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SubjectRe: [PATCH v3 1/3] driver core: Support asynchronous driver shutdown
[Beginning of thread: https://lore.kernel.org/r/20220517220816.1635044-2-tansuresh@google.com]
On Wed, May 18, 2022 at 12:50:02PM -0500, Bjorn Helgaas wrote:

> Devices have this async_suspend bit:
>
> struct device {
> struct dev_pm_info {
> unsigned int async_suspend:1;
>
> Drivers call device_enable_async_suspend() to set async_suspend if
> they want it. The system suspend path is something like this:
>
> suspend_enter
> dpm_suspend_noirq(PMSG_SUSPEND)
> dpm_noirq_suspend_devices(PMSG_SUSPEND)
> pm_transition = PMSG_SUSPEND
> while (!list_empty(&dpm_late_early_list))
> device_suspend_noirq(dev)
> dpm_async_fn(dev, async_suspend_noirq)
> if (is_async(dev))
> async_schedule_dev(async_suspend_noirq) # async path
>
> async_suspend_noirq # called asynchronously
> __device_suspend_noirq(dev, PMSG_SUSPEND, true)
> callback = pm_noirq_op(PMSG_SUSPEND) # .suspend_noirq()
> dpm_run_callback(callback) # async call
>
> __device_suspend_noirq(dev, pm_transition, false) # sync path
> callback = pm_noirq_op(PMSG_SUSPEND) # .suspend_noirq()
> dpm_run_callback(callback) # sync call
>
> async_synchronize_full # wait
>
> If a driver has called device_enable_async_suspend(), we'll use the
> async_schedule_dev() path to schedule the appropriate .suspend_noirq()
> method. After scheduling it via the async path or directly calling it
> via the sync path, the async_synchronize_full() waits for completion
> of all the async methods.

Correct me if I'm wrong: in the suspend scenario, there are several
phases, and async_synchronize_full() ensures that one phase finishes
before the next phase starts. For example:

dpm_suspend_end
dpm_suspend_late # phase 1
while (!list_empty(&dpm_suspended_list))
device_suspend_late
async_synchronize_full # finish phase 1
dpm_suspend_noirq # phase 2
dpm_noirq_suspend_devices
while (!list_empty(&dpm_late_early_list))
device_suspend_noirq
async_synchronize_full

The device .suspend_late() and .suspend_noirq() methods may all be
started asynchronously. So far there's nothing to order them within
the phase, but async_synchronize_full() ensures that all the
.suspend_late() methods finish before the .suspend_noirq() methods
start.

Obviously we do want a child's method to complete before we run the
parent's method. If I understand correctly, that parent/child
synchronization is done by a different method: __device_suspend_late()
and __device_suspend_noirq() call dpm_wait_for_subordinate(), which
waits for &dev->power.completion for all children:

__device_suspend_late
dpm_wait_for_subordinate
dpm_wait_for_children # wait for children .suspend_late()
device_for_each_child(dev, &async, dpm_wait_fn)
dpm_wait_fn
dpm_wait
wait_for_completion(&dev->power.completion)
dpm_run_callback # run parent method, e.g., ops->suspend_late
complete_all(&dev->power.completion) # note completion of parent

> I assume your suggestion is to do something like this:
>
> struct device {
> struct dev_pm_info {
> unsigned int async_suspend:1;
> + unsigned int async_shutdown:1;
>
> + void device_enable_async_shutdown(struct device *dev)
> + dev->power.async_shutdown = true;
>
> device_shutdown
> while (!list_empty(&devices_kset->list))
> - dev->...->shutdown()
> + if (is_async_shutdown(dev))
> + async_schedule_dev(async_shutdown) # async path
> +
> + async_shutdown # called asynchronously
> + dev->...->shutdown()
> +
> + else
> + dev->...->shutdown() # sync path
> +
> + async_synchronize_full # wait

In the shutdown case, I think we still probably need the
async_synchronize_full() to ensure that all the .shutdown() methods
complete before we turn the power off, reboot, or kexec.

But I think we also need a mechanism like dev->power.completion to
make sure all the child .shutdown() methods complete before we run a
parent's .shutdown().

There's not much overlap between the suspend path and the shutdown
path (probably none at all), so it's tempting to use the existing
dev->power.completion for shutdown as well.

But I don't think that's feasible because dev->power.completion is
tied up with dev->power.async_suspend, which is set by
device_enable_async_suspend(). That's a different concept than async
shutdown, and drivers will want one without the other.

Does this make sense?

Bjorn

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