drm/i915/crt: Do not rely upon the HPD presence pin

VGA hotplug detection "works" by measuring the resistance across
certain pins. A lot of kvm switches fumble this and wire up cheap
resistors with the wrong resistance or don't bother at all.

To accomodate these, also try to detect a connected monitor by trying
to grab the edid. Contrary to !HAS_HOTPLUG platforms we don't bother
with an actual load-detection cycle when the output is life - that
would be actual work to implement because things moved around. This is
the big difference to Chris Wilson's original approach:

commit 9e612a008fa7fe493a473454def56aa321479495
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Thu May 31 13:08:53 2012 +0100

    drm/i915/crt: Do not rely upon the HPD presence pin

This blew up on Linus' machine because it errornously detected a vga
screen (without and edid and hence only the default modes), leading to
it's prompt removal:

commit 8f53369b753f5f4c7684c2eb0b592152abb1dd00
Author: Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri Jun 8 14:53:06 2012 -0700

    Revert "drm/i915/crt: Do not rely upon the HPD presence pin"

Some digging around in Bspec shows the reason why load detect doesn't work on
newer chips - the legacy VGA load detect bit isn't wired up any longer:

Public Snb Bspec, Vol3 Part1, 1.1.1 ST00 Input Status 0, bit4:

"RGB Comparator / Sense. This bit is here for compatibility and will
always return one. Monitor detection must be done be done through the
programming of registers in the MMIO space.
0 = Below threshold
1 = Above threshold"

v2: Add a comment in the code that load detect on hotplug capable
machines is broken and pimp the commit message with a quote of Bspec
to show why.

Reported-and-tested-by: Matthieu LAVIE <boiteamadmax@hotmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=50501
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

diff --git a/drivers/gpu/drm/i915/intel_crt.c b/drivers/gpu/drm/i915/intel_crt.c
index 75a70c46ef1be81bf77fddbfd43e8e09a213eb71..5978490dac9265719e886219f65f3d021eab3e58 100644 (file)
--- a/drivers/gpu/drm/i915/intel_crt.c
+++ b/drivers/gpu/drm/i915/intel_crt.c
@@ -453,18 +453,27 @@ intel_crt_detect(struct drm_connector *connector, bool force)
        struct intel_load_detect_pipe tmp;
 
        if (I915_HAS_HOTPLUG(dev)) {
+               /* We can not rely on the HPD pin always being correctly wired
+                * up, for example many KVM do not pass it through, and so
+                * only trust an assertion that the monitor is connected.
+                */
                if (intel_crt_detect_hotplug(connector)) {
                        DRM_DEBUG_KMS("CRT detected via hotplug\n");
                        return connector_status_connected;
-               } else {
+               } else
                        DRM_DEBUG_KMS("CRT not detected via hotplug\n");
-                       return connector_status_disconnected;
-               }
        }
 
        if (intel_crt_detect_ddc(connector))
                return connector_status_connected;
 
+       /* Load detection is broken on HPD capable machines. Whoever wants a
+        * broken monitor (without edid) to work behind a broken kvm (that fails
+        * to have the right resistors for HP detection) needs to fix this up.
+        * For now just bail out. */
+       if (I915_HAS_HOTPLUG(dev))
+               return connector_status_disconnected;
+
        if (!force)
                return connector->status;