[PATCH] drm/i915: Fix DisplayPort Hotplug

[Palmer Dabbelt <palmer@dabbelt.com>]

DisplayPort no longer hotplugs on my machine (a 2014 MacBook Pro
attached to an ASUS PB287Q).  I believe the problem is that long pulses
are no longer triggering intel_dp_check_link_status.  I bisected the
problem down to (7d23e3c37 "drm/i915: Cleaning up intel_dp_hpd_pulse")
and it appears the only material change there was to remove one of those
calls.

This commit adds the check_link_status call back in, which causes hotplug to
work again for me.  I test this via a "xset dpms force off".  Note that this is
very similar to <https://bugs.freedesktop.org/show_bug.cgi?id=89453>, but it's
back again.

I've tested this against 4.9, and it applies against the current head.

See <https://bugs.freedesktop.org/show_bug.cgi?id=99766>.

Here's the commit that triggered the regression:

commit 7d23e3c37bb3fc6952dc84007ee60cb533fd2d5c
Author: Shubhangi Shrivastava <shubhangi.shrivastava@intel.com>
Date:   Wed Mar 30 18:05:23 2016 +0530

    drm/i915: Cleaning up intel_dp_hpd_pulse

    Current DP detection has DPCD operations split across
    intel_dp_hpd_pulse and intel_dp_detect which contains
    duplicates as well. Also intel_dp_detect is called
    during modes enumeration as well which will result
    in multiple dpcd operations. So this patch tries
    to solve both these by bringing all DPCD operations
    in one single function and make intel_dp_detect
    use existing values instead of repeating same steps.

Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>
---
 drivers/gpu/drm/i915/intel_dp.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
index 0b8e8eb..32ca4be 100644
--- a/drivers/gpu/drm/i915/intel_dp.c
+++ b/drivers/gpu/drm/i915/intel_dp.c
@@ -4831,6 +4831,10 @@ intel_dp_hpd_pulse(struct intel_digital_port *intel_dig_port, bool long_hpd)
 		      long_hpd ? "long" : "short");
 
 	if (long_hpd) {
+		drm_modeset_lock(&dev->mode_config.connection_mutex, NULL);
+		intel_dp_check_link_status(intel_dp);
+		drm_modeset_unlock(&dev->mode_config.connection_mutex);
+
 		intel_dp->detect_done = false;
 		return IRQ_NONE;
 	}
-- 
2.10.2

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