It is possible that some of the resources aren't available at the time
constraints are getting set and the boot constraints core will return
-EPROBE_DEFER for them. In order to retry adding the constraints at a
later point of time (after the resource is added and before any of its
users come up), this patch proposes two things:

- Each constraint is represented by a virtual platform device, so that
  it is re-probed again until the time all the dependencies aren't met.
  The platform device is removed along with the constraint, with help of
  the free_resources() callback.

- Enable early defer probing support by calling
  driver_enable_deferred_probe(), so that the core retries probing
  deferred devices every time any device is bound to a driver. This
  makes sure that the constraint is set before any of the users of the
  resources come up.

This is tested on ARM64 Hikey board where probe was deferred for a
device.

Tested-by: Rajendra Nayak <redacted>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
---
 drivers/base/dd.c                         |  12 ++
 drivers/boot_constraints/Makefile         |   2 +-
 drivers/boot_constraints/deferrable_dev.c | 235 ++++++++++++++++++++++++++++++
 include/linux/boot_constraint.h           |  14 ++
 4 files changed, 262 insertions(+), 1 deletion(-)
 create mode 100644 drivers/boot_constraints/deferrable_dev.c

diff --git a/drivers/base/dd.c b/drivers/base/dd.c
index 4eec27fe2b2b..19eff5d08b9a 100644
--- a/drivers/base/dd.c
+++ b/drivers/base/dd.c

@@ -228,6 +228,18 @@ void device_unblock_probing(void)
 	driver_deferred_probe_trigger();
 }
 
+/**
+ * driver_enable_deferred_probe() - Enable probing of deferred devices
+ *
+ * We don't want to get in the way when the bulk of drivers are getting probed
+ * and so deferred probe is disabled in the beginning. Enable it now because we
+ * need it.
+ */
+void driver_enable_deferred_probe(void)
+{
+	driver_deferred_probe_enable = true;
+}
+
 /**
  * deferred_probe_initcall() - Enable probing of deferred devices
  *

diff --git a/drivers/boot_constraints/Makefile b/drivers/boot_constraints/Makefile
index b7ade1a7afb5..a765094623a3 100644
--- a/drivers/boot_constraints/Makefile
+++ b/drivers/boot_constraints/Makefile

@@ -1,3 +1,3 @@
 # Makefile for device boot constraints
 
-obj-y := clk.o core.o pm.o supply.o
+obj-y := clk.o deferrable_dev.o core.o pm.o supply.o

diff --git a/drivers/boot_constraints/deferrable_dev.c b/drivers/boot_constraints/deferrable_dev.c
new file mode 100644
index 000000000000..04056f317aff
--- /dev/null
+++ b/drivers/boot_constraints/deferrable_dev.c

@@ -0,0 +1,235 @@
+/*
+ * Copyright (C) 2017 Linaro.
+ * Viresh Kumar <viresh.kumar@linaro.org>
+ *
+ * This file is released under the GPLv2.
+ */
+
+#define pr_fmt(fmt) "Boot Constraints: " fmt