Re: Re: [PATCH] Remove process freezer from suspend to RAM pathway

Paul Mackerras
2007-07-04T03:55:50Z

Alan Stern writes:

> USB already implements runtime PM. If a device is suspended at runtime
> and a task tries to access it, the device is automatically resumed.
> No problem there.
>
> The problem comes when the system is doing a STR. Right now the code
> doesn't keep track of the difference between a runtime suspend and a
> system suspend -- once the device is suspended, it's suspended, period.

Whether or not to resume a suspended device when an I/O request comes
in is a policy decision, and there could be cases where the user wants
I/O requests to be blocked, or to fail, or to be dropped while the
device is suspended, even for runtime power management. For example,
a sound card could be suspended due to a low-battery condition, and in
that case you would want the driver to just drop any data that
userspace tries to write to the soundcard.

> Yes, the code could be changed to keep track of the reason for a device
> suspend. But that just raises the old problem of what to do when
> there's an I/O request for a suspended device during STR.

Is this actually a real problem? I would think the policy would be
"block" for block devices (pun not intended :), "drop" for network
devices, etc.

> Consider a particularly troublesome case: During STR, a non-frozen task
> writes to /sys/bus/BBB/drivers/DDD/bind. The sysfs core grabs the
> device semaphore and calls the driver's probe routine. If the driver
> isn't PM-aware it simply tries to initialize the device and fails
> because the device is already suspended. That's no good; it isn't
> transparent.

How did the device get suspended if it didn't have a driver? If it
did have a driver, why didn't the bind attempt fail?

> So assume the driver is PM-aware. It tries to resume the device, which
> fails because STR is underway. Now what can it do? There's only one
> possibility: It must block until the resume call can succeed. But when
> is that?
>
> It has to be before the PM core tries to resume the device, because the
> core will try to acquire the device semaphore and will block waiting
> for the probe call to complete. But it has to be after the PM core
> resumes the device's parent, because obviously the device can't resume
> until its parent is awake.

Suppose the device-model core code simply blocked all bind and unbind
requests while suspend is under way, until resume is finished.
Wouldn't that solve the problem?

Paul.