Re: [PATCH v4 7/8] drm/i2c: tda998x: register as a drm bridge

[Russell King - ARM Linux <linux@armlinux.org.uk>]

On Fri, Jul 06, 2018 at 11:03:46AM +0100, Russell King - ARM Linux wrote:
> On Wed, Apr 25, 2018 at 11:01:15PM +0300, Jyri Sarha wrote:
> > Oh yes. But in this case the substandard solution is already there and
> > it is already widely used, despite it being severely broken. I am merely
> > trying to fix the existing substandard solution.
> > 
> > I admit that a full integration with component helpers would probably be
> > more elegant solution to the problem, but the amount of work is just too
> > much. The change would impact the way all the master drm drivers pull
> > them selves together. The drivers that already use the component helpers
> > for some internal stuff will add their own challenge. Separate component
> > matching implementations are needed for device-tree and ACPI (are ther
> > more flavors?) etc. I just do not see this happening any time soon (am
> > happy to be wrong about this).
> 
> The issue is actually worse than that:
> 
> - drivers that are already componentised can't use bridges
> - drivers that use bridges can't use componentised stuff
> 
> because bridges don't register themselves with the component helper,
> and the helpers in drm_of.c assume that all graph nodes will be
> components.
> 
> The whole thing about whether stuff is componentised or bridge based
> is really getting out of hand, and the push is towards bridge based
> stuff even though that is technically inferior when it comes to being
> able to develop and test (which involves being able to remove and
> re-insert modules.)
> 
> Consequently more and more code is being written for bridges, and
> the component helper ignored, and the problems with bridges are
> being ignored.  This is not healthy.
> 
> The problem is only going to get worse.  Someone needs to bite the
> bullet and fix bridges before the problem gets any more out of hand.

This patch (which is actually two patches locally) allows the component
helper to know what's going on inside the bridge code wrt bridge
availability, and takes the appropriate action at the correct time.
No need for device links or similar, or incompatibilities between
bridges and components.  The only requirement is that bridges set the
"device" member of struct drm_bridge to opt-in to this.

Tested with Armada converted to support bridges, TDA998x as a
componentised bridge, and dumb-vga-dac as a non-componentised bridge:

root@cubox:~# less /sys/kernel/debug/device_component/display-subsystem
master name                                            status
-------------------------------------------------------------
display-subsystem                                       bound

device name                                            status
-------------------------------------------------------------
port                                               registered
port                                               registered
hdmi-encoder                                       registered
vga-bridge                                         registered
root@cubox:~# dmesg |grep bound
[    1.921798] armada-drm display-subsystem: bound f1820000.lcd-controller (ops
armada_lcd_ops)
[    1.931014] armada-drm display-subsystem: bound f1810000.lcd-controller (ops
armada_lcd_ops)
[    2.069231] armada-drm display-subsystem: bound 1-0070 (ops tda998x_ops)
[    2.076059] armada-drm display-subsystem: bound vga-bridge (ops dummy_ops)

Without this, the same DT fails because "vga-bridge" is never added
to the component helpers.

diff --git a/drivers/base/component.c b/drivers/base/component.c
index 8946dfee4768..b14b3a3655ea 100644
--- a/drivers/base/component.c
+++ b/drivers/base/component.c
@@ -602,4 +602,32 @@ void component_del(struct device *dev, const struct component_ops *ops)
 }
 EXPORT_SYMBOL_GPL(component_del);
 
+static int component_dummy_bind(struct device *comp, struct device *master,
+				void *master_data)
+{
+	return 0;
+}
+
+static void component_dummy_unbind(struct device *comp, struct device *master,
+				   void *master_data)
+{
+}
+
+static const struct component_ops dummy_ops = {
+	.bind = component_dummy_bind,
+	.unbind = component_dummy_unbind,
+};
+
+int component_mark_available(struct device *dev)
+{
+	return component_add(dev, &dummy_ops);
+}
+EXPORT_SYMBOL_GPL(component_mark_available);
+
+void component_mark_unavailable(struct device *dev)
+{
+	component_del(dev, &dummy_ops);
+}
+EXPORT_SYMBOL_GPL(component_mark_unavailable);
+
 MODULE_LICENSE("GPL v2");
diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index 1638bfe9627c..ce3ccd327916 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -21,6 +21,7 @@
  * DEALINGS IN THE SOFTWARE.
  */
 
+#include <linux/component.h>
 #include <linux/err.h>
 #include <linux/module.h>
 #include <linux/mutex.h>
@@ -73,6 +74,9 @@ void drm_bridge_add(struct drm_bridge *bridge)
 	mutex_lock(&bridge_lock);
 	list_add_tail(&bridge->list, &bridge_list);
 	mutex_unlock(&bridge_lock);
+
+	if (bridge->device)
+		WARN_ON(component_mark_available(bridge->device));
 }
 EXPORT_SYMBOL(drm_bridge_add);
 
@@ -83,6 +87,9 @@ EXPORT_SYMBOL(drm_bridge_add);
  */
 void drm_bridge_remove(struct drm_bridge *bridge)
 {
+	if (bridge->device)
+		component_mark_unavailable(bridge->device);
+
 	mutex_lock(&bridge_lock);
 	list_del_init(&bridge->list);
 	mutex_unlock(&bridge_lock);
diff --git a/include/drm/drm_bridge.h b/include/drm/drm_bridge.h
index 3270fec46979..e863da14d4d9 100644
--- a/include/drm/drm_bridge.h
+++ b/include/drm/drm_bridge.h
@@ -268,6 +268,7 @@ struct drm_bridge {
 	struct drm_device *dev;
 	struct drm_encoder *encoder;
 	struct drm_bridge *next;
+	struct device *device;
 #ifdef CONFIG_OF
 	struct device_node *of_node;
 #endif
diff --git a/include/linux/component.h b/include/linux/component.h
index e71fbbbc74e2..a1c824485f54 100644
--- a/include/linux/component.h
+++ b/include/linux/component.h
@@ -16,6 +16,10 @@ struct component_ops {
 int component_add(struct device *, const struct component_ops *);
 void component_del(struct device *, const struct component_ops *);
 
+/* For subsystems where drivers do not call component_add()/component_del() */
+int component_mark_available(struct device *dev);
+void component_mark_unavailable(struct device *dev);
+
 int component_bind_all(struct device *master, void *master_data);
 void component_unbind_all(struct device *master, void *master_data);
 


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