[PATCH 3/6] devfreq: add support for suspend/resume of a devfreq device

[Lukasz Luba <l.luba@partner.samsung.com>]

The patch adds support for handling suspend/resume process.
It uses atomic variables to make sure no race condition
affects the process.

The patch draws on Tobias Jakobi's work posted ~2 years ago, who tried to
solve issue with devfreq device's frequency during suspend/resume.
During the discussion on LKML some corner cases and comments appeared
related to the design. This patch address them keeping in mind suggestions
from Chanwoo Choi.

Suggested-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Suggested-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Lukasz Luba <l.luba@partner.samsung.com>
---
 drivers/devfreq/devfreq.c | 45 +++++++++++++++++++++++++++++++++++++++------
 1 file changed, 39 insertions(+), 6 deletions(-)

diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
index cf9c643..7e09de8 100644
--- a/drivers/devfreq/devfreq.c
+++ b/drivers/devfreq/devfreq.c
@@ -872,14 +872,33 @@ EXPORT_SYMBOL(devm_devfreq_remove_device);
  */
 int devfreq_suspend_device(struct devfreq *devfreq)
 {
-	if (!devfreq)
-		return -EINVAL;
+        int ret;
+        unsigned long prev_freq;
+        u32 flags = 0;
+
+        if (!devfreq)
+                return -EINVAL;
+
+        if (devfreq->governor) {
+		ret = devfreq->governor->event_handler(devfreq,
+					DEVFREQ_GOV_SUSPEND, NULL);
+		if (ret)
+			return ret;
+	}
 
-	if (!devfreq->governor)
-		return 0;
+	if (devfreq->suspend_freq) {
+		if (atomic_inc_return(&devfreq->suspend_count) > 1)
+			return 0;
 
-	return devfreq->governor->event_handler(devfreq,
-				DEVFREQ_GOV_SUSPEND, NULL);
+		ret = devfreq_set_target(devfreq, devfreq->suspend_freq,
+					 &prev_freq, flags);
+		if (ret)
+			return ret;
+
+		devfreq->resume_freq = prev_freq;
+	}
+
+        return 0;
 }
 EXPORT_SYMBOL(devfreq_suspend_device);
 
@@ -893,9 +912,23 @@ EXPORT_SYMBOL(devfreq_suspend_device);
  */
 int devfreq_resume_device(struct devfreq *devfreq)
 {
+        int ret;
+        unsigned long prev_freq;
+        u32 flags = 0;
+
 	if (!devfreq)
 		return -EINVAL;
 
+	if (devfreq->suspend_freq) {
+		if (atomic_dec_return(&devfreq->suspend_count) >= 1)
+			return 0;
+
+		ret = devfreq_set_target(devfreq, devfreq->resume_freq,
+					 &prev_freq, flags);
+		if (ret)
+			return ret;
+	}
+
 	if (!devfreq->governor)
 		return 0;
 
-- 
2.7.4