[PATCH 1/4] dt-bindings: simplefb: Specify node location and handoff related properties

Hans de Goede hdegoede at redhat.com
Wed Nov 12 14:08:41 PST 2014

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

Signed-off-by: Hans de Goede <hdegoede at redhat.com>
 .../bindings/video/simple-framebuffer.txt          | 37 +++++++++++++++++++++-
 1 file changed, 36 insertions(+), 1 deletion(-)

diff --git a/Documentation/devicetree/bindings/video/simple-framebuffer.txt b/Documentation/devicetree/bindings/video/simple-framebuffer.txt
index 8f35718..95fe284 100644
--- a/Documentation/devicetree/bindings/video/simple-framebuffer.txt
+++ b/Documentation/devicetree/bindings/video/simple-framebuffer.txt
@@ -4,6 +4,26 @@ 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 (*). The primary display node must be named framebuffer0,
+additional nodes must be called framebuffer1, etc.
+If a simplefb node represents the preferred console for user interaction,
+then the chosen node's stdout-path property must point to it.
+If the devicetree contains nodes for the display hardware used by a simplefb,
+then one of the hw nodes must have a property called "framebuffer" pointing to
+the framebuffer# node in chosen, so that the operating system 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 contains the
+framebuffer property.
+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.
 Required properties:
 - compatible: "simple-framebuffer"
 - reg: Should contain the location and size of the framebuffer memory.
@@ -22,11 +42,26 @@ Optional properties:
-	framebuffer {
+chosen {
+	framebuffer0 {
 		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>;
+soc at 01c00000 {
+	lcdc0: lcdc at 1c0c000 {
+		compatible = "allwinner,sun4i-a10-lcdc";
+		framebuffer = <&framebuffer0>;
+	};
+*) Older devicetree files may have a compatible = "simple-framebuffer" node
+in a different place, operating systems must first enumerate any framebuffer#
+nodes found under chosen and then check for other compatible nodes.

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