Reinstate "drm/probe-helpers: Drop locking from poll_enable""

This reverts commit 54a07c7bb0da0343734c78212bbe9f3735394962,
and reinstates the original.

[airlied: this might be a bad plan for git].

commit 3846fd9b86001bea171943cc3bb9222cb6da6b42
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Wed Jan 11 10:01:17 2017 +0100

    drm/probe-helpers: Drop locking from poll_enable

    It was only needed to protect the connector_list walking, see

    commit 8c4ccc4ab6f64e859d4ff8d7c02c2ed2e956e07f
    Author: Daniel Vetter <daniel.vetter@ffwll.ch>
    Date:   Thu Jul 9 23:44:26 2015 +0200

        drm/probe-helper: Grab mode_config.mutex in poll_init/enable

    Unfortunately the commit message of that patch fails to mention that
    the new locking check was for the connector_list.

    But that requirement disappeared in

    commit c36a3254f7857f1ad9badbe3578ccc92be541a8e
    Author: Daniel Vetter <daniel.vetter@ffwll.ch>
    Date:   Thu Dec 15 16:58:43 2016 +0100

        drm: Convert all helpers to drm_connector_list_iter

    and so we can drop this again.

    This fixes a locking inversion on nouveau, where the rpm code needs to
    re-enable. But in other places the rpm_get() calls are nested within
    the big modeset locks.

    While at it, also improve the kerneldoc for these two functions a
    notch.

    v2: Update the kerneldoc even more to explain that these functions
    can't be called concurrently, or bad things happen (Chris).

1 files changed, 20 insertions, 31 deletions

diff --git a/drivers/gpu/drm/drm_probe_helper.c b/drivers/gpu/drm/drm_probe_helper.c
index 79519e4983c7..041a5504780f 100644
--- a/drivers/gpu/drm/drm_probe_helper.c
+++ b/drivers/gpu/drm/drm_probe_helper.c
@@ -115,25 +115,28 @@ static int drm_helper_probe_add_cmdline_mode(struct drm_connector *connector)
 
 #define DRM_OUTPUT_POLL_PERIOD (10*HZ)
 /**
- * drm_kms_helper_poll_enable_locked - re-enable output polling.
+ * drm_kms_helper_poll_enable - re-enable output polling.
  * @dev: drm_device
  *
- * This function re-enables the output polling work without
- * locking the mode_config mutex.
+ * This function re-enables the output polling work, after it has been
+ * temporarily disabled using drm_kms_helper_poll_disable(), for example over
+ * suspend/resume.
  *
- * This is like drm_kms_helper_poll_enable() however it is to be
- * called from a context where the mode_config mutex is locked
- * already.
+ * Drivers can call this helper from their device resume implementation. It is
+ * an error to call this when the output polling support has not yet been set
+ * up.
+ *
+ * Note that calls to enable and disable polling must be strictly ordered, which
+ * is automatically the case when they're only call from suspend/resume
+ * callbacks.
  */
-void drm_kms_helper_poll_enable_locked(struct drm_device *dev)
+void drm_kms_helper_poll_enable(struct drm_device *dev)
 {
 	bool poll = false;
 	struct drm_connector *connector;
 	struct drm_connector_list_iter conn_iter;
 	unsigned long delay = DRM_OUTPUT_POLL_PERIOD;
 
-	WARN_ON(!mutex_is_locked(&dev->mode_config.mutex));
-
 	if (!dev->mode_config.poll_enabled || !drm_kms_helper_poll)
 		return;
 
@@ -163,7 +166,7 @@ void drm_kms_helper_poll_enable_locked(struct drm_device *dev)
 	if (poll)
 		schedule_delayed_work(&dev->mode_config.output_poll_work, delay);
 }
-EXPORT_SYMBOL(drm_kms_helper_poll_enable_locked);
+EXPORT_SYMBOL(drm_kms_helper_poll_enable);
 
 static enum drm_connector_status
 drm_connector_detect(struct drm_connector *connector, bool force)
@@ -290,7 +293,7 @@ int drm_helper_probe_single_connector_modes(struct drm_connector *connector,
 
 	/* Re-enable polling in case the global poll config changed. */
 	if (drm_kms_helper_poll != dev->mode_config.poll_running)
-		drm_kms_helper_poll_enable_locked(dev);
+		drm_kms_helper_poll_enable(dev);
 
 	dev->mode_config.poll_running = drm_kms_helper_poll;
 
@@ -484,8 +487,12 @@ out:
  * This function disables the output polling work.
  *
  * Drivers can call this helper from their device suspend implementation. It is
- * not an error to call this even when output polling isn't enabled or arlready
- * disabled.
+ * not an error to call this even when output polling isn't enabled or already
+ * disabled. Polling is re-enabled by calling drm_kms_helper_poll_enable().
+ *
+ * Note that calls to enable and disable polling must be strictly ordered, which
+ * is automatically the case when they're only call from suspend/resume
+ * callbacks.
  */
 void drm_kms_helper_poll_disable(struct drm_device *dev)
 {
@@ -496,24 +503,6 @@ void drm_kms_helper_poll_disable(struct drm_device *dev)
 EXPORT_SYMBOL(drm_kms_helper_poll_disable);
 
 /**
- * drm_kms_helper_poll_enable - re-enable output polling.
- * @dev: drm_device
- *
- * This function re-enables the output polling work.
- *
- * Drivers can call this helper from their device resume implementation. It is
- * an error to call this when the output polling support has not yet been set
- * up.
- */
-void drm_kms_helper_poll_enable(struct drm_device *dev)
-{
-	mutex_lock(&dev->mode_config.mutex);
-	drm_kms_helper_poll_enable_locked(dev);
-	mutex_unlock(&dev->mode_config.mutex);
-}
-EXPORT_SYMBOL(drm_kms_helper_poll_enable);
-
-/**
  * drm_kms_helper_poll_init - initialize and enable output polling
  * @dev: drm_device
  *