Lyude Paul <cpaul@...hat.com>
To:     Chris Wilson <chris@...is-wilson.co.uk>, Lyude <lyude@...hat.com>
Cc:     Daniel Vetter <daniel.vetter@...el.com>,
        intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org
Subject: Re: [Intel-gfx] [PATCH 1/2] drm/i915: Remove redundant reprobe in
 i915_drm_resume

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@...hat.com>
> > > Cc: David Weinehall <david.weinehall@...ux.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.
> 
> So we're still polling the connectors when coming out of resume just like
> before, except now we're doing it without needlessly making the whole resume
> process stall until we're done. We're also no longer reprobing display
> connectors twice…
> 
> > 
> > 
> > which also depends upon us actually reseting the connector->status to
> > unknown in drm_mode_config_reset(), Daniel!
> > -Chris
> >