[Intel-gfx] [PATCH 13/19] drm: do not steal the display if we have a master

[Paulo Zanoni]

From: Paulo Zanoni <paulo.r.zanoni at intel.com>

Sometimes we want to disable all the screens on a system, because that
will allow the graphics card to be put into low-power states. The
problem is that, for example, while all screens are disabled, if we
get a hotplug interrupt, fbcon will decide to set a mode instead of
keeping everything disabled, which will remove us from our low power
states.

Let's assume that if there's a DRM master, it will be able to do
whatever is appropriate when we get the hotplug.

This problem can be reproduced by the runtime PM test program from
intel-gpu-tools: we disable all the screens so the graphics device can
be put into D3, then something triggers a hotplug interrupt, fbcon
sets a mode and breaks our test suite. The problem can be reproduced
more easily by the "i2c" subtest.

Other approaches considered for the problem:
    - Return "false" if "bound == 0" and the caller of
      drm_fb_helper_is_bound is a hotplug handler. This would break
      the case where the machine boots with no outputs connected, then
      the user plugs a monitor.
    - Add a new IOCTL to force fbcon to not set modes. This would keep
      all the current applications behaving the same, but adding a new
      IOCTL is not always the greatest idea.

Thanks to Daniel Vetter for the investigation, ideas and the
implementation of the hotplug alternative.

Credits-to: Daniel Vetter <daniel.vetter at ffwll.ch>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni at intel.com>
---
 drivers/gpu/drm/drm_fb_helper.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index 0a19401..199d0c0 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -368,6 +368,12 @@ static bool drm_fb_helper_is_bound(struct drm_fb_helper *fb_helper)
 
 	if (bound < crtcs_bound)
 		return false;
+
+	/* Sometimes user space wants everything disabled, so don't steal the
+	 * display if there's a master. */
+	if (bound == 0 && dev->primary->master)
+		return false;
+
 	return true;
 }
 
-- 
1.8.3.1