[PATCH V6 13/13] boot_constraint: Add documentation

[Viresh Kumar]

This adds boot constraint documentation.

Signed-off-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
---
 .../driver-api/boot-constraint/constraints.rst     | 98 ++++++++++++++++++++++
 Documentation/driver-api/boot-constraint/index.rst |  4 +
 Documentation/driver-api/index.rst                 |  1 +
 3 files changed, 103 insertions(+)
 create mode 100644 Documentation/driver-api/boot-constraint/constraints.rst
 create mode 100644 Documentation/driver-api/boot-constraint/index.rst

diff --git a/Documentation/driver-api/boot-constraint/constraints.rst b/Documentation/driver-api/boot-constraint/constraints.rst
new file mode 100644
index 000000000000..036bb77a26f9
--- /dev/null
+++ b/Documentation/driver-api/boot-constraint/constraints.rst
@@ -0,0 +1,98 @@
+.. include:: <isonum.txt>
+
+=========================
+Boot Constraint Subsystem
+=========================
+
+:Copyright: |copy| 2017 Viresh Kumar <vireshk@xxxxxxxxxx>, Linaro Ltd.
+
+Introduction
+============
+
+A lot of devices are configured and powered ON by the bootloader before passing
+on control to the operating system, Linux in our case.  It is important for some
+of them to keep working until the time a Linux device driver probes the device
+and reconfigure it.
+
+A typical example of that can be the LCD controller, which is used by the
+bootloaders to show image(s) while the platform boots Linux.  The LCD controller
+can be using resources like clk, regulators, etc, that are shared with other
+devices. These shared resources should be configured in such a way that they
+satisfy need of all the user devices.  If another device's (X) driver gets
+probed before the LCD controller driver, then it may end up disabling or
+reconfiguring these resources to ranges satisfying only the current user (device
+X) and that may make the LCD screen unstable and present a bad user experience.
+
+Another case can be a debug serial port (earlycon) enabled from the bootloader,
+which may be used to debug early kernel oops.
+
+There are also cases where the resources may not be shared, but the kernel will
+disable them forcefully as no users may have appeared until a certain point in
+the kernel boot.
+
+Of course we can have more complex cases where the same resource is getting used
+by multiple devices while the kernel boots and the order in which the devices
+get probed wouldn't matter as the other devices may break because of the chosen
+configuration of the first probed device.
+
+Adding boot constraints
+=======================
+
+A boot constraint defines a configuration requirement set for the device by the
+boot loader. For example, if a clock is enabled for a device by the bootloader
+and we want the device to be working as is until the time the device is probed
+by its driver, then keeping this clock enabled is one of the boot constraint.
+
+Following are the different type of boot constraints supported currently by the
+core:
+
+.. kernel-doc:: include/linux/boot_constraint.h
+   :functions: dev_boot_constraint_type
+
+
+A single boot constraint can be added using the following helper:
+
+.. kernel-doc:: drivers/boot_constraint/core.c
+   :functions: dev_boot_constraint_add
+
+
+The second parameter to this routine describes the constraint to add and is
+represented by following structures:
+
+.. kernel-doc:: include/linux/boot_constraint.h
+   :functions: dev_boot_constraint dev_boot_constraint_info
+
+The power domain boot constraint doesn't need any data, while the clock and
+power supply boot constraint specific data is represented by following
+structures:
+
+.. kernel-doc:: include/linux/boot_constraint.h
+   :functions: dev_boot_constraint_supply_info dev_boot_constraint_clk_info
+
+
+In order to simplify adding multiple boot constraints for a platform, the boot
+constraints core supplies another helper which can be used to add all
+constraints for the platform.
+
+.. kernel-doc:: drivers/boot_constraint/deferrable_dev.c
+   :functions: dev_boot_constraint_add_deferrable_of
+
+
+The argument of this routine is described by following structure:
+
+.. kernel-doc:: include/linux/boot_constraint.h
+   :functions: dev_boot_constraint_of
+
+
+Removing boot constraints
+=========================
+
+Once the boot constraints are added, they will be honored by the boot constraint
+core until the time a driver tries to probe the device. The constraints are
+removed by the driver core if either the driver successfully probed the device
+or failed with an error value other than -EPROBE_DEFER. The constraints are kept
+as is for deferred probe. The driver core removes the constraints using the
+following helper, which must not be called directly by the platforms:
+
+.. kernel-doc:: drivers/boot_constraint/core.c
+   :functions: dev_boot_constraints_remove
diff --git a/Documentation/driver-api/boot-constraint/index.rst b/Documentation/driver-api/boot-constraint/index.rst
new file mode 100644
index 000000000000..d6fce17626a2
--- /dev/null
+++ b/Documentation/driver-api/boot-constraint/index.rst
@@ -0,0 +1,4 @@
+.. toctree::
+   :maxdepth: 1
+
+   constraints
diff --git a/Documentation/driver-api/index.rst b/Documentation/driver-api/index.rst
index d17a9876b473..e591c172b5b5 100644
--- a/Documentation/driver-api/index.rst
+++ b/Documentation/driver-api/index.rst
@@ -47,6 +47,7 @@ available subsections can be seen below.
    gpio
    misc_devices
    dmaengine/index
+   boot-constraint/index
 
 .. only::  subproject and html
 
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