linux-kernel - [PATCH] regmap: allow to disable all locking mechanisms

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Date:   Wed,  6 Dec 2017 15:26:21 +0100
From:   Bartosz Golaszewski <brgl@...ev.pl>
To:     Mark Brown <broonie@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, Bartosz Golaszewski <brgl@...ev.pl>
Subject: [PATCH] regmap: allow to disable all locking mechanisms

We have a use case in the at24 EEPROM driver (recently converted to
using regmap instead of raw i2c/smbus calls) where we read from/write
to the regmap in a loop, while protecting the entire loop with
a mutex.

Currently this implicitly makes us use two mutexes - one in the driver
and one in regmap. While browsing the code for similar use cases I
noticed a significant number of places where locking *seems* redundant.

Allow users to completely disable any locking mechanisms in regmap
config.

Signed-off-by: Bartosz Golaszewski <brgl@...ev.pl>
---
 drivers/base/regmap/regmap.c | 9 ++++++++-
 include/linux/regmap.h       | 5 +++++
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/drivers/base/regmap/regmap.c b/drivers/base/regmap/regmap.c
index 8d516a9bfc01..72917b2fc10e 100644
--- a/drivers/base/regmap/regmap.c
+++ b/drivers/base/regmap/regmap.c
@@ -459,6 +459,11 @@ static void regmap_unlock_hwlock_irqrestore(void *__map)
 }
 #endif
 
+static void regmap_lock_unlock_empty(void *__map)
+{
+
+}
+
 static void regmap_lock_mutex(void *__map)
 {
 	struct regmap *map = __map;
@@ -669,7 +674,9 @@ struct regmap *__regmap_init(struct device *dev,
 		goto err;
 	}
 
-	if (config->lock && config->unlock) {
+	if (config->disable_locking) {
+		map->lock = map->unlock = regmap_lock_unlock_empty;
+	} else if (config->lock && config->unlock) {
 		map->lock = config->lock;
 		map->unlock = config->unlock;
 		map->lock_arg = config->lock_arg;
diff --git a/include/linux/regmap.h b/include/linux/regmap.h
index 15eddc1353ba..072a90229e34 100644
--- a/include/linux/regmap.h
+++ b/include/linux/regmap.h
@@ -264,6 +264,9 @@ typedef void (*regmap_unlock)(void *);
  *                field is NULL but precious_table (see below) is not, the
  *                check is performed on such table (a register is precious if
  *                it belongs to one of the ranges specified by precious_table).
+ * @disable_locking: This regmap is either protected by external means or
+ *                   is guaranteed not be be accessed from multiple threads.
+ *                   Don't use any locking mechanisms.
  * @lock:	  Optional lock callback (overrides regmap's default lock
  *		  function, based on spinlock or mutex).
  * @unlock:	  As above for unlocking.
@@ -333,6 +336,8 @@ struct regmap_config {
 	bool (*readable_reg)(struct device *dev, unsigned int reg);
 	bool (*volatile_reg)(struct device *dev, unsigned int reg);
 	bool (*precious_reg)(struct device *dev, unsigned int reg);
+
+	bool disable_locking;
 	regmap_lock lock;
 	regmap_unlock unlock;
 	void *lock_arg;
-- 
2.15.1