[PATCH][RFC] mfd: intel-lpss: Avoid resuming runtime-suspended lpss unnecessarily

[Chen Yu]

We have report that the intel_lpss_prepare() takes too much time during
suspend, and this is because we first resume the devices from runtime
suspend by resume_lpss_device(), to make sure they are in proper state
before system suspend, which takes 100ms for each LPSS devices(PCI power
state from D3_cold to D0). And since resume_lpss_device() resumes the
devices synchronously, we might get huge latency if we have many
LPSS devices.

So first try is to use pm_request_resume() instead, to make the runtime
resume process asynchronously. Unfortunately the asynchronous runtime
resume relies on pm_wq, which is freezed at early stage. So we choose
another method, that is to avoid resuming runtime-suspended devices,
if they are already runtime suspended. This is safe because for LPSS
driver, the runtime suspend and system suspend are of the same
hook - i.e., intel_lpss_suspend(). And moreover, this device is
neither runtime wakeup source nor system wakeup source.

Cc: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
Cc: Mika Westerberg <mika.westerberg@xxxxxxxxxxxxxxx>
Cc: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
Signed-off-by: Chen Yu <yu.c.chen@xxxxxxxxx>
---
 drivers/mfd/intel-lpss.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/mfd/intel-lpss.c b/drivers/mfd/intel-lpss.c
index 41b1138..6dcc9a0 100644
--- a/drivers/mfd/intel-lpss.c
+++ b/drivers/mfd/intel-lpss.c
@@ -485,6 +485,15 @@ static int resume_lpss_device(struct device *dev, void *data)
 int intel_lpss_prepare(struct device *dev)
 {
 	/*
+	 * This is safe because:
+	 * 1. The runtime suspend and system suspend
+	 * are of the same hook.
+	 * 2. This device is neither runtime wakeup source
+	 * nor system wakeup source.
+	 */
+	if (pm_runtime_status_suspended(dev))
+		return 1;
+	/*
 	 * Resume both child devices before entering system sleep. This
 	 * ensures that they are in proper state before they get suspended.
 	 */
-- 
2.7.4