[PATCH 6.6 1/1] PM: sleep: Restore asynchronous device resume optimization

[Yenchia Chen]

>> From: "Rafael J. Wysocki" <rafael.j.wysocki at intel.com>
>> 
>> commit 3e999770ac1c7c31a70685dd5b88e89473509e9c upstream.
>> 
>> Before commit 7839d0078e0d ("PM: sleep: Fix possible deadlocks in core
>> system-wide PM code"), the resume of devices that were allowed to resume
>> asynchronously was scheduled before starting the resume of the other
>> devices, so the former did not have to wait for the latter unless
>> functional dependencies were present.
>> 
>> Commit 7839d0078e0d removed that optimization in order to address a
>> correctness issue, but it can be restored with the help of a new device
>> power management flag, so do that now.
>> 
>> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki at intel.com>
>> Reviewed-by: Stanislaw Gruszka <stanislaw.gruszka at linux.intel.com>
>> Signed-off-by: Yenchia Chen <yenchia.chen at mediatek.com>
>> ---
>>  drivers/base/power/main.c | 117 +++++++++++++++++++++-----------------
>>  include/linux/pm.h        |   1 +
>>  2 files changed, 65 insertions(+), 53 deletions(-)

>Why does this need to be backported?  What bug is it fixing?

>confused,

>greg k-h

Below is the scenario we met the issue:
1) use command 'echo 3 > /proc/sys/vm/drop_caches'
   and enter suspending stage immediately.
2) power on device, our driver try to read mmc after leaving resume callback
   and got stucked.

We found if we did not drop caches, mmc_blk_resume will be called and
system works fine.

If we drop caches before suspending, there is a high possibility that
mmc_blk_resume not be called and our driver stucked at filp_open.

We still try to find the root casue is but with this patch, it works.

Since it has been merged in mainline, we'd like to know it is ok to merge to stable.