linux-kernel - Re: [PATCH v3] PM: sleep: core: Avoid setting power.must

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Date:   Mon, 23 Aug 2021 06:03:41 -0700
From:   psodagud@...eaurora.org
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     rjw@...ysocki.net, len.brown@...el.com,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        pavel@....cz
Subject: Re: [PATCH v3] PM: sleep: core: Avoid setting power.must_resume to
 false

On 2021-08-13 00:23, Greg KH wrote:
> On Tue, Aug 10, 2021 at 01:05:38PM -0700, Prasad Sodagudi wrote:
>> There are variables(power.may_skip_resume and dev->power.must_resume)
>> and DPM_FLAG_MAY_SKIP_RESUME flags to control the resume of devices 
>> after
>> a system wide suspend transition.
>> 
>> Setting the DPM_FLAG_MAY_SKIP_RESUME flag means that the driver allows
>> its "noirq" and "early" resume callbacks to be skipped if the device
>> can be left in suspend after a system-wide transition into the working
>> state. PM core determines that the driver's "noirq" and "early" resume
>> callbacks should be skipped or not with dev_pm_skip_resume() function 
>> by
>> checking power.may_skip_resume variable.
>> 
>> power.must_resume variable is getting set to false in 
>> __device_suspend()
>> function without checking device's DPM_FLAG_MAY_SKIP_RESUME and
>> dev->power.usage_count variables. In problematic scenario, where
>> all the devices in the suspend_late stage are successful and some
>> device can fail to suspend in suspend_noirq phase. So some devices
>> successfully suspended in suspend_late stage are not getting chance
>> to execute __device_suspend_noirq() to set dev->power.must_resume
>> variable to true and not getting resumed in early_resume phase.
>> 
>> Add a check for device's DPM_FLAG_MAY_SKIP_RESUME flag before
>> setting power.must_resume variable in __device_suspend function.
>> 
>> Fixes: 6e176bf8d461 ("PM: sleep: core: Do not skip callbacks in the 
>> resume phase")
>> Signed-off-by: Prasad Sodagudi <psodagud@...eaurora.org>
>> ---
>>  V2 -> V3: Format issues patch posting
>>  V1 -> V2: Fixed indentation and commit text to include scenario
>>  drivers/base/power/main.c | 6 +++++-
>>  1 file changed, 5 insertions(+), 1 deletion(-)
>> 
>> diff --git a/drivers/base/power/main.c b/drivers/base/power/main.c
>> index d568772..9ee6987 100644
>> --- a/drivers/base/power/main.c
>> +++ b/drivers/base/power/main.c
>> @@ -1642,7 +1642,11 @@ static int __device_suspend(struct device *dev, 
>> pm_message_t state, bool async)
>>  	}
>> 
>>  	dev->power.may_skip_resume = true;
>> -	dev->power.must_resume = false;
>> +	if ((atomic_read(&dev->power.usage_count) <= 1) &&
>> +	     (dev_pm_test_driver_flags(dev, DPM_FLAG_MAY_SKIP_RESUME)))
>> +		dev->power.must_resume = false;
>> +	else
>> +		dev->power.must_resume = true;
> 
> Again, what happens if the usage_count changes right after reading the
> value?  What protects that from happening?

Hi Gregh KH,
Yes. you are right.  I think, relying on  the usage_count at the  
__device_suspend stage may not be correct.
Devices IRQs are still enabled and usage_count can be changed even after 
reading.
I will send next patchset without power.usage_count check.

@@ -1649,7 +1651,10 @@ static int __device_suspend(struct device *dev, 
pm_message_t state, bool async)
         }

         dev->power.may_skip_resume = true;
-       dev->power.must_resume = false;
+       if (dev_pm_test_driver_flags(dev, DPM_FLAG_MAY_SKIP_RESUME))
+               dev->power.must_resume = false;
+       else
+               dev->power.must_resume = true;


> 
> thanks,
> 
> greg k-h