drivers/pinctrl: grab default handles from device core

This makes the device core auto-grab the pinctrl handle and set
the "default" (PINCTRL_STATE_DEFAULT) state for every device
that is present in the device model right before probe. This will
account for the lion's share of embedded silicon devcies.

A modification of the semantics for pinctrl_get() is also done:
previously if the pinctrl handle for a certain device was already
taken, the pinctrl core would return an error. Now, since the
core may have already default-grabbed the handle and set its
state to "default", if the handle was already taken, this will
be disregarded and the located, previously instanitated handle
will be returned to the caller.

This way all code in drivers explicitly requesting their pinctrl
handlers will still be functional, and drivers that want to
explicitly retrieve and switch their handles can still do that.
But if the desired functionality is just boilerplate of this
type in the probe() function:

struct pinctrl  *p;

p = devm_pinctrl_get_select_default(&dev);
if (IS_ERR(p)) {
   if (PTR_ERR(p) == -EPROBE_DEFER)
        return -EPROBE_DEFER;
        dev_warn(&dev, "no pinctrl handle\n");
}

The discussion began with the addition of such boilerplate
to the omap4 keypad driver:
http://marc.info/?l=linux-input&m=135091157719300&w=2

A previous approach using notifiers was discussed:
http://marc.info/?l=linux-kernel&m=135263661110528&w=2
This failed because it could not handle deferred probes.

This patch alone does not solve the entire dilemma faced:
whether code should be distributed into the drivers or
if it should be centralized to e.g. a PM domain. But it
solves the immediate issue of the addition of boilerplate
to a lot of drivers that just want to grab the default
state. As mentioned, they can later explicitly retrieve
the handle and set different states, and this could as
well be done by e.g. PM domains as it is only related
to a certain struct device * pointer.

ChangeLog v4->v5 (Stephen):
- Simplified the devicecore grab code.
- Deleted a piece of documentation recommending that pins
  be mapped to a device rather than hogged.
ChangeLog v3->v4 (Linus):
- Drop overzealous NULL checks.
- Move kref initialization to pinctrl_create().
- Seeking Tested-by from Stephen Warren so we do not disturb
  the Tegra platform.
- Seeking ACK on this from Greg (and others who like it) so I
  can merge it through the pinctrl subsystem.
ChangeLog v2->v3 (Linus):
- Abstain from using IS_ERR_OR_NULL() in the driver core,
  Russell recently sent a patch to remove it. Handle the
  NULL case explicitly even though it's a bogus case.
- Make sure we handle probe deferral correctly in the device
  core file. devm_kfree() the container on error so we don't
  waste memory for devices without pinctrl handles.
- Introduce reference counting into the pinctrl core using
  <linux/kref.h> so that we don't release pinctrl handles
  that have been obtained for two or more places.
ChangeLog v1->v2 (Linus):
- Only store a pointer in the device struct, and only allocate
  this if it's really used by the device.

Cc: Felipe Balbi <balbi@ti.com>
Cc: Benoit Cousson <b-cousson@ti.com>
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Mitch Bradley <wmb@firmworks.com>
Cc: Ulf Hansson <ulf.hansson@linaro.org>
Cc: Rafael J. Wysocki <rjw@sisk.pl>
Cc: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Cc: Rickard Andersson <rickard.andersson@stericsson.com>
Cc: Russell King <linux@arm.linux.org.uk>
Reviewed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
[swarren: fixed and simplified error-handling in pinctrl_bind_pins(), to
correctly handle deferred probe. Removed admonition from docs not to use
pinctrl hogs for devices]
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

2 files changed, 29 insertions, 4 deletions

diff --git a/drivers/pinctrl/core.c b/drivers/pinctrl/core.c
index e6373f30aeeb..5a2fe9aae20f 100644
--- a/drivers/pinctrl/core.c
+++ b/drivers/pinctrl/core.c
@@ -14,6 +14,7 @@
 #define pr_fmt(fmt) "pinctrl core: " fmt
 
 #include <linux/kernel.h>
+#include <linux/kref.h>
 #include <linux/export.h>
 #include <linux/init.h>
 #include <linux/device.h>
@@ -727,6 +728,8 @@ static struct pinctrl *create_pinctrl(struct device *dev)
                 return ERR_PTR(ret);
         }
 
+        kref_init(&p->users);
+
         /* Add the pinctrl handle to the global list */
         list_add_tail(&p->node, &pinctrl_list);
 
@@ -740,9 +743,17 @@ static struct pinctrl *pinctrl_get_locked(struct device *dev)
         if (WARN_ON(!dev))
                 return ERR_PTR(-EINVAL);
 
+        /*
+         * See if somebody else (such as the device core) has already
+         * obtained a handle to the pinctrl for this device. In that case,
+         * return another pointer to it.
+         */
         p = find_pinctrl(dev);
-        if (p != NULL)
-                return ERR_PTR(-EBUSY);
+        if (p != NULL) {
+                dev_dbg(dev, "obtain a copy of previously claimed pinctrl\n");
+                kref_get(&p->users);
+                return p;
+        }
 
         return create_pinctrl(dev);
 }
@@ -798,13 +809,24 @@ static void pinctrl_put_locked(struct pinctrl *p, bool inlist)
 }
 
 /**
- * pinctrl_put() - release a previously claimed pinctrl handle
+ * pinctrl_release() - release the pinctrl handle
+ * @kref: the kref in the pinctrl being released
+ */
+void pinctrl_release(struct kref *kref)
+{
+        struct pinctrl *p = container_of(kref, struct pinctrl, users);
+
+        pinctrl_put_locked(p, true);
+}
+
+/**
+ * pinctrl_put() - decrease use count on a previously claimed pinctrl handle
  * @p: the pinctrl handle to release
  */
 void pinctrl_put(struct pinctrl *p)
 {
         mutex_lock(&pinctrl_mutex);
-        pinctrl_put_locked(p, true);
+        kref_put(&p->users, pinctrl_release);
         mutex_unlock(&pinctrl_mutex);
 }
 EXPORT_SYMBOL_GPL(pinctrl_put);
diff --git a/drivers/pinctrl/core.h b/drivers/pinctrl/core.h
index 232a9f6db4aa..fdd350d639f6 100644
--- a/drivers/pinctrl/core.h
+++ b/drivers/pinctrl/core.h
@@ -9,6 +9,7 @@
  * License terms: GNU General Public License (GPL) version 2
  */
 
+#include <linux/kref.h>
 #include <linux/mutex.h>
 #include <linux/radix-tree.h>
 #include <linux/pinctrl/pinconf.h>
@@ -58,6 +59,7 @@ struct pinctrl_dev {
  * @state: the current state
  * @dt_maps: the mapping table chunks dynamically parsed from device tree for
  *      this device, if any
+ * @users: reference count
  */
 struct pinctrl {
         struct list_head node;
@@ -65,6 +67,7 @@ struct pinctrl {
         struct list_head states;
         struct pinctrl_state *state;
         struct list_head dt_maps;
+        struct kref users;
 };
 
 /**