Re: [Intel-gfx] [PATCH] drm/i915: Don't send MST hotplugs until after resume

[Imre Deak]

On Fri, Jan 25, 2019 at 08:24:35PM -0500, Lyude Paul wrote:
> Turns out we are sending a lot more hotplug events then we need, and
> this is causing some pretty serious issues. Currently, we call
> intel_dp_mst_resume() in i915_drm_resume() well before we have any sort
> of hotplugging setup.

We call hpd_irq_setup() before calling intel_dp_mst_resume(). The only
purpose of that part (lifted out from intel_hpd_init()) is to provide
the short HPD interrupt functionality MST AUX transfers need.

But you are right in that - as a side-effect - we'll also enable generic
hotplug functionality that is independent of the above MST requirement.
Doing that kind of generic hotplug processing before
intel_display_resume() is probably not a good idea, it can interfere at
least with the mode restore in __intel_display_resume().

> This is a pretty big problem, because in practice it will generally
> result in throwing the power domain refcounts out of wack.
> 
> For instance: On my T480s, removing a previously connected topology
> before the system finishes resuming causes
> drm_kms_helper_hotplug_event() to be called before HPD is setup again,
> which causes us to do a connector reprobe, which then causes
> intel_dp_detect() to be called on all DP devices -including- the eDP
> display. From there, intel_dp_detect() is run on the eDP display which
> triggers DPCD transactions. Those DPCD transactions then cause us to
> call edp_panel_vdd_on(), which then causes us to grab an additional
> wakeref to the relevant power wells (PORT_DDI_A_IO on this machine).
> From there, this wakeref is never released which then causes the next
> suspend/resume cycle to entirely fail due to the hardware not being
> powered off correctly.
> 
> This sucks really badly, and I don't see any decent way to actually fix
> this in intel_dp_detect() easily. Additionally, I don't even think it'd
> be worth the time now since we're not expecting to handle any kind of
> connector reprobing at the point in which we call intel_dp_mst_resume(),
> but we also can't move intel_dp_mst_resume() any higher in the resume
> process since MST topologies need to be resumed before
> intel_display_resume() is called.
> 
> However, there's a light at the end of the tunnel! After reading through
> a lot of code dozens of times, it occurred to me that we -never-
> actually need to send hotplug events when calling
> drm_dp_mst_topology_mgr_set_mst() since we send hotplug events in
> drm_dp_destroy_connector_work(). Imagine that!
> 
> So, since we only seem to call intel_dp_mst_check_status() to disable
> MST on the encoder in question and then send a hotplug, get rid of this
> and instead just disable MST mode when a hub fails in
> intel_dp_mst_resume(). From there, drm_dp_destroy_connector_work() will
> eventually send the hotplug event.
> 
> Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx>
> Fixes: 0e32b39ceed6 ("drm/i915: add DP 1.2 MST support (v0.7)")
> Cc: Todd Previte <tprevite@xxxxxxxxx>
> Cc: Dave Airlie <airlied@xxxxxxxxxx>
> Cc: Jani Nikula <jani.nikula@xxxxxxxxxxxxxxx>
> Cc: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
> Cc: Rodrigo Vivi <rodrigo.vivi@xxxxxxxxx>
> Cc: intel-gfx@xxxxxxxxxxxxxxxxxxxxx
> Cc: <stable@xxxxxxxxxxxxxxx> # v3.17+

Not knowing enough about the MST code, but we do need to prevent
generic hotplug processing at this point:

Acked-by: Imre Deak <imre.deak@xxxxxxxxx>


> ---
>  drivers/gpu/drm/i915/intel_dp.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> index 681e88405ada..c2399acf177b 100644
> --- a/drivers/gpu/drm/i915/intel_dp.c
> +++ b/drivers/gpu/drm/i915/intel_dp.c
> @@ -7096,7 +7096,10 @@ void intel_dp_mst_resume(struct drm_i915_private *dev_priv)
>  			continue;
>  
>  		ret = drm_dp_mst_topology_mgr_resume(&intel_dp->mst_mgr);
> -		if (ret)
> -			intel_dp_check_mst_status(intel_dp);
> +		if (ret) {
> +			intel_dp->is_mst = false;
> +			drm_dp_mst_topology_mgr_set_mst(&intel_dp->mst_mgr,
> +							false);
> +		}
>  	}
>  }
> -- 
> 2.20.1
> 
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