drm: Introduce drm_fb_helper_prepare()

To implement hotplug detection in a race-free manner, drivers must call
drm_kms_helper_poll_init() before hotplug events can be triggered. Such
events can be triggered right after any of the encoders or connectors
are initialized. At the same time, if the drm_fb_helper_hotplug_event()
helper is used by a driver, then the poll helper requires some parts of
the FB helper to be initialized to prevent a crash.

At the same time, drm_fb_helper_init() requires information that is not
necessarily available at such an early stage (number of CRTCs and
connectors), so it cannot be used yet.

Add a new helper, drm_fb_helper_prepare(), that initializes the bare
minimum needed to allow drm_kms_helper_poll_init() to execute and any
subsequent hotplug events to be processed properly.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

1 files changed, 37 insertions, 10 deletions

diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index 6cd687a20cd5..bdee6eb0b24a 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -49,10 +49,11 @@ static LIST_HEAD(kernel_fb_helper_list);
  * helper functions used by many drivers to implement the kernel mode setting
  * interfaces.
  *
- * Initialization is done as a three-step process with drm_fb_helper_init(),
- * drm_fb_helper_single_add_all_connectors() and drm_fb_helper_initial_config().
- * Drivers with fancier requirements than the default behaviour can override the
- * second step with their own code.  Teardown is done with drm_fb_helper_fini().
+ * Initialization is done as a four-step process with drm_fb_helper_prepare(),
+ * drm_fb_helper_init(), drm_fb_helper_single_add_all_connectors() and
+ * drm_fb_helper_initial_config(). Drivers with fancier requirements than the
+ * default behaviour can override the third step with their own code.
+ * Teardown is done with drm_fb_helper_fini().
  *
  * At runtime drivers should restore the fbdev console by calling
  * drm_fb_helper_restore_fbdev_mode() from their ->lastclose callback. They
@@ -63,6 +64,19 @@ static LIST_HEAD(kernel_fb_helper_list);
  *
  * All other functions exported by the fb helper library can be used to
  * implement the fbdev driver interface by the driver.
+ *
+ * It is possible, though perhaps somewhat tricky, to implement race-free
+ * hotplug detection using the fbdev helpers. The drm_fb_helper_prepare()
+ * helper must be called first to initialize the minimum required to make
+ * hotplug detection work. Drivers also need to make sure to properly set up
+ * the dev->mode_config.funcs member. After calling drm_kms_helper_poll_init()
+ * it is safe to enable interrupts and start processing hotplug events. At the
+ * same time, drivers should initialize all modeset objects such as CRTCs,
+ * encoders and connectors. To finish up the fbdev helper initialization, the
+ * drm_fb_helper_init() function is called. To probe for all attached displays
+ * and set up an initial configuration using the detected hardware, drivers
+ * should call drm_fb_helper_single_add_all_connectors() followed by
+ * drm_fb_helper_initial_config().
  */
 
 /**
@@ -528,6 +542,24 @@ static void drm_fb_helper_crtc_free(struct drm_fb_helper *helper)
 }
 
 /**
+ * drm_fb_helper_prepare - setup a drm_fb_helper structure
+ * @dev: DRM device
+ * @helper: driver-allocated fbdev helper structure to set up
+ * @funcs: pointer to structure of functions associate with this helper
+ *
+ * Sets up the bare minimum to make the framebuffer helper usable. This is
+ * useful to implement race-free initialization of the polling helpers.
+ */
+void drm_fb_helper_prepare(struct drm_device *dev, struct drm_fb_helper *helper,
+			   const struct drm_fb_helper_funcs *funcs)
+{
+	INIT_LIST_HEAD(&helper->kernel_fb_list);
+	helper->funcs = funcs;
+	helper->dev = dev;
+}
+EXPORT_SYMBOL(drm_fb_helper_prepare);
+
+/**
  * drm_fb_helper_init - initialize a drm_fb_helper structure
  * @dev: drm device
  * @fb_helper: driver-allocated fbdev helper structure to initialize
@@ -539,8 +571,7 @@ static void drm_fb_helper_crtc_free(struct drm_fb_helper *helper)
  * nor register the fbdev. This is only done in drm_fb_helper_initial_config()
  * to allow driver writes more control over the exact init sequence.
  *
- * Drivers must set fb_helper->funcs before calling
- * drm_fb_helper_initial_config().
+ * Drivers must call drm_fb_helper_prepare() before calling this function.
  *
  * RETURNS:
  * Zero if everything went ok, nonzero otherwise.
@@ -555,10 +586,6 @@ int drm_fb_helper_init(struct drm_device *dev,
 	if (!max_conn_count)
 		return -EINVAL;
 
-	fb_helper->dev = dev;
-
-	INIT_LIST_HEAD(&fb_helper->kernel_fb_list);
-
 	fb_helper->crtc_info = kcalloc(crtc_count, sizeof(struct drm_fb_helper_crtc), GFP_KERNEL);
 	if (!fb_helper->crtc_info)
 		return -ENOMEM;