regmap: Make regmap-mmio usable from atomic contexts

regmap-mmio uses a spinlock with spin_lock() and spin_unlock() for locking.
To be able to use the regmap API from different contexts (atomic vs non-atomic),
without the risk of race conditions, we need to use spin_lock_irqsave() and
spin_lock_irqrestore() instead. A new field, the spinlock_flags field, is added
to regmap struct to store the flags between regmap_{,un}lock_spinlock(). The
spinlock_flags field itself is also protected by the spinlock.

Thanks to Stephen Warren for the suggestion of this particular solution.
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

drivers/base/regmap/internal.h

@@ -52,6 +52,7 @@ struct regmap_async {
 struct regmap {
 	struct mutex mutex;
 	spinlock_t spinlock;
+	unsigned long spinlock_flags;
 	regmap_lock lock;
 	regmap_unlock unlock;
 	void *lock_arg; /* This is passed to lock/unlock functions */

drivers/base/regmap/regmap.c

@@ -302,13 +302,16 @@ static void regmap_unlock_mutex(void *__map)
 static void regmap_lock_spinlock(void *__map)
 {
 	struct regmap *map = __map;
-	spin_lock(&map->spinlock);
+	unsigned long flags;
+
+	spin_lock_irqsave(&map->spinlock, flags);
+	map->spinlock_flags = flags;
 }
 
 static void regmap_unlock_spinlock(void *__map)
 {
 	struct regmap *map = __map;
-	spin_unlock(&map->spinlock);
+	spin_unlock_irqrestore(&map->spinlock, map->spinlock_flags);
 }
 
 static void dev_get_regmap_release(struct device *dev, void *res)