regulator: core: Reduce busy-wait looping

Keep busy-wait looping to a minimum while waiting for a regulator to
ramp-up to the target voltage. This follows the guidelines set forth
in Documentation/timers/timers-howto.txt and assumes that regulators
are never enabled in atomic context.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Mark Brown <broonie@linaro.org>

1 files changed, 33 insertions, 5 deletions

diff --git a/drivers/regulator/core.c b/drivers/regulator/core.c
index ad154247bb90..5075ed1e94a6 100644
--- a/drivers/regulator/core.c
+++ b/drivers/regulator/core.c
@@ -1740,11 +1740,39 @@ static int _regulator_do_enable(struct regulator_dev *rdev)
          * together.  */
         trace_regulator_enable_delay(rdev_get_name(rdev));
 
-        if (delay >= 1000) {
-                mdelay(delay / 1000);
-                udelay(delay % 1000);
-        } else if (delay) {
-                udelay(delay);
+        /*
+         * Delay for the requested amount of time as per the guidelines in:
+         *
+         *     Documentation/timers/timers-howto.txt
+         *
+         * The assumption here is that regulators will never be enabled in
+         * atomic context and therefore sleeping functions can be used.
+         */
+        if (delay) {
+                unsigned int ms = delay / 1000;
+                unsigned int us = delay % 1000;
+
+                if (ms > 0) {
+                        /*
+                         * For small enough values, handle super-millisecond
+                         * delays in the usleep_range() call below.
+                         */
+                        if (ms < 20)
+                                us += ms * 1000;
+                        else
+                                msleep(ms);
+                }
+
+                /*
+                 * Give the scheduler some room to coalesce with any other
+                 * wakeup sources. For delays shorter than 10 us, don't even
+                 * bother setting up high-resolution timers and just busy-
+                 * loop.
+                 */
+                if (us >= 10)
+                        usleep_range(us, us + 100);
+                else
+                        udelay(us);
         }
 
         trace_regulator_enable_complete(rdev_get_name(rdev));