[RFC PATCH] dt: describe base reset signal binding

[Stephen Warren]

From: Stephen Warren <swarren at nvidia.com>

This binding is intended to represent the hardware reset signals present
internally in most IC (SoC, FPGA, ...) designs.

Such a binding would allow the creation of a "reset subsystem", which
could replace APIs such as the following Tegra-specific API:

void tegra_periph_reset_deassert(struct clk *c);
void tegra_periph_reset_assert(struct clk *c);

(Note that at present, Tegra couples reset assertion with the clock for
the affected peripheral module. However, reset and clocking are two
separate, yet admittedly related, concepts).

Signed-off-by: Stephen Warren <swarren at nvidia.com>
---
What do people think of this? Does it sound like a good idea to go ahead
with a reset subsystem? Should we simply add a new API to the common clock
subsystem instead (and assume that reset and clock domains match 1:1).
Should this be implemented as part of the generic power management domains;
see include/linux/pm_domain.h instead?

 Documentation/devicetree/bindings/reset/reset.txt |   75 +++++++++++++++++++++
 1 files changed, 75 insertions(+), 0 deletions(-)
 create mode 100644 Documentation/devicetree/bindings/reset/reset.txt

diff --git a/Documentation/devicetree/bindings/reset/reset.txt b/Documentation/devicetree/bindings/reset/reset.txt
new file mode 100644
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+= Reset Signal Device Tree Bindings =
+
+This binding is intended to represent the hardware reset signals present
+internally in most IC (SoC, FPGA, ...) designs. Reset signals for whole
+standalone chips are most likely better represented as GPIOs, although there
+are likely to be exceptions to this rule.
+
+Hardware blocks typically receive a reset signal. This signal is generated by
+a reset provider (e.g. power management or clock module) and received by a
+reset consumer (the module being reset, or a module managing when a sub-
+ordinate module is reset). This binding exists to represent the provider and
+consumer, and provide a way to couple the two together.
+
+A reset signal is represented by the phandle of the provider, plus a reset
+specifier - a list of DT cells that represents the reset signal within the
+provider. The length (number of cells) and semantics of the reset specifier
+are dictated by the binding of the reset provider, although common schemes
+are described below.
+
+A word on where to place reset signal consumers in device tree: It is possible
+in hardware for a reset signal to affect multiple logically separate HW blocks
+at once. In this case, it would be unwise to represent this reset signal in
+the DT node of each affected HW block, since if activated, an unrelated block
+may be reset. Instead, reset signals should be represented in the DT node
+where it makes most sense to control it; this may be a bus node if all
+children of the bus are affected by the reset signal, or an individual HW
+block node for dedicated reset signals. The intent of this binding is to give
+appropriate software access to the reset signals in order to manage the HW,
+rather than to slavishly enumerate the reset signal that affects each HW
+block.
+
+= Reset providers =
+
+Required properties:
+#reset-cells:	Number of cells in a reset specifier; Typically 0 for nodes
+		with a single reset output and 1 for nodes with multiple
+		reset outputs.
+
+For example:
+
+	rst: reset-controller {
+		#reset-cells = <1>;
+	};
+
+= Reset consumers =
+
+Required properties:
+resets:		List of phandle and reset specifier pairs, one pair
+		for each reset signal that affects the device, or that the
+		device manages. Note: if the reset provider specifies '0' for
+		#reset-cells, then only the phandle portion of the pair will
+		appear.
+
+Optional properties:
+reset-names:	List of reset signal name strings sorted in the same order as
+		the resets property. Consumers drivers will use reset-names to
+		match reset signal names with reset specifiers.
+
+For example:
+
+	device {
+		resets = <&rst 20>;
+		reset-names = "reset";
+	};
+
+This represents a device with a single reset signal named "reset".
+
+	bus {
+		resets = <&rst 10> <&rst 11> <&rst 12> <&rst 11>;
+		reset-names = "i2s1", "i2s2", "dma", "mixer";
+	};
+
+This represents a bus that controls the reset signal of each of four sub-
+ordinate devices. Consider for example a bus that fails to operate unless no
+child device has reset asserted.
-- 
1.7.0.4