dt-bindings: simplefb: Specify node location and handoff related properties

Since simplefb nodes do not relate directly to hw typically they have been placed in the root of the devicetree. As the represent runtime information having them as sub-nodes of /chosen is more logical, specify this.

Also specify when to set the chosen stdout-path property to a simplefb node.

For reliable handover to a hardware specific driver, that driver needs to know which simplefb to unregister when taking over, specify how the hw driver can find the matching simplefb node.

Last add some advice on how to fill and use simplefb nodes from a firmware pov.

--
Changes in v2:
-Add stdout-path to the example code
Changes in v3:
-Specify that the node name must be "framebuffer@<address>" -Specify that the link to link simplefb node and display hw nodes to one
another for handover goes into the simplefb node, and must be called "display"
-Specify how aliases may be used to tell the OS how to number displays -Update the example to reflect these changes
Changes in v4:
-Keep the size of the reg property in the example as it was before Changes in v5:
-Specify that stdout-path property should work the same way as the aliases,
or just point to an alias
-Document naming scheme for pre-populated simplefb nodes

---
 .../bindings/video/simple-framebuffer.txt          | 58 +++++++++++++++++++++-
 1 file changed, 57 insertions(+), 1 deletion(-)

index 8f35718..f8fb7e6 100644
@@ -4,6 +4,40 @@ A simple frame-buffer describes a frame-buffer setup by firmware or
 the bootloader, with the assumption that the display hardware has already
 been set up to scan out from the memory pointed to by the reg property.
 
+Since simplefb nodes represent runtime information they must be sub-nodes of
+the chosen node (*). Simplefb nodes must be named "framebuffer@<address>".
+
+If the devicetree contains nodes for the display hardware used by a simplefb,
+then the simplefb node must contain a property called "display", which
+contains a phandle pointing to the primary display hw node, so that the OS
+knows which simplefb to disable when handing over control to a driver for the
+real hardware. The bindings for the hw nodes must specify which node is
+considered the primary node.
+
+It is advised to add display# aliases to help the OS determine how to number
+things. If display# aliases are used, then if the simplefb node contains a
+"display" property then the /aliases/display# path must point to the display
+hw node the "display" property points to, otherwise it must point directly
+to the simplefb node.
+
+If a simplefb node represents the preferred console for user interaction,
+then the chosen node's stdout-path property should point to it, or to the
+primary display hw node, as with display# aliases. If display aliases are
+used then it should be set to the alias instead.
+
+It is advised that devicetree files contain pre-filled, disabled framebuffer
+nodes, so that the firmware only needs to update the mode information and
+enable them. This way if e.g. later on support for more display clocks get
+added, the simplefb nodes will already contain this info and the firmware
+does not need to be updated.
+
+If pre-filled framebuffer nodes are used, they should be named
+"framebuffer#-<output>", e.g. "framebuffer0-hdmi". The output should be
+included in the name since different outputs typically require different
+clocks and the clocks are part of the pre-populated nodes. The firmware must
+rename the nodes to the standard "framebuffer@<address>" name using the
+runtime chosen address when enabling the nodes.
+
 Required properties:
 - compatible: "simple-framebuffer"
 - reg: Should contain the location and size of the framebuffer memory.

@@ -19,14 +53,36 @@ Optional properties:
            are expected to already be configured correctly. The OS must
            ensure these clocks are not modified or disabled while the
            simple framebuffer remains active.
+- display : phandle pointing to the primary display hardware node
 
 Example:
 
-	framebuffer {
+aliases {
+	display0 = &lcdc0;
+}
+
+chosen {
+	framebuffer0: framebuffer at 1d385000 {
 		compatible = "simple-framebuffer";
 		reg = <0x1d385000 (1600 * 1200 * 2)>;
 		width = <1600>;
 		height = <1200>;
 		stride = <(1600 * 2)>;
 		format = "r5g6b5";
+		clocks = <&ahb_gates 36>, <&ahb_gates 43>, <&ahb_gates 44>;
+		display = <&lcdc0>;
+	};
+	stdout-path = "display0";
+};
+
+soc at 01c00000 {
+	lcdc0: lcdc at 1c0c000 {
+		compatible = "allwinner,sun4i-a10-lcdc";
+		...
 	};
+};
+
+
+*) Older devicetree files may have a compatible = "simple-framebuffer" node
+in a different place, operating systems must first enumerate any compatible
+nodes found under chosen and then check for other compatible nodes.

