[RFC v2 5/6] drivers: boot_constraint: Add initial DT bindings

[Viresh Kumar]

This adds device tree bindings for boot constraints. Only power supply
constraint types are supported currently.

Signed-off-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
---
 .../devicetree/bindings/boot-constraints.txt       | 68 ++++++++++++++++++++++
 1 file changed, 68 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/boot-constraints.txt

diff --git a/Documentation/devicetree/bindings/boot-constraints.txt b/Documentation/devicetree/bindings/boot-constraints.txt
new file mode 100644
index 000000000000..9a01ea1e6e72
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+BOOT CONSTRAINTS
+================
+
+Some devices are powered ON by the bootloader before the bootloader handovers
+control to the Operating System (OS). It maybe important for those devices to
+keep working until the time the OS takes over and starts configuring the devices
+again.
+
+A typical example of that can be the LCD controller, which is used by the
+bootloaders to show image(s) while the platform is booting into the Operating
+System. The LCD controller can be using some resources, like clk, supplies, etc,
+that are shared between several devices. These shared resources should be
+configured to satisfy need of all the users. If another device's (X) driver gets
+probed before the LCD controller driver in this case, then it may end up
+reconfiguring these resources to ranges satisfying the current users (only
+device X) and that can make the LCD screen unstable.
+
+This document describes the binding used to specify such boot constraints to the
+OS.
+
+Power Supply Constraints:
+-------------------------
+
+This describes the binding of constraints for the power supply resources. These
+must be present directly in the consumer device's node.
+
+Required properties:
+- boot-constraint-supplies:
+
+  This contains an array of (one or more) strings, each of which must match with
+  the <name> of a corresponding <name>-supply property in the same device node.
+  This is required for the OS to know about the power supplies that are
+  configured (and enabled) by the bootloader for the consumer device.
+
+  It is assumed that the power supply is already enabled by the bootloader.
+
+- boot-constraint-uV:
+
+  This contains an array of {min max} microvolt tuples for the power supplies in
+  the same order in which they are present in "boot-constraint-supplies"
+  property. Here, min is the smallest and max is the largest voltage that the
+  consumer (corresponding to the device node where this property is present) may
+  set.
+
+Example of a consumer device node (mmc) referencing two regulators and setting
+their boot constraints (twl_reg1 and twl_reg2):
+
+	twl_reg1: regulator@0 {
+		...
+		...
+		...
+	};
+
+	twl_reg2: regulator@1 {
+		...
+		...
+		...
+	};
+
+	mmc: mmc@0x0 {
+		...
+		...
+		vmmc-supply = <&twl_reg1>;
+		vmmcaux-supply = <&twl_reg2>;
+		boot-constraint-supplies = "vmmc", "vmmcaux";
+		boot-constraint-uV = <1800000 2000000>, /* vmmc */
+				     <2000000 2000000>; /* vmmcaux */
+	};
-- 
2.13.0.71.gd7076ec9c9cb