drm: Take lock around probes for drm_fb_helper_hotplug_event

We need to hold the dev->mode_config.mutex whilst detecting the output
status. But we also need to drop it for the call into
drm_fb_helper_single_fb_probe(), which indirectly acquires the lock when
attaching the fbcon.

Failure to do so exposes a race with normal output probing. Detected by
adding some warnings that the mutex is held to the backend detect routines:

[   17.772456] WARNING: at drivers/gpu/drm/i915/intel_crt.c:471 intel_crt_detect+0x3e/0x373 [i915]()
[   17.772458] Hardware name: Latitude E6400
[   17.772460] Modules linked in: ....
[   17.772582] Pid: 11, comm: kworker/0:1 Tainted: G        W 2.6.38.4-custom.2 #8
[   17.772584] Call Trace:
[   17.772591]  [<ffffffff81046af5>] ? warn_slowpath_common+0x78/0x8c
[   17.772603]  [<ffffffffa03f3e5c>] ? intel_crt_detect+0x3e/0x373 [i915]
[   17.772612]  [<ffffffffa0355d49>] ?  drm_helper_probe_single_connector_modes+0xbf/0x2af [drm_kms_helper]
[   17.772619]  [<ffffffffa03534d5>] ?  drm_fb_helper_probe_connector_modes+0x39/0x4d [drm_kms_helper]
[   17.772625]  [<ffffffffa0354760>] ?  drm_fb_helper_hotplug_event+0xa5/0xc3 [drm_kms_helper]
[   17.772633]  [<ffffffffa035577f>] ? output_poll_execute+0x146/0x17c [drm_kms_helper]
[   17.772638]  [<ffffffff81193c01>] ? cfq_init_queue+0x247/0x345
[   17.772644]  [<ffffffffa0355639>] ? output_poll_execute+0x0/0x17c [drm_kms_helper]
[   17.772648]  [<ffffffff8105b540>] ? process_one_work+0x193/0x28e
[   17.772652]  [<ffffffff8105c6bc>] ? worker_thread+0xef/0x172
[   17.772655]  [<ffffffff8105c5cd>] ? worker_thread+0x0/0x172
[   17.772658]  [<ffffffff8105c5cd>] ? worker_thread+0x0/0x172
[   17.772663]  [<ffffffff8105f767>] ? kthread+0x7a/0x82
[   17.772668]  [<ffffffff8100a724>] ? kernel_thread_helper+0x4/0x10
[   17.772671]  [<ffffffff8105f6ed>] ? kthread+0x0/0x82
[   17.772674]  [<ffffffff8100a720>] ? kernel_thread_helper+0x0/0x10

Reported-by:  Frederik Himpe <fhimpe@telenet.be>
References: https://bugs.freedesktop.org/show_bug.cgi?id=36394
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_fb_helper.c

@@ -1516,17 +1516,33 @@ bool drm_fb_helper_initial_config(struct drm_fb_helper *fb_helper, int bpp_sel)
 }
 EXPORT_SYMBOL(drm_fb_helper_initial_config);
 
-bool drm_fb_helper_hotplug_event(struct drm_fb_helper *fb_helper)
+/**
+ * drm_fb_helper_hotplug_event - respond to a hotplug notification by
+ *                               probing all the outputs attached to the fb.
+ * @fb_helper: the drm_fb_helper
+ *
+ * LOCKING:
+ * Called at runtime, must take mode config lock.
+ *
+ * Scan the connectors attached to the fb_helper and try to put together a
+ * setup after *notification of a change in output configuration.
+ *
+ * RETURNS:
+ * 0 on success and a non-zero error code otherwise.
+ */
+int drm_fb_helper_hotplug_event(struct drm_fb_helper *fb_helper)
 {
+	struct drm_device *dev = fb_helper->dev;
 	int count = 0;
 	u32 max_width, max_height, bpp_sel;
 	bool bound = false, crtcs_bound = false;
 	struct drm_crtc *crtc;
 
 	if (!fb_helper->fb)
-		return false;
+		return 0;
 
-	list_for_each_entry(crtc, &fb_helper->dev->mode_config.crtc_list, head) {
+	mutex_lock(&dev->mode_config.mutex);
+	list_for_each_entry(crtc, &dev->mode_config.crtc_list, head) {
 		if (crtc->fb)
 			crtcs_bound = true;
 		if (crtc->fb == fb_helper->fb)
@@ -1535,7 +1551,8 @@ bool drm_fb_helper_hotplug_event(struct drm_fb_helper *fb_helper)
 
 	if (!bound && crtcs_bound) {
 		fb_helper->delayed_hotplug = true;
-		return false;
+		mutex_unlock(&dev->mode_config.mutex);
+		return 0;
 	}
 	DRM_DEBUG_KMS("\n");
 
@@ -1546,6 +1563,7 @@ bool drm_fb_helper_hotplug_event(struct drm_fb_helper *fb_helper)
 	count = drm_fb_helper_probe_connector_modes(fb_helper, max_width,
 						    max_height);
 	drm_setup_crtcs(fb_helper);
+	mutex_unlock(&dev->mode_config.mutex);
 
 	return drm_fb_helper_single_fb_probe(fb_helper, bpp_sel);
 }

include/drm/drm_fb_helper.h

@@ -127,7 +127,7 @@ void drm_fb_helper_fill_fix(struct fb_info *info, uint32_t pitch,
 
 int drm_fb_helper_setcmap(struct fb_cmap *cmap, struct fb_info *info);
 
-bool drm_fb_helper_hotplug_event(struct drm_fb_helper *fb_helper);
+int drm_fb_helper_hotplug_event(struct drm_fb_helper *fb_helper);
 bool drm_fb_helper_initial_config(struct drm_fb_helper *fb_helper, int bpp_sel);
 int drm_fb_helper_single_add_all_connectors(struct drm_fb_helper *fb_helper);
 int drm_fb_helper_debug_enter(struct fb_info *info);