[RFC] mmc:change mmc_init workqueue into a freezable workqueue

[Russell King - ARM Linux]

On Thu, Jan 08, 2015 at 06:06:32PM +0800, Wang, Yalin wrote:
> This patch fix the mmc driver suspend/resume conflict problems,
> mmc workqueue will queue mmc_rescan(), and it will call some
> pm_runtime_* functions, this will conflict with suspend path sometimes,
> and will result in some strange behavior:
> 
> Suspend path:
> 	-000 |context_switch(inline)
> 	-000 |__schedule()
> 	-001 |schedule_preempt_disabled()
> 	-002 |spin_lock(inline)
> 	-002 |__mutex_lock_common(inline)
> 	-002 |__mutex_lock_slowpath(lock_count = 0xEDCD0F48)
> 	-003 |__mutex_fastpath_lock(inline)
> 	-003 |mutex_lock(lock = 0xEDCD0F48)
> 	-004 |sdhci_do_set_ios(host = 0xEDCD0CC0, ios = 0xEDCD0A70)
> 	-005 |sdhci_set_ios(?, ios = 0xEDCD0A70)
> 	-006 |mmc_set_ios(host = 0xEDCD0800)
> 	-007 |mmc_delay(inline)
> 	-007 |mmc_power_off(host = 0xEDCD0800)
> 	-008 |mmc_suspend_host(inline)
> 	-008 |mmc_suspend_host(host = 0xEDCD0800)
> 	-009 |mmc_host_suspend(dev = 0xEDCD0808)
> 	-010 |dpm_run_callback(cb = 0xC0627A88, dev = 0xEDCD0808, state = (event = 2), info = 0xC0B6EF9B)
> 	-011 |__device_suspend(dev = 0xEDCD0808, state = (event = 2), ?)
> 	-012 |device_suspend(inline)

Suspend can't complete until device_suspend() returns.  Hence, mmc_power_off()
must finish.

> mmc_rescan() resume path:
> 	 -000 |context_switch(inline)
> 	 -000 |__schedule()
> 	 -001 |do_undefinstr(regs = 0xD12242F0)
> 	 -002 |__und_svc(asm)
> 	  --> |exception

I assume this is what you mean by "strange behaviour" ?  If so, please
give the full oops.  Have you fully diagnosed the failure?

> most mmc power callback function don't check this special case, and
> will cause problems, make sure the workqueue is stopped during suspend
> is more safe.

I think there's a bad side effect of this: if you have swap on a SD card,
and you ask the system to hibernate, you don't want this thread to freeze.

What you do need to do is to ensure that new requests can't be processed
while the host is suspended.

A hibernate works (approximately) as:

- freeze all tasks
- push as much out to swap as possible
- suspend devices
- create system snapshot
- resume devices
- write system snapshot to swap
- power down

If the MMC thread is frozen, and you have swap on SD, then it can't write
the image to swap.

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