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.

Suggested-by: Rafael J. Wysocki <redacted>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Mika Westerberg <mika.westerberg@linux.intel.com>
Cc: Rafael J. Wysocki <redacted>
Cc: Lee Jones <redacted>
Signed-off-by: Chen Yu <yu.c.chen@intel.com>
---
 drivers/mfd/intel-lpss.c | 9 +++++++++
 include/linux/pm.h       | 3 +++
 2 files changed, 12 insertions(+)

diff --git a/drivers/mfd/intel-lpss.c b/drivers/mfd/intel-lpss.c
index 41b1138..2583db8 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 DPM_DIRECT_COMPLETE;
+       /*
         * Resume both child devices before entering system sleep. This
         * ensures that they are in proper state before they get suspended.
         */

diff --git a/include/linux/pm.h b/include/linux/pm.h
index 06eb353..5606ad9 100644
--- a/include/linux/pm.h
+++ b/include/linux/pm.h

@@ -786,4 +786,7 @@ enum dpm_order {
        DPM_ORDER_DEV_LAST,
 };

+/* The device is OK to remain runtime-suspended during suspend.*/