[PATCH 16/16] drm/fb-helper: don't sleep for screen unblank when an oopps is in progress

[Daniel Vetter]

Otherwise the system will burn even brighter and worse, leave the user
wondering what's going on exactly.

Since we already have a panic handler which will (try) to restore the
entire fbdev console mode, we can just bail out. Inspired by a patch
from Konstantin Khlebnikov. The callchain leading to this, cut&pasted
from Konstantin's original patch:

callstack:
panic()
bust_spinlocks(1)
unblank_screen()
vc->vc_sw->con_blank()
fbcon_blank()
fb_blank()
info->fbops->fb_blank()
drm_fb_helper_blank()
drm_fb_helper_dpms()
drm_modeset_lock_all()
mutex_lock(&dev->mode_config.mutex)

Note that the entire locking in the fb helper around panic/sysrq and
kdbg is ... non-existant. So we have a decent change of blowing up
everything. But since reworking this ties in with funny concepts like
the fbdev notifier chain or the impressive things which happen around
console_lock while oopsing, I'll leave that as an exercise for braver
souls than me.

Cc: Konstantin Khlebnikov <khlebnikov at openvz.org>
Cc: Andrew Morton <akpm at linux-foundation.org>
References: https://patchwork.kernel.org/patch/1878181/
Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>
---
 drivers/gpu/drm/drm_fb_helper.c |    8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index b774101..a3b4ecf 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -371,6 +371,14 @@ static void drm_fb_helper_dpms(struct fb_info *info, int dpms_mode)
 	int i, j;
 
 	/*
+	 * fbdev->blank can be called from irq context in case of a panic.
+	 * Since we already have our own special panic handler which will
+	 * restore the fbdev console mode completely, just bail out early.
+	 */
+	if (oops_in_progress)
+		return -EBUSY;
+
+	/*
 	 * For each CRTC in this fb, turn the connectors on/off.
 	 */
 	drm_modeset_lock_all(dev);
-- 
1.7.10.4