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

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Date:   Mon, 12 Sep 2016 16:17:04 +0100
From:   Lee Jones <lee.jones@...aro.org>
To:     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>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [PATCH][RFC] mfd: intel-lpss: Avoid resuming runtime-suspended
 lpss unnecessarily

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'?

> +	/*
>  	 * Resume both child devices before entering system sleep. This
>  	 * ensures that they are in proper state before they get suspended.
>  	 */

-- 
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
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