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

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Message-ID: <be1f07d1-4078-8963-2297-42e39c942681@intel.com>
Date:   Tue, 13 Sep 2016 14:15:36 +0200
From:   "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
To:     Lee Jones <lee.jones@...aro.org>, Chen Yu <yu.c.chen@...el.com>
Cc:     linux-kernel@...r.kernel.org,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>
Subject: Re: [PATCH][RFC] mfd: intel-lpss: Avoid resuming runtime-suspended
 lpss unnecessarily

On 9/13/2016 10:24 AM, Lee Jones wrote:
> On Mon, 12 Sep 2016, Chen Yu wrote:
>> On Mon, Sep 12, 2016 at 04:17:04PM +0100, Lee Jones wrote:
>>> On Sun, 04 Sep 2016, Chen Yu wrote:
>>>
>>>> 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@...ux.intel.com>
>>>> Cc: Mika Westerberg <mika.westerberg@...ux.intel.com>
>>>> Cc: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>>>> Signed-off-by: Chen Yu <yu.c.chen@...el.com>
>>>> ---
>>>>   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;
>>> What's '1'?
>>>
>> According to the comment in device_prepare():
>>
>> A positive return value from ->prepare() means "this device appears
>> to be runtime-suspended and its state is fine, so if it really is
>> runtime-suspended, you can leave it in that state provided that you
>> will do the same thing with all of its descendants".
> Are there no defines for this?
>

Not at the moment, but I guess they can be added if really necessary. :-)

But that said it doesn't have to be 1 or any specific value. Any 
positive number will have the same effect.


Thanks,

Rafael