2.1.0

Keep the extern declaration of of_allnodes and friends, when building without of support, this way code using them can be written like this:

	if (IS_ENABLED(CONFIG_OF_PLATFORM) && of_chosen) {
		for_each_child_of_node(of_chosen, np)
			...
	}

And rely on the compiler optimizing it away, avoiding the need for #ifdef-ery.

---
 include/linux/of.h | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

index 6545e7a..f83ca9d 100644
@@ -105,8 +105,6 @@ static inline struct device_node *of_node_get(struct device_node *node)
 static inline void of_node_put(struct device_node *node) { }
 #endif /* !CONFIG_OF_DYNAMIC */
 
-#ifdef CONFIG_OF
-
 /* Pointer for first entry in chain of all nodes. */
 extern struct device_node *of_allnodes;
 extern struct device_node *of_chosen;

@@ -114,6 +112,7 @@ extern struct device_node *of_aliases;
 extern struct device_node *of_stdout;
 extern raw_spinlock_t devtree_lock;
 
+#ifdef CONFIG_OF
 static inline bool of_have_populated_dt(void)
 {
 	return of_allnodes != NULL;

2.1.0

Update simplefb to support the new preferred location for simplefb dt nodes under /chosen.

--
Changes in v2:
-Make name array larger in case we ever encounter more then 10000 framebuffers Changes in v3:
-Switch to for_each_child_of_node to loop over the nodes under chosen Changes in v4:
-Wrap the chosen enumeration code in #ifdef CONFIG_OF, as simplefb is used on
non devicetree platforms too
-simplefb can only be built-in, so drop the module_exit function calling
platform_driver_unregister
Changes in v5:
-Cleanup #ifdef CONFIG_OF handling

---
 drivers/video/fbdev/simplefb.c | 23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

index d192946..74c5edd 100644
@@ -27,6 +27,7 @@
 #include <linux/platform_data/simplefb.h>
 #include <linux/platform_device.h>
 #include <linux/clk-provider.h>
+#include <linux/of_platform.h>
 
 static struct fb_fix_screeninfo simplefb_fix = {
 	.id		= "simple",

@@ -392,7 +393,27 @@ static struct platform_driver simplefb_driver = {
 	.probe = simplefb_probe,
 	.remove = simplefb_remove,
 };
-module_platform_driver(simplefb_driver);
+
+static int __init simplefb_init(void)
+{
+	int ret;
+	struct device_node *np;
+
+	ret = platform_driver_register(&simplefb_driver);
+	if (ret)
+		return ret;
+
+	if (IS_ENABLED(CONFIG_OF) && of_chosen) {
+		for_each_child_of_node(of_chosen, np) {
+			if (of_device_is_compatible(np, "simple-framebuffer"))
+				of_platform_device_create(np, NULL, NULL);
+		}
+	}
+
+	return 0;
+}
+
+module_init(simplefb_init);
 
 MODULE_AUTHOR("Stephen Warren <[email protected]>");
 MODULE_DESCRIPTION("Simple framebuffer driver");

2.1.0

Various fb drivers register themselves before module_init so as to have a console as early as possible, this is of little use if fbcon does not initialize early too.

Fbcon cannot initialize earlier then fs_initcall, because then the creation of /sys/class/graphics/fbcon will fail.

---
 drivers/video/console/fbcon.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index eb976ee..ea43724 100644
@@ -3624,7 +3624,7 @@ static int __init fb_console_init(void)
 	return 0;
 }
 
-module_init(fb_console_init);
+fs_initcall(fb_console_init);
 
 #ifdef MODULE
 

2.1.0

One of the reasons for having the simplefb nodes in /chosen, and doing explicit enumeration of the nodes there, is too allow enumerating them sooner, so that we get a console earlier on.

Doing this earlier then fs_initcall is not useful, since the fb only turns into a console when fbcon intializes, which is a fs_initcall too.

--
Changes in v4 (of the patch-set):
-Drop the comment about using fs_initcall vs module_exit, as we no longer have
a module_exit

---
 drivers/video/fbdev/simplefb.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 74c5edd..8be9754 100644
@@ -413,7 +413,7 @@ static int __init simplefb_init(void)
 	return 0;
 }
 
-module_init(simplefb_init);
+fs_initcall(simplefb_init);
 
 MODULE_AUTHOR("Stephen Warren <[email protected]>");
 MODULE_DESCRIPTION("Simple framebuffer driver");

2.1.0

Ok, so I have pushed this and the other simplefb series to fbdev for-next.

I can still refresh the whole series if need be, but if possible I'd rather take any new changes as patches on top of the current ones.

Tomi

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